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Introduction Section 1.0.0

Glory Bowl Avalanche Incident

Photograph of the Glory Bowl avalanche pathIn the early morning of December 1, 2000, a 28-year old snowboarder named Joel Roof began his ascent of Mt. Glory, intending to snowboard down from the summit of the main Glory Bowl run, a large east-facing bowl.

Mt. Glory is at the southern end of the Teton Mountain Range in western Wyoming. It’s a popular backcountry destination for skiers and snowboarders, due to its relatively easy access from the top of Teton Pass. The bowl is fairly wide, with aspects ranging from southeasterly to northeasterly. Its average steepness is just over 35 degrees (the slope angle of a challenging, black diamond, ski run).

Glory Bowl received 22 inches (56 cm) of snow during the week prior to the incident, with 3 inches (8 cm) falling the evening before. Joel was greeted with a fresh blanket of snow and clear skies when he began his ascent of Glory Bowl early in the morning on December 1. He started down from the summit around 8:00 AM, choosing a bold line almost directly down the center of the east-facing bowl. Veering slightly to the right with his first few turns, he moved onto the more northeast-facing aspect.

When Joel was only several hundred feet (100 m) below the summit, he triggered an avalanche, which took down almost the entire right side of the bowl. Joel was carried approximately 2,000 vertical feet (600 m) and buried under four to five feet (1 to 2 m) of snow at the bottom of the avalanche, just short of the highway. The avalanche actually ran across the highway, which was busy with morning commuters. Witnesses started searching for him almost immediately. Joel’s body was found within one and a half hours using probes and search dogs. He was not wearing an avalanche transceiver.

Photograph of the bottom of avalanche debris where a snowboarder was buriedWhat led to the formation of the avalanche? How long had it been in the making? Why did it go on that particular day? Did one person really set it off?

To answer these questions, you need to understand the area’s weather history, snowpack, and terrain, for these are the main factors that create avalanches.

Let’s take a look at Glory Bowl’s weather history from the start of the cold season. The avalanche had been in the making since early October, when the first snow fell and stayed on the ground in the mountains. Several days of warmer temperatures in mid-October led to a few melt-freeze cycles, which created a thick crust near the ground. By the end of October, the upper reaches of the northeast-facing slope had less than a foot (30 cm) of settled snow. The adjacent southeast-facing aspect was almost bare, which is noteworthy because that half of the bowl didn’t avalanche.

Another brief warm up in early November created a new crust on the snow surface. Light snow fell during mid-November, adding less than a foot (<30 cm) of new snow to the pack. Since temperatures were unusually cold for the time of year (0° to 20° F or -18° to -7°C) at 10,000 feet (3000 m), the snow density was very low. The light snows were followed by several days of clear and even colder temperatures (0° to -5° F or -18° to -15°C) at 10,000 feet (3000 m).

A week before the avalanche, the snowpack was shallow, with less than 2 feet (60 cm) of settled snow on the ground at 9,000 to 10,000 feet (2700 to 3000 m). Recall the two crusts that formed earlier in the season; these were now covered with mostly unconsolidated, poorly bonded, cold snow crystals.

From November 24 to 30, just under two feet (60 cm) of new snow were recorded at 9,500 feet (2900 m) at the nearby Jackson Hole Mountain Resort. The snow contained almost 1.5 inches (38 mm) of water, making it moderately dense. The heaviest 24-hour total occurred on November 27, when seven inches (18 cm) of new, very dense snow fell containing 0.71 inches (18 mm) of water. During the storm, ridge-top winds were primarily out of the west, although there were periods of south-to-southwest winds. The winds loaded much more snow onto the leeward east- and northeast-facing aspects of Glory Bowl. By the end of November, the avalanche was “ready to go,” needing only a little provocation, which the snowboarder provided.

As you go through the module, we’ll refer back to this incident, helping you understand why the sequence of events created such dangerous conditions. You’ll understand how precipitation, wind, and temperature can combine to create an unstable snowpack, one that’s prone to avalanches.

Avalanche Impacts

Avalanche fatalities have been on the increase in the last 20 years due to the rising number of climbers, skiers, snowboarders, and snowmobiliers in the backcountry.

Photograph of people probing avalanche debris for a skier buried in a slab avalanche, Hyalite Range, Bozeman, MT

On average, about 150 people worldwide die annually, with 30 of those deaths in the United States and 14 in Canada.

Graph of US Annual Avalanche Fatalities from 1950/51 to 2009/10

Avalanche forecast centers throughout the world play a significant role in keeping those numbers from growing much further. Warnings and avalanche danger ratings are issued by professional avalanche forecasters on a daily basis for an increasing number of mountainous locations around the world. The ratings include:

  • Extreme (5): Avoid all avalanche terrain
  • High (4): Very dangerous avalanche conditions; do not travel in avalanche terrain
  • Considerable (3): Dangerous avalanche conditions; evaluate snowpack and plot routes very carefully and be conservative when making decisions
  • Moderate (2): Avalanche conditions are heightened in specific types of terrain; evaluate terrain and snow carefully and identify features of concern
  • Low (1): Avalanche conditions are generally safe but look out for unstable snow on isolated terrain features

Avalanches also impact highways and railroads and have caused millions of dollars of damage to buildings and other structures. More diligent planning, improved mitigation techniques, and better forecasting have also helped limit property damage in recent years.

Avalanche Weather Forecasts

Photograph of an avalanche in motionMeteorologists work with avalanche forecasters, providing detailed, area-specific weather information for mountainous areas. Avalanche forecasters integrate this information with their knowledge of snowpack and terrain to create a more comprehensive avalanche hazard forecast that describes the stabillity of the snowpack and likelihood of avalanche occurrence. Even without detailed snowpack data, avalanche potential can be estimated based on weather information alone (although the forecasts are not as complete or accurate).

Like weather forecasting, avalanche forecasting is part science and art, part fact and judgment. Currently, no computer models can forecast avalanches with any degree of accuracy. The forecast problem is compounded by the number of weather variables involved, the complexity of the terrain, and the lack of onsite weather and snowpack information. This puts the burden on avalanche forecasters, who need skills honed by years of experience to make accurate decisions about the likelihood of avalanche occurrence.

About the Module

The module begins by introducing avalanches, including the role that weather plays in their formation. This sets the stage for part two, where we present a process for forecasting avalanche weather. The process is divided into four parts:

  • Pre-forecast preparation
  • Current weather
  • Future weather
  • Making the final forecast

After the process is described, you’ll have several opportunities to apply it to different avalanche-prone mountainous locations.

Thumbnail image for COMET's Snowpack and Its Assessment moduleBy the end of the module, you should be able to determine whether avalanche potential is increasing, decreasing, or staying the same based on the weather. Specifically, you’ll be able to:

  • Define an avalanche and describe its components, the types of weather that produce it, the different types of avalanches, and avalanche triggers
  • Identify the weather parameters that play a key role in avalanche formation
  • Outline the avalanche weather forecast process
  • Given a request for an avalanche weather forecast, describe the background information that's important to gather about the area's climate, terrain, and snowpack, and its weather and avalanche history
  • Describe the process of assessing current and future avalanche weather conditions for avalanche potential
  • Describe how the information from the previous steps is integrated to produce a final avalanche weather forecast

It is recommended that you go through COMET’s Snowpack and Its Assessment module (http://www.meted.ucar.edu/afwa/snowpack/) before starting this one. The Assessment module explores snowpack science and describes various assessment techniques. However, it does not provide a detailed procedure for doing snowpack stability tests; you’ll need to get that information elsewhere. In fact, neither module substitutes for a formal avalanche training course. To find out about avalanche field courses, visit http://www.avalanche.org.

Basic Avalanche Information

Introduction

small photograph of an avalanches
  • What’s the relationship between climate and avalanches?
  • How many types of avalanches are there? Which are the most dangerous and destructive?
  • How does weather contribute to their development?
  • What are the components of an avalanche?
  • What causes an avalanche to release?
  • How large and destructive can avalanches be?
  • How can you tell if you’re in avalanche country?

These are some of the questions that will be addressed in this section. We’ll introduce avalanches and then focus on their main components and the types of weather that produce them.

Avalanche Climate

Climate determines why certain parts of the world have lots of snow while other places at similar latitudes are arid. Various classification schemes describe the types of snowpack found around the world.

Global snowpack types based on the Sturm et al classification scheme, with maps showing the type for each region of Northern America

When it comes to the types of climate in which avalanches occur, we use another set of terms. Avalanche climate refers to average winter weather patterns that cause certain kinds of snowpack conditions to develop. There are three primary types, maritime (coastal), continental, and intermountain, which are differentiated by average overall snowpack depth, average winter temperatures, and frequency and average density of new snowfall. In general, maritime avalanche climates lie within coastal mountain ranges; continental avalanche climates are located in higher-elevation, interior mountain ranges; and intermountain avalanche climates are transitional zones, usually found between coastal and interior mountain ranges.

Google map of the Western US with the avalanche climates zones (maritime, intermountain, continental) labeled for many of the major mountain ranges

Types of Avalanches

An avalanche is a mass of snow that moves rapidly down a steep mountain slope. There are two main types to be concerned about: loose snow and slab avalanches.

A loose snow avalanche (a.k.a. a slough or sluff) is simply loose snow that originates at a single point on a slope and gathers cohesionless snow on the surface of the pack as it descends. (Cohesionless snow is loose, unconsolidated, individual snow crystals that are not well bonded to each other.)

Photograph of a large cornice formed by prevailing winds over a ridgetop, Alaska

Photograph of a large cornice formed by prevailing winds over a ridgetop, Alaska

Loose snow avalanches often appear as an inverted ‘V” pattern on the snow slope. They are capable of burying a person or carrying someone over a cliff but rarely are large enough to do significant property damage.

A slab avalanche occurs when a cohesive layer of snow slides down a slope. (Cohesive snow consists of very well bonded and consolidated snow crystals throughout the entire layer.) Since slab avalanches often form from new snow and wind, they are referred to as “wind slabs.”

Photograph of a slab avalanche

Slabs can consist of snow from a single storm or from multiple layers of snow from several storms. As you’d expect, slab avalanches cause the most fatalities and do the most property damage.

Slabs have several parts:

  • A fracture line at the upper limit on the slope (the “crown”)
  • Flanks, which are continuations of the fracture lines down both sides of the slab
  • A stauchwall, which is the bottom or lower limit of the slab; this is often obliterated as the avalanche moves downhill
  • A bed surface upon which the avalanche slides; this is usually smooth and planar, although it can be the ground itself

Photograph of a slab avalanche with the parts labeled

Slab avalanches are further categorized as “soft” or “hard.”

Soft slabs form when winds are relatively light and/or the snow has relatively low density. They break up easily and become more powdery as they run downhill.

Photograph of a skier-released, medium sized, soft slab avalanche

Hard slabs form when winds are relatively strong and/or the snow is of higher density. They maintain large blocks of snow as they descend to the bottom of the slope.

Photograph of a small, hard slab avalanche

Both loose snow and slab avalanches may consist of dry or wet snow.

Photo of a wet snow avalanche in early spring in Alaska

Question

Photograph of the Glory Bowl avalanche path

The Glory Bowl avalanche discussed earlier was a slab avalanche. What evidence supports this? (Choose all that apply, then click Done.)

The correct answers are A, B, and D.

The Glory Bowl avalanche encompassed a large area, almost the entire right side of the bowl (when looking down from the top). The photograph from the road below shows a crown (partially visible on the upper left side of the Bowl) where the fracturing began. The exposed vegetation and rocks mark the bed surface.

Avalanche Paths

An avalanche path is a fixed area within which avalanches travel. Avalanche paths have three main sections:

  • The starting zone, which is the uppermost part of the avalanche path; for a loose snow avalanche, it’s where the first snow grain starts to move downhill; for a slab avalanche, it’s where the crown is located
  • The track, which is the area within which a particular avalanche travels; the track is generally downhill of the starting zone and is usually treeless or has trees smaller than the surrounding vegetation
  • The runout zone, where the debris from the avalanche accumulates at the bottom of the slope
Photograph of an avalanche with the parts labeled (starting zone, track, runout zone)

Avalanche Terrain

Most avalanches occur on mountain slopes with a steepness between 30 and 45 degrees. Avalanches only occur on slopes of less than 30 degrees if the snowpack is very unstable. Slope angles steeper than 45 degrees usually shed snow regularly as it accumulates through sluffing or shallow-depth slab avalanches, which prevents larger avalanches from occurring. This is especially true in continental and intermountain avalanche climates. In maritime climates, slab avalanches can sometimes build to greater depths on steeper slopes, between 45 and 60 degrees.

Graphical depiction showing the window of terrain steepness in which avalanches can form

Avalanche terrain is also defined by other factors, such as:

  • The aspect (direction that a tilted ground surface faces); aspect affects how wind deposits snow on a slope and the intensity of solar radiation on that slope
  • The elevation (altitude); elevation impacts the type of snow that an area gets and its temperatures and wind regime

Note that upper slopes can have different snowpack conditions, exposure to wind and sun, and vegetation than lower slopes. For more information on avalanche terrain, see the COMET module “Snowpack and Its Assessment.”

Avalanche Triggers

Natural avalanches are initiated or triggered by something in nature, such as new snow overloading an existing snowpack, a cornice breaking off, snowmelt, a rockfall, or even an earthquake.

Photograph of a large cornice formed by prevailing winds over a ridgetop, Alaska

In contrast, artificial avalanches are triggered by people and wildlife. Avalanche hazard reduction personnel intentionally trigger avalanches using explosives in order to keep roads, railway corridors, and ski areas safe for use.

Photograph of an avalanche crew firing a howitzer as an avalanche control method on Sylvan Pass in Yellowstone National Park

Natural and artificial avalanches can occur at any time.

  • Direct-action avalanches occur during a storm or just after it’s ended
  • Delayed-action avalanches occur more than 24 hours after a storm has stopped

Avalanche Sizes

Avalanches are classified by their relative size and destructive force.

Size relative to path takes the whole avalanche path into account and can include its horizontal and vertical extent as well as the depth of the fracture.

Destructive force is a measure of the avalanche’s potential to damage trees, people, and property in its track or runout zone.

Table of avalanche size ratings (size relative to path vs. destructive force)

What’s the highest rating that the Glory Bowl avalanche would probably be given? (Choose the best answer.)

The correct answer is C.

Since the avalanche buried and killed a person, had a path length closer to 3,300 feet than 330 (1000 m than 100), and could have easily buried a car on the highway, it was a medium avalanche.

Avalanche Development

Avalanche educators like to say, “weather is the architect of all avalanches.” After all, it’s the weather that builds the snowpack and causes changes to occur within it. And it’s the weather that can eventually overburden a snowpack to the point of failure, resulting in an avalanche.

Since slab avalanches are the most dangerous type of avalanche, we’ll focus on them.

A slab avalanche has three basic components: a slab, weak layer, and sliding surface. An avalanche can occur without all three, but the presence of each one increases its potential.

  • The slab is the consolidated mass of snow that’s put into motion as a unit when the avalanche releases
  • The weak layer is made up of unconsolidated or poorly bonded snow that can easily collapse under stress; most avalanches occur when a weak layer fails, putting the slab above it in motion down the slope
  • A sliding surface under a weak layer provides a relatively smooth surface upon which the avalanche can readily and rapidly move downhill

Let’s look at the weather conditions that promote the development of each one.

Slab Formation

Slabs usually form when wind transports new snow from the windward side of a mountain onto a leeward slope. This most commonly occurs through turbulent suspension in the air above the snow slope but also through rolling or saltation along the snow surface. (Saltation is a process in which snow crystals move forward through a series of jumps or skips, like a game of leap-frog.)

Wind fractures the new snowflakes in the air or the snow crystals that it picks up from the ground. It breaks them into smaller pieces and deposits them on the leeward slope as a dense, cohesive layer of snow.

Although slabs usually form during storms, they can also develop between storms if the wind is strong enough to move the snow on the ground from a windward to leeward slope. The lower the density of the snow, the lighter the wind required to move it.

Photograph of a large cornice formed by prevailing winds over a ridgetop, Alaska

Snow transported by wind blowing perpendicular to a ridge crest sometimes forms a cornice. Cornices are good visual indicators of the prevailing wind direction, pointing towards wind-loaded leeward slopes.

Key points

  • New snow combined with wind forms slabs
  • It doesn’t need to be snowing to form a new slab, you just need wind

Weak Layer Formation

Weak layers can form through several processes that generally occur between storm periods when clear skies and cold temperatures are present. The longer the clear, cold weather, the weaker a weak layer can become. Temperature gradients drive these processes, both within the snowpack and at the surface. There are three main types of weak layers: surface hoar, near-surface facets, and depth hoar.

1. Surface hoar

Closeup photograph of surface hoar crystals
  • The frozen winter equivalent of dew
  • Isn’t a problem until it becomes buried by the next snowfall
  • Forms through “deposition” where water vapor (a gas) is directly transformed into an ice crystal (a solid) without first becoming a liquid
  • Develops when relatively warm, moist air very close to a cold snow surface becomes oversaturated, reaching the frost point, and deposits onto the colder snow surface, forming feathery ice crystals on the surface; these crystals can range in size from a baby’s fingernail to an oak leaf, with the size dependent on how long the weather remains clear, cold, and calm
  • Can only form with near-calm winds (less than 10 mph)
  • Can form in just a few minutes although it typically forms overnight
  • Can continue forming during the day on north-facing, shaded slopes
  • Will sublimate (go from a solid back to a vapor) when contacted by warm air or direct sunlight

2. Near-surface facets

Photograph of faceted snow grains
  • Commonly referred to as “facets” or “faceted snow”
  • Not a problem until the layer is buried by the next snowfall
  • Develop with a large temperature gradient (over 1°C per 10 cm) in the upper 20 cm of the pack; the gradient drives vapor transport within snowpack from warmer towards colder temperatures, transforming new snow crystals near top of the pack into very small, angular, snow grains, usually less than 2 mm in size
  • Can occur day or night and will continue developing as long as air temperatures remain cold
  • Develops regardless of wind speed

3. Depth hoar

Photograph showing depth hoar from the bottom of a snowpack
  • Highly faceted snow that creates a weak basal layer (a weak layer at the base or bottom of the pack)
  • Develops near the bottom of a snowpack whenever there’s a large temperature gradient (greater than 1°C per 10 cm) between the ground and pack above; note that even in mid-winter, ground temperatures remain at or just below freezing due to Earth’s heat and the insulating properties of snow
  • Requires relatively shallow snowpack (less than about three feet or 1 m deep) for the metamorphic process to continue; any more snow will stop the process, usually because the temperature gradient will be too small
  • Snow crystals near the ground are sometimes metamorphosed into very large, angular, cup-shaped grains that can exceed 5 mm in diameter

Key points

  • Weak layers form when air temperatures are cold and skies are clear
  • Surface hoar, near-surface facets, and depth hoar are known as “persistent” weak layers; they can remain in the snowpack and cause delayed-action avalanches weeks or even months after forming

At what point can we be sure that the snowpack on Glory Bowl contained a weak layer? (Choose the best answer.)

The correct answer is C.

The cold, low-density snow followed by several days of clear and even colder temperatures in mid-November likely led to the formation of near-surface facets or depth hoar. The new snowfall at the end of November would have buried the weak layer in the pack.

The third component of an avalanche, the sliding surface, is discussed on the next page.

Sliding Surfaces

Any hard or slick snow surface, such as an ice or rain crust, makes a good sliding surface for an avalanche.

Photograph of 2 people holding an ice crust cut out of a snowpack

There are three main types of crusts.

  1. Ice crusts form when relatively warm rain refreezes as it strikes a cold (below freezing) snow surface.
  2. Rain crusts form when warm rain falls on a relatively warm snow surface (at or just above freezing) and the liquid water penetrates into the upper portion of the snowpack. If temperatures drop below freezing, the water in the upper part of the pack will freeze into a very hard and well-anchored crust.
  3. Melt-freeze crusts form at or near the surface of a snowpack when temperatures get above freezing for a period of time and melt the surface snow. Once temperatures return to below freezing, the melted snow re-freezes into a firm crust. Thicker, stronger melt-freeze crusts develop when the process continues for several days. During springtime, solar radiation on a slope also accelerates the melting process during the day and forms a sun crust once temperatures cool below freezing overnight.

Problems arise once a good, firm sliding surface is buried by new snowfall. This is especially true when a weak layer forms on top of a good sliding surface prior to the next snowstorm.

How would you characterize the two crusts that formed in the Glory Bowl snowpack in October and November? (Choose the best answer.)

The correct answer is A.

The periods of warmer temperatures in mid-October and early November briefly melted the snow surface. This was quickly followed by colder temperatures, which refroze it.

Stable vs. Unstable Snowpacks

Photograph of unstable snow structure with weaker snow at the bottom of the snow profile and stronger/denser snow above Having a slab on a weak layer that’s on top of a good sliding surface constitutes an unstable snowpack, the kind most prone to avalanching. An unstable snowpack contains both weak and strong snow layers. A weak layer consists of poorly bonded or unconsolidated snow, while a strong snow layer has well consolidated snow, such as a slab.

A stable snowpack is generally more homogeneous and lacks significant density differences throughout the pack. It can contain:

  • Snow that’s all strong
  • Snow that’s all weak
  • A layer of weak snow on top of a layer of strong snow

Over time, the bonds between weak layers and slabs may strengthen, stabilizing the pack. A stable snowpack is not prone to avalanching.

 

Snow Bonding

Avalanche forecasters evaluate how well the layers in a snowpack are bonded together at their interfaces by conducting strength and stability tests in the field.

Weather forecasters can gain insight into how well new snow will bond to the existing snow surface by evaluating weather conditions immediately prior to the arrival of the next storm.

Good bonding is likely if:

  • The snow surface is relatively warm (air temperatures near or above freezing) just before new snow falls; then new snow of almost any temperature or crystal type will bond with it
  • There’s a crust on the surface and new snow occurs at relatively warm temperatures
  • Weak snow and warmer temperatures precede a warmer snowfall or rain event

Poor bonding is likely if:

  • The new snow surface is relatively cold and consists of a weak snow type
  • The new snow surface consists of preserved, colder snow crystals, such as stellars and dendrites
  • The new snow surface is a crust and air temperatures remain cold as new snow begins falling

Key points:

  • Good bonding creates a strong layer of snow, and good bonding between all layers makes a stable snowpack
  • Poor bonding means that a weak layer of snow exists, and poor bonding between layers makes an unstable snowpack
  • Warmer temperatures promote bonding

Avalanche Weather Forecast Process

The Forecast Process

The sequence of weather events throughout the winter season determines whether a snowpack is prone to avalanching. Careful monitoring of the types of weather that lead to the development of slab, weak, and sliding layers is paramount in forecasting avalanche weather. This is accomplished by continually evaluating the main weather parameters in your forecast area. These include precipitation, wind, temperature, and cloud cover.

The next four sections of the module outline a process for forecasting avalanche weather, the goals of which are to determine if:

  • Critical thresholds for the weather parameters are being met or exceeded
  • The potential for avalanches is increasing, decreasing, or staying the same based on the current and forecasted weather

The steps in the process include:

  • Pre-forecast preparation, where you gather basic information about your area, such as its climate, terrain, snowpack, and weather history
  • Assessing current weather, where you evaluate the weather over the last 24 hours or so
  • Forecasting future weather, where you make a weather forecast for the important avalanche weather parameters
  • Making the avalanche weather forecast, where you evaluate the impact of the current and forecasted weather on your area’s avalanche potential and determine if it is increasing, decreasing, or remaining the same

Before we get started, note the following:

  • Since an avalanche weather forecaster cannot do a good job without some knowledge of terrain and snowpack, we will include information about those topics when appropriate; for more information on snowpack, see the COMET module “Snowpack and Its Assessment.”
  • Every forecast situation is different. In some cases, you’ll have abundant weather and snowpack observations. In other cases, local weather and snowpack data will be sparse to non-existent and you’ll have to rely solely on the forecasted weather conditions to assess avalanche potential. While this is challenging, it’s not impossible. Knowing what weather criteria are conducive to avalanche production is key to estimating avalanche potential.

Forecasting Guide and Data Form

We will use the Avalanche Weather Forecast Guide to walk us through the process of forecasting avalanche weather. The Guide summarizes each step, describing the data to evaluate and what the results mean for avalanche potential. Take a few minutes to review it now.

Once you’ve finished the module, we encourage you to download the Guide and use it whenever you need to make an avalanche weather forecast. The Guide comes with an Avalanche Weather Forecasting Data Collection Form and instructions. These resources are available in the Summary and Resources / Resources section. The Guide is also accessible from the lower left menu.

Pre-forecast Preparation

Overview

Before you start looking at weather data, you need to gather general avalanche information about your forecast area, including:

  • Its type of avalanche climate
  • Any areas that are prone to avalanches
  • Its weather history from the start of the cold season
  • Any snowpack observations about the snow surface and stability of the pack
  • Any reports of recent avalanche activity

This Pre-Forecast Preparation portion of the Avalanche Weather Forecast Guide provides more detailed information about these indicators and their impact on avalanche potential. Review the table, then we’ll explore the indicators in more detail.

Table outlining some of the factors that impact avalanche formation, and the values for each factor that increase or decrease avalanche potential or have no effect

Avalanche Climate

Knowing your region’s climate is important, particularly when forecasting for a new area, for it gives you an idea of the:

  • Types of weather to expect
  • Average winter temperatures
  • Average kind of snow that falls
  • Types of avalanches that may occur

The three major climate types are described below.

Maritime (coastal) avalanche climate

  • Deep snowpack with settled mid-winter snow depths of 9 feet (3 m) or more
  • Frequent storms produce moderate to heavy snowfall
  • Relatively mild regime throughout winter, producing a denser snowpack
  • Often rains on the snowpack throughout the winter
  • Consistent and relatively mild air masses along with more frequent storms create more consistent snowpack compared to the other climates
  • Usually has direct-action avalanches

Continental avalanche climate

  • Relatively shallow snowpack with settled mid-winter snow depths less than 5 feet (1.5 m)
  • Less frequent storms with lower-density snow than the maritime climate
  • Long periods of drought with very cold temperatures during winter
  • Temperatures variable throughout winter, leading to more varied layers within the snowpack
  • Both direct action avalanches and delayed-action avalanches are common and often involve layers deep within the snowpack; the pack can remain unstable and avalanches may be triggered even weeks after last significant storm

Intermountain (transitional) avalanche climate

  • Intermediate mid-winter settled snow depths ranging from 5 to 9 feet (1.5 to 3 m)
  • Storms are more frequent and dry periods are not as prolonged as in continental climates
  • Temperatures are closer to maritime climate but have less frequent rain events
  • Both direct- and delayed-action avalanches can occur and sometimes involve deep layers
Google map of the Western US with the avalanche climates zones (maritime, intermountain, continental) labeled for many of the major mountain ranges

Note that interior ranges can assume the characteristics of a maritime climate during unusually warm and wet winters, while coastal ranges can have a continental-type climate during unusually dry and cold winters. In addition, avalanche climates can change with elevation within an individual mountain range, going from an intermountain climate at lower elevations to a continental climate much higher up. This is common when large disparities in elevation over short linear distances exist.

The highlighted row of the table summarizes the relationship between climate and avalanche potential.

Relationship betwen avalanche climate and avalanche potential

Think back on the Glory Bowl Avalanche. What type of avalanche climate does this part of the Tetons in Wyoming have? (Choose all that apply.)

Both A and B are correct.

Generally, the area’s elevation and inland location give it an intermountain climate. However, the early season conditions and shallow snowpack give it characteristics of a continental climate.

Avalanche Terrain

You need to be familiar with the terrain in your forecast area. Plotting avalanche paths and risk zones on aerial photos or topographic maps is very helpful.

Several instruments, such as a slopemeter and inclinometer, can be used to determine the angle of a slope in the field. If you’re not in the field, you can determine slope angle using a topographic map, ruler, and scientific calculator (select INV, then TAN).

  1. From the map, determine the elevation gain of the slope in question (the rise), then measure the horizontal distance (the run) using the map scale and a ruler
  2. Using the calculator, divide the rise by the run (in feet or meters) and take the arctangent of that value. If your calculator does not have an arctangent function, use the following table to get an approximate value:
    Table for getting an approximate arctangent value

As an example, we’ll determine the angle of the Glory Bowl area. The straight blue line on the topographic map below marks the approximate location of where Joel Roof was snowboarding. To determine the slope along the blue line, get the rise and the run from the map.

  • For the rise, the starting point (left end) is along the 9,880-ft contour, the ending point (right end) is along the 9,600-foot contour
  • Subtracting the two yields a rise of 280 feet (85 m)
  • To determine the run, the map scale shows that the blue line corresponds to a 400-ft long linear distance
  • Therefore, the calculation for the slope is: arctan(rise/run) = arctan(280/400) = arctan(0.70)=35 degrees

Topo map of the Glory Bowl area annotated with information for determining slope angle

Summary table:

Relationship betwen terrain and avalanche potential

Weather History

Avalanche forecasting begins with a good understanding of past weather, from the very beginning of the “winter” season when the first snow stays put in the mountains.

If you’re in a formal avalanche forecasting situation and will be forecasting for, say, a highway department, ski area, or helicopter skiing operation, ideally you’ll want the full complement of daily historical weather data from the start of the winter season. This includes:

  • Precipitation (daily snowfall and its water content or SWE)
  • Maximum and minimum temperatures at forecast-area elevations
  • Winds near ridge-top level or at all forecast-area elevations
  • Average cloud cover each day over the forecast area

You can get daily weather observations from ski areas, transportation agencies, and automated weather sites. Some historical information can be obtained from climate station records, satellite data, and SNOTEL sites. (SNOTEL is an extensive, automated system for collecting snowpack and related climatic data in the Western United States and Alaska run by the U.S. Natural Resources Conservation Service.)

Two photographs, one showing an onsite SNOTEL site, the other showing a snowpit

Display the data in a graphical format so you can roughly reconstruct snowpack depths and structure.

Hand drawn snow history calendar for the Snake River Range, 2005 - 2006

If detailed historical data are not available, try to find out about the region’s general weather. Has it been wetter or drier than climatic norms? Warmer or colder? Warmer and wetter conditions can create deep mountain snowpack that may be strong and consolidated. Colder and drier than normal-early season weather tends to produce shallow and perhaps weaker snowpack.

Try to get snowpack observations or snow pit profiles from someone trained to perform snowpack assessments. The data can help you figure out:

  • If the current snowpack is stable or unstable. If it’s unstable, the potential for avalanches may already be high; if it’s stable, the potential will be low to start with.
  • If the snow surface is hard and crusty or soft and powdery, if it’s very rough looking or smooth and planar; and if it’s relatively warm or cold. New snow bonds well to rough, soft, or warm snow surfaces but not to smooth, hard, or cold snow surfaces.

Photo of avalanche workers in a snow pit

Summary table:

Relationship betwen weather history and avalanche potential

Recent Avalanches

Keep a record of any reported avalanches, identifying the elevation, aspect, and relative size of each event. Recent avalanche activity is one of the best indicators of an unstable snowpack. Look for clues of recent avalanche activity:

  • Debris piles at the bottom of slopes
  • Recent crown lines
  • Newly damaged trees in avalanche paths
Photograph of debris from a dry slab avalanche

Previous avalanches ran for a reason and another load of snow may start another cycle or release new, even larger avalanches.

Summary table:

Relationship betwen avalanche activity and avalanche potential

Current Weather

Overview

Once you’ve gathered pre-forecast information, you’re ready to evaluate the current weather situation, looking at conditions during the preceding 24 hours. Specifically, you’ll examine weather variables that have the greatest impact on snowpack and avalanche potential (precipitation, wind, temperature, and cloud cover) and determine if they’ve exceeded critical thresholds. Solar radiation is also important but will be considered in combination with cloud cover.

This portion of the guide summarizes the important weather parameters and shows how to interpret the values for avalanche potential. Review the table, then we’ll discuss the items in more detail.

table summarizing the relationship between current weather variables that are important to avalanche weather and avalanche potential

Precipitation

Since most avalanches are associated with recent or newly fallen precipitation, you should collect the following information for any recent and/or ongoing precipitation events:

  • Amount of new snowfall
  • Rate of accumulation
  • Snow water equivalent
  • New snow density

New snow amount: New snowfall is usually recorded to the nearest inch (cm) at 24-hour intervals or sometimes more often during storms. Measurements are taken manually from a storm board (snow board) at the site or automatically, using a remote sensing device, such as an acoustic snow height sensing instrument.

Snow accumulation rate: We measure the water content or SWE of new snow to help us determine its density and learn about the weight of new snow being added to the pack. Record SWE to the nearest one-hundredth of an inch (or tenth of a millimeter). Measure it using a melting precipitation gauge or by bringing a snow sample from a standard precipitation gauge indoors, melting it, and measuring the height of the water. It can also be measured by weighing a core sample of new snow using a calibrated scale designed for weighing snow collected with a snow tube. For more information, see COMET’s Snowmelt Processes module at http://www.meted.ucar.edu/hydro/basic/Snowmelt/.

Photograph of snow water equivalent being measured after snow is melted from a sample tube

New snow density: Snow density is another way of expressing how much weight is being added to the snowpack. For a given snow depth, high-density snow adds more weight (water) than low-density snow. Snow density is usually expressed as a dimensionless ratio of snow water content to snow depth (such as 1:10) or as a percentage of snow water content to snow depth (such as 10%).

To calculate density, divide the amount of water contained in the snow by the depth of the new snow. The higher the value, the more water is in the snow.

Here are a few exercises.

What’s the density of 20 inches (50 cm) of snow that contains 2 inches (5 cm) of water? (Choose the best answer.)

The correct answer is A.

If you divide the snow water content (2 in or 5 cm) by the snow depth (20 in or 50 cm), you get 1:10 or 10%.

Which snowpack weighs more? (Choose the best answer.)

The correct answer is A.

The first pack has a density of 10% (2 inches / 20 = 1:10 or 10%), whereas the second has a density of 5% (1 / 20 = 1:20 or 5%). The snow depths are the same but the denser snow contains twice as much weight as the less dense snow.

For more exercises like these, see the COMET module Snowpack and Its Assessment at http://www.meted.ucar.edu/afwa/snowpack/.

Storm trend: During the course of a storm, temperature fluctuations will produce snowfalls with varying new snow densities.

  • Storms that begin with warmer temperatures and higher-density snowfall and end with colder temperatures and lower-density snowfall produce a more stable layer. These are known as right-side-up storms because lighter snow overlays heavier snow.
  • Storms that begin with colder temperatures and lower-density snow followed by warmer temperatures and higher-density snow produce a more unstable layer. These are referred to as upside-down storms since heavier snow overlays lighter snow.

When tracking incoming storms, be sure to consider the nature of the underlying or old snow surface. If new snow falls on surface hoar or near-surface facets, an unstable situation may develop regardless of the new snow’s density.

Rainfall: Rainfall adds weight to a pack. Especially in maritime climates, you should closely monitor the snow/rain level during storms since rain on snow almost always causes avalanches.

Summary table:

Table showing the relationship between precipitation and avalanche potential

You’ve finished the background information on precipitation. Scroll up and access the Operational Information tab.

Ideally, precipitation data should come from sites near the elevations of historical avalanche starting zones but below ridge-top level, in locations somewhat protected from the wind.

New snow amount: Gather all new snow depth measurements for the past few days. Determine if it’s snowing anywhere and if the reported snowfall exceeds the critical threshold.

Snow accumulation rate: Depending on the frequency of your new snow depth reports, determine snow accumulation rates for 6-, 12-, and 24-hr periods. Use the table to interpret the impact of your rates on avalanche potential.

Snow water equivalent: Collect SWE data and see if the amount of water in the new snow exceeds the threshold.

New snow density: Determine the density of newly fallen snow from the SWE and snow depth using this formula: New snow density = snow water content / new snow depth.

Storm trend: If it’s snowed, when you’re ready to start looking at current temperatures, determine if they warmed or cooled during the snowfall event. Rising temperatures during a snowfall destabilize the storm snow layer (an upside-down situation) whereas falling temperatures stabilize it (a right-side up situation).

Rainfall: Determine if the area has received any rain because it will have added significant weight to the pack. Rain has a particularly destabilizing impact.

Table showing the relationship between precipitation and avalanche potential

Wind

After precipitation, wind is the next most important weather variable affecting avalanche potential. It is fundamental to slab development and determines where wind-transported snow accumulates.

Wind speed:

Macro photograph showing snow being transported by wind at the snow surface

Wind speed determines the amount and rate of wind loading. Winds between 20 and 60 mph are optimal for transporting snow, while those less than 20 mph can only transport very low-density snow (< 5%) on the ground.

Very high winds (> 60 mph) often disperse snow beyond the starting zones of avalanche paths, depositing it farther down the slope. There it will stay, beyond the point where it can form dangerous wind slabs on critical slope angles. Very high winds, especially during periods with low relative humidity (such as when skies are clear) also cause snow to sublimate into the atmosphere before it can fall out and form a thick slab layer.

Photograph of high winds at Big Sky Ski Area transporting snow a great distance

Average wind speeds between 20 and 60 mph transport and deposit snow of almost any density onto the upper reaches of avalanche starting zones. Average speeds between 30 and 40 mph have the greatest potential to build dangerous slabs in these areas.

Remember that new snow is not needed to form an avalanche. As long as there’s snow on the ground that the wind can transport, new slabs can form on leeward slopes.

Wind and snow accumulation rates: The combination of new snow and wind-deposited snow during a storm increases the snow accumulation rate significantly, easily doubling or even quadrupling it on leeward slopes. For example, if snow falls at a rate of 1 inch (2.5 cm) per hour for 8 hours, you’d expect 8 inches (20 cm) of accumulation on the ground. But if ridge-top winds average 30 to 40 mph from a consistent direction throughout that period, you might get snow depths of 16 to 32 inches (40 and 80 cm) on leeward slopes.

Photograph of wind loading of new snow from a windward (left) to leeward slope (right), which produces a slab avalancheWind direction: Wind direction indicates which slope aspects are susceptible to snow being deposited from wind loading.

Say, for example, that a storm begins with strong southwesterly winds that load leeward, northeast-facing slopes. If it is followed by northwesterly wind flow after, for example, the passage of a cold front or an upper-level trough of low pressure (in the Northern Hemisphere), southeast-facing aspects will become the leeward slopes and get the most loading.

When wind blows over a ridge top and transports snow from the windward to leeward side, it’s known as top-loading. Wind blowing parallel to a ridgeline may dump wind-deposted snow on avalanche paths. This cross-loading may be difficult to detect or monitor without observations from mid-slope and valley anemometers.

If the wind is strong enough to transport snow, the longer it blows from a steady direction, the greater the buildup of snow on favored leeward slopes.

If the wind is strong enough to transport snow, the longer that it blows from a steady direction, the more snow will build up on favored leeward slopes.

Summary table:

Relationship between wind speed and direction and avalanche potential

Select Operational Information at the top.

Data: Wind data should come from anemometers on an exposed, representative ridge top or atop several peaks in the area. Naturally, the more reports, the better. Hourly or more frequent data make it easier to determine the persistence of wind speed and direction. Be aware that remote unheated anemometers are susceptible to collecting rime ice, which can cause data and reporting errors. (Rime ice forms when supercooled water droplets strike a cold, sub-freezing surface.)

Photograph of a ridge-top weather station for measuring wind and temperatures

Wind speed: Gather wind data for the previous 24 hours, noting periods when the average wind speed is optimal for transporting snow (between 20 and 60 mph). Notice if there’s fresh or low-density snow on the ground that can be easily transported by the wind. If it snowed in the last 24 hours while the wind speed was between 20 and 60 mph, consider doubling or quadrupling the estimated snowfall depths on leeward slopes.

Wind speed conversion table from MPH to KTS to M/S

Wind direction: Gather wind data for the previous 24 hours, noting the average direction and how long it blew from various directions. Pay particular attention to long periods with a consistent wind direction.

Summary table:

Relationship between wind speed and direction and avalanche potential

Temperature

Air temperature not only determines the type of snow crystals that fall from the sky, but the metamorphic processes that occur within the snowpack.

You need to track:

  • Daily temperature values (highs and lows) and range
  • Temperature trend (increasing or decreasing) and rate and duration of change

Daily temperature values: Use daily high and low temperatures to determine freezing levels in the mountains as well as snow and rain levels during storms. Note the following.

  • Crusts may form if temperatures rise above freezing during the day and drop below freezing at night; the crust can create a potential sliding layer for the next storm
  • When temperatures remain above freezing for more than 24 hours, the upper snowpack becomes saturated from melt water and may cause wet snow avalanching
  • When temperatures stay well below freezing for long periods of time and high and low temperatures never rise above 15°F (-10°C) day and night, weak layers may develop on the surface or near the ground in a shallow snowpack; this can create a layer that fails with the next load of new snow

Key points:

  • Avalanches may occur when air temperatures rise above freezing and stay there
  • Weak layers often develop when temperatures get very cold and remain that way

Temperature trend: As temperatures increase, the snowpack undergoes settlement, deforming and becoming denser. Settlement on an incline results in creep, a slow downhill motion.

Photograph showing snow settlement around the base of an aspen tree
  • If air temperatures increase rapidly (more than 15°F or 8°C) in less than 12 hours, the rate of creep increases, which can lead to avalanching. This is most critical when temperatures are near or above freezing because the rate of creep increases exponentially with rising temperature.
  • If temperatures increase slowly over several days, settlement rates are slower. If air temperatures never rise above freezing for very long, the snowpack deforms slowly and creep rates are slower.
  • As temperatures decrease, settlement and creep rates also decrease.
  • Prolonged periods of cold temperatures promote the metamorphic processes that form weak layers.
  • Warm days and cold nights strengthen the pack, especially overnight and during early morning hours.
  • Several days of warmer temperatures followed by colder temperatures also strengthen the pack.

Key points:

  • Rapid rises in temperature can destabilize a pack, while slow rises stabilize it (especially if a cooling trend follows)
  • Long periods of cold temperatures build weaker snow, which isn’t a problem until the next significant snowfall

Summary table:

Table describing relationship between temperature and avalanche potential

Select Operational Information at the top.

Data: You can get temperature data from thermometers at precipitation and wind instrumentation sites. During clear conditions, surface temperatures can also be estimated from high-resolution infrared channels onboard weather satellites.

Gather temperature information across your forecast area for the previous 24-hour period, noting:

  • Maximums and minimums
  • Temperatures at different elevations
  • Temperatures near avalanche starting zones
  • 24-hour and longer-term temperature trends

If it snowed, determine the freezing level and if it was at or above avalanche starting zones.

Temperature trend:

  • Increasing temperatures: Notice when air temperatures warm by about 15°F (10°C) or more during the day, especially when they’re near the freezing mark; the pack may settle too rapidly, causing avalanching; the faster temperatures rise, the more unstable the snowpack will be
  • Melt-freeze: Note when a warm-up is followed by a cooling or days with melt-freeze cycles, since this will help stabilize the pack

Warm temperatures: Monitor temperatures, especially near avalanche starting zones, to determine if they will remain above freezing for more than 24 hours. The longer this goes on, the more likely wet slab avalanches will develop.

If there’s a storm, compare the freezing level with the avalanche starting zone elevations. Rain at a starting zone will quickly destabilize the snowpack.

Cold temperatures: Evaluate whether weak layers of surface hoar, near-surface facets, or depth hoar could have formed.

  • Surface hoar forms during clear, cold, calm weather, mainly at night with wind speeds less than 10 mph (8 kts or 5 m/s) and air temperatures at or below 14°F (-10°C)
  • Near-surface facets form with clear, cold weather, mainly at night even with wind speeds above 10 mph (8 kts or 5 m/s) and air temperatures at or below 14°F (-10°C)
  • Depth hoar forms from low air temperatures, usually at or below 14°F (-10°C), mainly at night over a shallow snowpack (one that’s less than about 3 feet or 1 m deep)

Also monitor extended periods of clear and cold weather. The longer the weather remains clear and cold, the weaker the weak layers will become.

Summary table:

Table describing relationship between temperature and avalanche potential

Cloud Cover & Solar Radiation

Cloud cover affects both the amount of radiational cooling that can occur at night and the amount of direct solar heating that can occur during the day.

Clear, cold nights are accompanied by rapid radiative cooling and can create weak layers in the snowpack, especially when very cold temperatures persist for several nights. Even thin layers of high cloudiness at night will inhibit radiative cooling.

Daytime direct solar radiation warms the snow surface and can cause melting or sublimation, especially as the sun gets higher in the sky in late winter and spring. During this time, higher temperatures combined with increased solar radiation can lead to rapid snowmelt, especially near rock outcroppings and cliff bands and on south-facing slopes (in the Northern Hemisphere). All but a thick overcast will allow some solar radiation to reach the surface and potentially cause melting.

Late in spring, once the free water has percolated all the way down through the snowpack, climax avalanches can occur, removing what remains of the season’s snowpack. (A climax avalanche involves all or most of a snowpack, exposing the ground.)

The duration of cloud cover determines how much it reduces nighttime radiative cooling and daytime direct solar radiation.

Summary table:

Table showing relationship between cloud cover and solar radiation and avalanche potential

Select Operational Information at the top.

Data: Observed cloud cover information for a specific location can be extrapolated from manual and automated surface observations at mountain airports, ski areas, and mountaintops that have instrumentation (a rare situation).

Due to the scarcity of these observations, you can use data from weather satellites and/or radar to estimate cloud cover over a particular mountainous location. Satellite data have many advantages:

  • Wide availability
  • High resolution worldwide
  • Can detect and differentiate multiple levels of cloudiness
  • Can be used to detect atmospheric water vapor in the absence of cloudiness

Amount and duration: Gather as much cloud data as possible for the preceding 24 hours. Note periods of clear skies, dense overcast, and any cloudiness whatsoever at night.

Summary table:

Table showing relationship between cloud cover and solar radiation and avalanche potential

Forecast Weather

Overview

When forecasting future weather conditions, you analyze the same weather variables (precipitation, wind, air temperature, and cloud cover) but use weather forecast models to predict how they may change in the future. Choose a model that works well in your area, preferably a mesoscale or high-resolution regional model.

Most avalanche weather forecasts focus on the next 24 hours. However, longer-range forecasts of up to 48 hours or more are needed when, for example, planning long-range travel or future avalanche hazard reduction work.

This table outlines the standard upper-air constant pressure levels to use based on the elevation of your mountains. Choose the appropriate model forecast maps to approximate mountain elevations and interpolate between pressure levels to create forecasts as necessary.

Table of the most useful upper-air maps to use for various elevations

Precipitation

First and foremost, you need to predict the amount of new snowfall and its snow water equivalent across your forecast area.

Snow water equivalent: Usually it’s easiest to start by forecasting the amount of liquid precipitation (SWE) expected to fall over your forecast area. To do so, you’ll need to evaluate Quantitative Precipitation Forecast (QPF) output from one or more weather models.

  • Forecast for 12-hour periods at the most, determining the 12- and 24-hour amounts of liquid equivalent precipitation for each period
  • Use even shorter periods (such as 3 or 6 hours) to calculate snowfall intensity rates and identify the heaviest snowfall periods
  • Modify the model results based on potential areas of orographic enhancement or the effect of mountain shadowing; note that model results can vary based on a model’s coordinate system and its ability to accurately identify a place’s location and altitude; for more information, see the COMET module Impact of Model Structure and Dynamics module at http://www.meted.ucar.edu/nwp/model_structure/

New snow amount: Some weather forecast models explicitly forecast new snow depth. If you don’t have such guidance, estimate snowfall amount for each time period using the forecasted amount of liquid precipitation and the forecasted temperature at the desired elevation. Temperatures can be estimated from model forecasted temperatures on constant pressure surfaces nearest to elevations of interest. Snow falling within certain temperature ranges has an approximate density range.

For example, you can estimate the amount of snow for a designated time interval from the model QPF amount and average forecasted air temperature at the forecast elevation.

  • Use the table below to estimate new snow density based on the temperature forecast
  • Divide the density into the forecasted liquid water amount to determine new snowfall depth
Table showing how to estimate snow density from air temperature Example showing how to determine new snowfall depth by dividing density into the forecasted liquid water amount

Wind

You need to forecast ridge-top winds (both speed and direction) for your area of interest.

  • Predict the average wind speed and direction over 6- or 12-hour time intervals
  • Also produce a 24-hour average
    • Use model-forecasted winds on constant pressure surfaces closest to ridge-top elevation
    • If you break model forecasts of wind speed and direction into smaller time periods (3- or 6-hour), you can determine the duration and intensity of resultant wind loading more specifically
    • Note changes in wind speed and direction over time
  • If it’s snowing or if there’s fresh snow on the ground, adjust your forecasted snowfall depth higher for leeward slopes if the wind’s between 20 and 60 mph
  • Note periods of calmer winds, especially at night with clear skies, since surface hoar can form

Temperature

Forecast the maximum and minimum temperatures expected in the next 24 hours at or near the elevations of avalanche starting zones. Note the general day-to-day trend as well as the diurnal spread from minimum to maximum. If possible, also forecast temperatures at 3- or 6-hour intervals since it will help you:

  • Analyze same-day trends
  • Examine how the temperature is expected to change during a precipitation event
  • Calculate new snowfall densities during a storm

You can use model temperature data for this forecast, such as:

  • Model-predicted surface temperatures (2 m or 10 m above ground level)
  • Forecasted temperatures on upper-level constant pressure surfaces
  • Forecast soundings and time-height cross sections

You also need to forecast the snow level within mountainous terrain. You can often extract this directly from a model forecast of the height above the surface of the 32°F (0°C) wet-bulb temperature (wet bulb zero level). If not available, use this rule of thumb: the snow level will be about 300 m (1,000 feet) lower in elevation than the forecasted freezing level.

Cloud Cover & Solar Radiation

Forecast cloud cover amount for each 6- to 12-hour interval, noting daytime vs. nighttime cloudiness. Some models provide explicit cloud cover forecasts. If not, you can use relative humidity (RH) forecasts at various pressure levels to estimate the cloud cover expected over the area. You may need to analyze RH at all significant pressure levels at or above the elevation for which you are forecasting, for example, 850, 700, 500, and 300 mb. Meteograms depicting model forecasted RH vs. time at all atmospheric levels are excellent graphical tools for evaluating model results with respect to RH and possible cloudiness.

Note that model forecasted RH values greater than 60% typically imply some amount of cloud cover.

Real-time satellite imagery can also be very helpful for short-term cloudiness forecasts. By extrapolating cloud areas (using thermal infrared imagery) or moisture layers (using water vapor imagery) upstream of the forecast area, you can generate rough estimates of cloud cover for short-range (12- to 18-hour) forecasts. Even longer extrapolations of synoptic weather features, such as storm systems, frontal cloudiness, and short wave troughs, can provide a first approximation of possible cloud conditions as much as a day in advance.

Pay particular attention to long durations of clear skies at night and clear or thin overcast skies during the day. The former is a precursor to weak layer formation, the latter still allows direct solar insolation to melt the snow surface.

Making the Avalanche Weather Forecast

Now it’s time to pull together all of the information that you’ve analyzed (and hopefully tracked on the Avalanche Weather Forecast Data Sheet).

  • Check the findings from your pre-forecast preparations. Do they indicate that the area is prone to avalanches? Has the season’s weather set the stage for their formation?
  • Review your weather data for the last 24 hours. Is the weather increasing, decreasing, or not changing avalanche potential?
  • Analyze your 24-hour forecasts and determine their impact on avalanche potential
  • Look at the combined data and identify the trend. Does the potential for avalanche formation appear to be increasing, decreasing, or remaining the same based on weather conditions?

It’s much easier to forecast the trend if you know what the avalanche potential was prior to this point in time. If not, be conservative and assume that it’s increasing unless most indicators imply otherwise.

Finally, be on the lookout for high-risk situations that make an area particularly prone to avalanches.

  • Heavy, dense snowfall of 6 inches (15 cm) or more produces avalanches, especially on steeper terrain with slopes greater than 40 degrees
  • 12 inches (30 cm) of new snow produce sluffing on steeper terrain at a minimum
  • 12 inches (30 cm) or more of new snow, especially with higher densities, may overload an already weak snowpack and produce deeper slab releases
  • New slabs created with 6 inches (15 cm) or more of new snow and winds from a consistent direction for 6 hours or more averaging 20 to 60 mph (~17 to 52 kts or 10 to 30 m/s) pose high risks
  • Rain on snow almost always produces avalanching, with heavier rain producing larger avalanches
  • Storms beginning with cold temperatures and low density snow and ending with warm temperatures and higher-density snow often initiate direct action avalanches
  • Rapidly warming temperatures during the day increase snowpack settlement rates and can cause avalanches without any new snow loading
  • Warm surface temperatures (at or above freezing) combined with intense solar radiation produce wet snow avalanches
  • Water percolates deeper into the snowpack if continuous above-freezing temperatures last for more than 24 hours; the longer the situation continues, the deeper the avalanches may be (even reaching the ground)
  • One day of clear, cold, calm weather followed by a significant snowfall can bury a layer of surface hoar, posing risks to the future stability of the pack
  • Longer spells of dry, relatively cold weather followed by a significant snowfall often will bury near-surface faceted snow or depth hoar and may pose a risk if more loading occurs

Forecast Exercises

Introduction

This section contains five mini-cases that enable you to apply aspects of the avalanche weather forecast process to different avalanche-prone mountainous locations.

The first three cases provide background information about the areas’ settings and weather histories and pose questions for you to answer.

The last two cases provide weather data and ask you to produce weather forecasts and evaluate their effect on avalanche potential. This will give you some experience with forecasting with little background information, a situation that you may encounter.

Note that you can review the Forecast Guide at any point by clicking the link in the lower left menu.

Case 1: South-Central Alaska

You’re forecasting for a highway corridor in south-central Alaska where several south-facing avalanche paths impact the road. The starting zones are at approximately 3,000 feet (900 meters) and the road is just above sea level. Slope angles in the starting zones average around 40 degrees.

Topo map of the area around Seward, Alaska

It’s been a snowy winter. From the third week of November through December, the area was hit with a continuous series of storms, bringing snow down to near sea level. By the end of December, settled snow depths were about 5 feet (1.5 m) between 2,000 and 3000 feet (600 and 900 m).

It rained on January 1 up to near 3,000 feet (900 m). This was immediately followed by several days of high pressure, cooler temperatures (15°F to 25°F, -9°C to -4°C), and northerly winds of 30 to 50 mph (13 to 22 m/s). Clouds increased and temperatures warmed up to 30°F (-1°C) at 3,000 feet (900 m) by the end of the first week in January.

During a five-day period in the second week in January, a series of storms brought more snow to the area.

  • They began with warm, southwesterly flow and temperatures around 30°F (-1°C) at 3,000 feet (900 m)
  • The winds turned southeasterly at the end of the period, with temperatures around 25°F (-4°C)
  • Ridge-top winds averaged between 30 and 40 mph (13-18 m/s) throughout the storm period
  • The five days of storminess produced 3 feet (1 m) of snow and a little less than 3 inches (76 mm) of water at 2,500 feet (750 m)
  • The average snow density was around 10%

Click the Questions tab and answer the questions, then access the Synopsis tab and review the case.

1. What type of avalanche climate does the area have? (Choose the best answer.)

The correct answer is C.

The deep snowpack, frequent storms, and occasional winter rain give the area a maritime avalanche climate.

2. How would you characterize the overall snowpack between 2,000 and 3,000 feet (600 and 900 m) just after January 1? (Choose the best answer.)

The correct answer is B.

The five feet (1.5 m) of snow in December and almost continuous storms in January built a consistent snowpack.

3. During the first week of Janaury, what type of layer formed on the snow surface in the starting zones? (Choose the best answer.)

The correct answer is B.

Rain to almost 3,000 feet (900 m) on January 1 was followed by a hard freeze, causing a rain crust to form.

4. Could surface hoar or near-surface facets have formed on the southerly aspects just prior to the storms during the second week of January? (Choose the best answer.)

The correct answer is B.

It was windy when it was coldest, which inhibited surface hoar growth. There were no new snow crystals on top of the hard frozen rain crust that could become faceted. It warmed up to near freezing before the next storm.

5. Which slope aspects were most likely to be wind loaded during the storms that occurred in the second week of January? (Choose all that apply.)

The correct answers are B and C.

Winds were first out of the southwest, then southeast. Thus, the northeast- and northwest-facing slopes were leeward.

6. Would you expect avalanche potential to be high on south-facing slopes (those that threaten the highway) after the last series of storms? (Choose the best answer.)

The correct answer is B.

South-facing slopes were windward. Other than some possible cross loading, most of the wind loading went to the opposite side of the ridge. Three feet (1 m) of snow over five days was probably insufficient to increase avalanche potential a lot.

7. Assume it’s now the third week in January. The storms have ended, high pressure is building over western Alaska, and ridge-top winds are coming out of the north at 50 to 60 mph (22 to 27 m/s). They are expected to remain northwesterly or northeasterly for the next 24 hours. How should this impact avalanche potential on south-facing slopes during this period? (Choose the best answer.)

The correct answer is A.

A lot of snow is available for transport on the northwest and northeast slopes, which are now windward. The snow on the south-facing slopes is sitting on top of a slick rain crust. If the wind transports more snow to those slopes, avalanche potential will increase. Remember to review the synopsis before proceeding to the next case!

This low-elevation coastal mountain range has a maritime avalanche climate. It had a relatively deep snowpack (5 feet or 1.5 m) by the end of December. We can assume that the pack was strong based on its depth and the storms that hit the area continually from late November through December. This prevented crusts from forming on top of the pack and weak layers from forming within it.

The rain on January 1 may have caused some avalanching but it would have stopped when the cold air arrived and froze the surface snow. In addition, the rain that percolated down into the upper snowpack would have frozen, increasing the strength of the pack. Overall, January began with a strong snowpack. The only worrisome layer was the rain crust, which could provide a good sliding surface for any new snow. However, the high northerly winds prevented any weak layers from forming on top of the crust during the cold period. In addition, the warming temperatures would have sublimated any surface hoar that might have formed on south-facing slopes.

The storm began to warm but it’s questionable if the snow could have bonded with the crust if the snow remained relatively cold and hard. The potential for direct-action avalanches would have been higher on south-facing slopes if the three feet (1 m) of snow had accumulated faster or if the storm had been upside-down. Since the snow fell at relatively warm and consistent temperatures over the five-day period, it probably had time to settle and bond. The storm also finished right-side-up with slightly cooler temperatures and lower-density snow. In addition, the wind direction during the storm was not favorable for loading the south-facing slopes since they were actually windward at that time. However, the avalanche potential would certainly increase once the strong northerly winds transported the new snow back over the ridge and onto those south-facing slopes that had a good sliding surface below.

Case 2: Rocky Mountains

You’re planning a backcountry hut-to-hut ski trip from December 23 to 26 in the Rocky Mountains of Colorado. You’ll be skiing between elevations of about 10,000 feet (3050 m) to over 11,500 feet (3500 m). The surrounding peaks are higher, with tops ranging from 12,500 to 14,000 feet (3800 to 4300 m). Your route will take you through avalanche terrain and you’ll be crossing many avalanche paths that could produce D2 and D3 avalanches. You’ll also need to ski beneath several D4 paths on your approach to the huts.

Topo map of the Aspen area in CO

In mid-October, the area got between 2 and 3 feet (0.6 to 1 m) of snow at approximately 11,000 feet (3350 m). From mid-October through mid-November, conditions were very dry (under high pressure) and temperatures at higher elevations were relatively low, especially at night. Temperature rose during the day but still remained below freezing.

Several smaller storm systems occurred during the latter half of November. Each storm brought 2 to 3 inches (5 to 8 cm) of snow, accompanied by high winds from the southwest. On December 1, the settled snow depth at 11,000 feet (3350 m) was 30 inches (76 cm).

From December 1 through 20, no new snow was recorded; skies were generally clear; winds were moderate (30 mph or 13 m/s) to high at times (gusts over 100 mph or 45 m/s); and temperatures were very cold (0°F to 10°F or -18°C to -12°C) between 10,000 and 12,000 feet (3050 to 3650 m).

On December 21, temperatures began warming into the teens Fahrenheit (-12°C to -7°C) between 10,000 and 12,000 feet (3050 to 3650 m) and clouds increased.

On December 22, the area got 4 inches (10 cm) of new snow at 11,000 feet (3350 m); the SWE was 0.20 inches (5 mm); the snow density was 5%; and ridge-top winds were westerly to southwesterly, averaging 20 mph (9 m/s).

It’s now December 22 and you’re getting ready to leave on your trip. Here’s the forecast.

December 23:

  • At 11,000 feet (3350 m)
    • Approximately 8 inches (20 cm) of new snow expected over the entire day (from midnight to midnight)
    • Snow water equivalent (SWE): 0.40 inches (10 mm)
    • New snow density: 5%
    • Temperatures: In the teens Fahrenheit (-12°C to -7°C)
  • At 13,000 feet (4000 m): Southerly to southwesterly winds at 25 mph (11 m/s)

December 24:

  • At 11,000 feet (3350 m)
    • 12 inches (30 cm) of snow expected from noon to midnight
    • SWE: Up to 0.80 inches (20 mm)
    • New snow density: 6 to 7%
    • Temperatures: Around 20°F (-7°C)
  • At 13,000 feet (4000 m): Westerly winds at 40 mph (18 m/s)

December 25:

  • At 11,000 feet (3350 m)
    • Another 16 inches (40 cm) of snow from midnight to 3 PM
    • SWE: 1.25 inches (32 mm)
    • New snow density: Almost 8%
    • Temperatures: Between 20 and 25°F (-7°C and -4°C)
  • At 13,000 feet (4000 m): Southwesterly winds at 40 to 50 mph (18 to 22 m/s)

December 26:

  • At 11,000 feet (3350 m)
    • No new snow
    • Temperatures: Teens to single digits °F (-15°C to -9°C)
  • At 13,000 feet (4000 m): Northwesterly winds at 20 mph (9 m/s)

Click the Questions tab and answer the questions, then access the Synopsis and review the case.

1. What type of avalanche climate does the area have? (Choose the best answer.)

The correct answer is A.

This is a cold, alpine region at or above timberline. It has a relatively shallow snowpack, cold temperatures, and extended periods of drought, all characteristic of a continental avalanche climate.

2. How would you characterize the early season snowpack in the mountains above 11,000 feet (3350 m)? (Choose the best answer.)

The correct answer is A.

The settled snow depth was only 30 inches (76 cm) on December 1. There were two extended periods of very dry weather: from mid-October to mid-November and during the first three weeks of December. During these periods, weak layers could have formed in the snowpack.

3. In general, what would you expect the snowpack structure to look like just prior to December 25? (Choose the best answer.)

The correct answer is B.

With a shallow snowpack and long periods of dry, cold weather, depth hoar probably formed. Even if it did not, other weak layers, such as near-surface facets or surface hoar, probably formed.

4. Was there sufficient loading on December 23 to significantly increase the potential for avalanches on all of the mountains’ slopes? (Choose the best answer.)

The correct answer is B.

Twelve inches (30 cm) of new snow over 48 hours, SWE of 0.60 or 15 mm, and new snow density of 5% probably do not add enough load to increase avalanche potential on all slopes. But, there’s been sufficient loading on wind-loaded aspects and in gullies (north to northeast in this case) for avalanche potential to have increased.

5. Which aspect would have some potential for smaller slab avalanches late on December 23? (Choose the best answer.)

The correct answer is C.

Soft slabs could develop on north- and northeast-facing slopes steeper than 35 degrees with 25 mph (11 m/s) winds from the south and southwest.

6. Based on the forecast through December 26, when might more widespread natural avalanche activity occur on slopes with angles between 30 and 45 degrees? (Choose the best answer.)

The correct answer is B.

The 28 inches (71 cm) of new snow expected from noon on December 24 to 3 PM on December 25 is enough to increase avalanche potential on December 25. In addition, the snow that day is expected to be heavier (higher density) than the previous day’s snow, further increasing avalanche potential.

7. Near the end of the December 25 storm, how deep would you expect the crowns of any avalanches on wind-loaded slopes to be? (Choose the best answer.)

The correct answer is C.

From December 22 to 25, the area’s total storm snowfall was 42 inches (107 cm), with a SWE of over 4 inches (100 mm). If wind loading added more snow, any slabs on leeward slopes could easily exceed 4 feet (1.2 m) in depth, even with some snow settlement over the three-day period. Access the Synopsis to review the case.

This is a typical setup for developing a weak layer of depth hoar. The area has a high-altitude, continental avalanche climate and it was early in the season. There were less than three feet (1 m) of snow on the ground and temperatures were very cold for an extended period of time. The depth hoar would probably be a persistent weak layer throughout the winter.

Then the upside-down storm created an unstable snowpack within the new snow layers. The following factors compounded the situation:

  • Wind speeds increased through the snowiest time period
  • There was a lot of wind loading
  • The new snow sat on top of weak-faceted snow or depth hoar

Under these conditions, it would probably have been prudent to cancel the hut trip!

Case 3: Western Montana

It’s early February and you’re forecasting for a search and rescue (SAR) operation that’s looking for a snowmobiler lost in a remote area of the mountains of western Montana. The SAR crew is to be flown into the mountains early in the morning, during a break between storm systems. The weather is forecast to worsen by late morning. There are known avalanche paths in the area and elevations are between 6,500 and 8,500 feet (2000 and 2600 m).

Topo map of the area in western Montana

All that we know about the weather is that snow has been on the ground down to 6,500 feet (2000 m) since mid-November. At the end of November, there was one day with a brief, light rain/mist up to around 8,000 feet (2450 m), which was followed by several days of cold temperatures and very low-density snow in early December.

The area had a week-long spell of cold and dry weather in mid-December, but we don’t know if it was windy. Smaller storms hit the area from the latter half of December through the end of January. Each lasted from two to four days, producing 2 to 4 inches (5 to 10 cm) of snow at a time. There were one-to-three day clearing periods between storms.

Temperatures during and between the storms varied widely, from single digits to the upper 20s F (-15°C to -3°C). Winds were unknown, but climatology suggests that they were probably from the southwest, west, and northwest during the storm periods.

FORECAST FOR WESTERN MONTANA MOUNTAIN WEATHER at 8,500 feet (2600 m):

Precipitation:

  • 12 to 16 inches (30 to 40 cm) of new snow at 8,000 feet (2450 m)
  • Water content: From 1.00 to 1.50 inches (25 to 38 mm)
  • Average new snow densities: Around 9%
  • The heaviest snowfall period will be during the 12- to 24-hr forecast period, with snowfall rates approaching 1 inch per hour (2.5 cm/hr) during that period

Wind:

  • Starts out westerly at 25 to 35 mph (11-16 m/s) through the first 24 hours
  • Switches to northwesterly at 15 mph (7 m/s) for the last 12 hours

Temperatures:

  • Start out between 25° and 30°F (-4° to -1°C) for the first 24 hours
  • Slowly cool down to between 20 and 25°F (-7° to -4°C) for the last 12 hours

Select the Questions tab and go through the questions, then read the Synopsis.

1. What type of avalanche climate does the area have? (Choose the best answer.)

The correct answer is B.

The elevations, geographic location, and mix of frequent storms and dry periods make this an intermountain avalanche climate.

2. At the end of November, the light rain/mist followed by cold temperatures formed a thin crust on top of the pack up to 8,000 feet (2450 m). What type of crust was it? (Choose the best answer.)

The correct answer is C.

The light rain/mist followed by cold temperatures would have created an ice crust.

3. What type of layer likely formed on the snow surface in mid-December? Recall that we didn’t have any wind information then. (Choose the best answer.)

The correct answer is C.

The low-density snow that fell in early December would not have bonded with the crusty snow surface. The one-week cold spell that followed would have probably turned the snow into near-surface facets. Surface hoar may also have formed but we cannot say for sure without any wind information.

4. Was this an upside-down storm? (Choose the best answer.)

The correct answer is B.

The period started out warm and ended cold, with higher-density snow early in the storm followed by lower-density snow later on. Therefore, lighter snow overlay heavier snow, which is a right-side-up situation.

5. Is the snow loading forecasted for the next 24 to 36 hours sufficient to increase the potential for avalanches during the storm? (Choose the best answer.)

The correct answer is A.

The new snow will meet or exceed almost all critical thresholds: 12 inches (30 cm) of new snow in 24 hours; one inch (25 mm) of water in 24 hours; snowfall rates of one inch per hour (2.5 cm/hr); more than 6 inches (15 cm) of 9% or greater new snow density; wind speeds in the critical range; and a consistent wind direction for first 24 hour period.

6. Which aspect should have the highest potential for avalanching during the forecast period? (Choose the best answer.)

The correct answer is B.

Westerly winds at 25 to 35 mph (11-16 m/s) will primarily load east-facing slopes. Northwest winds at 15 mph (7 m/s) for the last 12 hours of the forecast period are not sufficient to transport significant amounts of snow to southeast-facing slopes, especially with new snow densities around 9%.

7. What layer in the snowpack is most likely to fail, possibly producing larger, more dangerous avalanches given a sufficient trigger? (Choose the best answer.)

The correct answer is C.

We don’t know how deep the individual layers are, but most of the snowpack since late November has sat atop the ice crust and near-surface facet layer, which developed in late November and early December. Layers of surface hoar could also be interspersed with layers of near-surface facets higher up in the pack, which formed between the smaller storms from late December thru January. However, we do not know for sure. Remember to read the synopsis before moving on to the next case.

The elevations and geographic location make this an intermountain avalanche climate even though we do not know the overall depth of the snowpack. The fluctuations in temperature produced a lot of variability in the snowpack, which is also typical of intermountain avalanche climates.

The most suspect weak layer is probably the near-surface faceted layer that formed in mid-December. It would sit upon a fairly good sliding surface (the ice crust that formed in late November after the rain/mist event).

The lower-density snow that originally fell on the ice crust would probably not have bonded to it very well. The subsequent cold spell would have made that layer even weaker as it turned to near-surface facets. This was all buried deeper in the snowpack by the late December and January snowfalls.

The potential for avalanches (especially on wind-loaded, east-facing slope aspects) would increase with the amount of snow and water and the wind loading. The new snow would probably avalanche but the really dangerous slab avalanches would come from the faceted snow layer above the ice crust. This layer could persist throughout the winter.

Surface hoar layers probably formed between storms when it was clear, especially on open slopes protected from the wind. Without snowpit test results to verify the depth and weakness/strength of these layers, our pre-season weather information leads us to treat the facets on the crust as the most likely weak layer to fail with any avalanches that might occur.

Case 4: Teton Mountains

Links for the case: Data Viewer, Forecast Guide, Data Form

You’ve been asked to make an avalanche weather forecast for Jackson Hole Mountain Resort in Wyoming. You have little to no information about the area’s snowpack and weather history. All you know is that:

  • Jackson Hole is near latitude 43°N, longitude 111°W, with a ridge-top elevation of approximately 10,000 feet (3050 m)
  • The forecast elevation is 9,500 feet (2900 m)
  • You’ll be forecasting from 1200Z April 1 to 1200Z April 2
Topo map of eastern Idaho, western Wyoming with contours every 2500 feet; the location of Jackson Hole Mountain Resort is noted

Here’s the process to follow:

Step 1: Open the Data Viewer and examine the following data.

  • Satellite imagery: 0000Z to 1500Z on April 1
  • Composite radar reflectivity: 0000Z to 1500Z on April 1
  • GFS model output: From 1200Z April 1 through 1200Z April 2; six-hourly forecast charts beginning with the initial analysis at 1200Z April 1 and going through 1200Z April 2
    • 300-mb charts (near jet-stream level)
      • Geopotential heights (m): Black lines with contour intervals of 120 m
      • Isotachs (kts): Color-filled contours beginning at 50 kts (57 mph, 26 m/s) with contour intervals of 20 kts (23 mph, 10 m/s)
    • 500-mb, 700-mb, 850-mb charts
      • Geopotential heights (m): Black lines with contour intervals of 60 m
      • Winds (kts): Standard wind barbs in black (pennant = 50 kts or 26 m/s; long staff = 10 kts or 5 m/s; short staff = 5 kts or 3 m/s)
      • Temperatures (°C): Red lines, dashed for < 0°C, solid otherwise, with contour intervals of 5°C (9°F)
      • Relative humidity (%): Color-filled contours beginning at 50%, with contour intervals of 10%
Table of the most useful upper-air maps to use for various elevations
    • QPF charts: 6-hour accumulated liquid precipitation; color-filled contours beginning at 0.05 inch (1.3 mm), with contour intervals of 0.05 inch (1.3 mm)

Step 2: Forecast the following avalanche weather variables, using the Data Form to keep track of your findings. (Note that you can enter data in the form but not save it. It’s for single-session use only.)

  • Total 24-hour water amount (add up each 6-hour period)
  • Minimum and maximum temperature range (convert to °F)
  • From your temperature forecast, estimate the new snow density using the temperature-density table; divide the estimated total water by the estimated density to calculate a 24-hr snowfall accumulation
Table showing how to estimate snow density from air temperature
  • Average wind speed and direction for the 24-hr period
  • Average sky condition for 24 hours

Step 3: When you’ve finished examining the data, access the Questions tab. We’ll walk you through the avalanche weather forecast process to determine the impact of expected weather conditions on avalanche potential.

Step 4: When done, access the Synopsis tab and review the case.

Links for the case: Data Viewer, Forecast Guide, Data Form

1. What’s the estimated total accumulated water amount for the 24-hr forecast period? (Choose the best answer.)

The correct answer is C.

The GFS QPF shows about 0.15 inches during each of the first two 6-hour forecast periods, a small amount (0.02 in) during the third period, and no precipitation in the fourth period. Adjusting the total of 0.32 inches slightly upwards to account for orographic enhancement yields a forecast of at least 0.40 inches. Actual instrument data recorded 0.42 inches.

2. What’s the best approximation for the minimum/maximum temperature range for the 24-hr forecast period? (Choose the best answer.)

The correct answer is A.

The forecast elevation is closest to 700 mb. Using the 700-mb forecast charts, the area is under cold advection through most of the forecast period. The GFS 700-mb temperature drops from about -9°C at 1200Z April 1 to -13°C at 1200Z April 2. Adjusting for weak daytime heating and nighttime cooling yields a max/min of approximately -8°C/-14°C (18°F/7°F). Actual instrument data had a maximum of -8°C (17°F) on April 1 and a minimum of -15°C (5°F) at 1200Z on April 2.

3. What’s the estimated new snow density based on the temperature-density table? (Choose the best answer.)

The correct answer is C.

Temperatures between -8°C and -14°C support an estimated snow density of about 0 .04 or 0.05. Actual data from the snow study plot at 9,500 feet showed an average snow density of 0.04.

4. What’s the average wind speed for the 24-hr forecast period? (Choose the best answer.)

The correct answer is B.

Using the GFS 700-mb forecast charts, wind speeds are forecasted to be in the range of 10 to 20 kts (12 to 23 mph or 5 to 10 m/s) and strongest between 0000Z and 0600Z on April 2. Actual instrument data from that day had average wind speeds of 18 mph (8 m/s) and gusts to 44 mph (20 m/s).

5. What’s the average wind direction for the 24-hour forecast period? (Choose the best answer.)

The correct answer is C.

The winds will start off from the north but change to northwest and west-northwest. Actual instrument data from that day had average wind direction from WNW.

6. What’s the average sky condition for the 24-hr forecast period? (Choose the best answer.)

The correct answer is A.

The GFS RH forecast at both 700 and 500 mb average above 90% through 0600Z on April 2. After that, drier air begins to move into the forecast area from the west. Overcast skies can be expected with such high RH values. Actual observations showed mostly cloudy skies until just after 0600Z with clearing thereafter.

7. Is enough new snow, water, or wind loading forecasted in the next 24 hours to increase avalanche potential? (Choose the best answer.)

The correct answer is B.

To calculate the 24-hour total new snowfall, divide the estimated total water by the estimated new snow density (0.30 to 0.40 water / 0.04 density). This yields a snowfall forecast of 7.5 to 10 inches (18 to 25 cm). The actual 24-hr total snowfall at 9,500 feet (2900 m) was 10 inches (25 cm). Given that the forecast for the 24-hour period calls for less than 12 inches (31 cm) of new snow, less than one inch (25 mm) of water equivalent, and not exceedingly strong winds, avalanche potential is not expected to increase.

8. Is there enough snow to cause sluffing on slopes steeper than 45 degrees? (Choose the best answer.)

The correct answer is A.

Some minor sluffing could occur due to the low-density nature of the forecasted snow.

9. Given the low-density snow that’s expected to fall, could the 20 mph (9 m/s) average winds create new soft slabs on southeasterly facing slopes? (Choose the best answer.)

The correct answer is A.

The winds are expected to be strong enough to transport the low-density snow. Soft slabs may be created on leeward slopes but shouldn’t be much more than a foot or so deep.

10. Based on all of this information, what do you expect avalanche potential to be for the next day? (Choose the best answer.)

The correct answer is A.

The avalanche potential is expected to stay the same. See the synopsis for more information.

Links for the case: Data Viewer, Forecast Guide, Data Form

Based on 20 inches (50 cm) of new snow and high winds on March 29 and 30, the Bridger-Teton National Forest Avalanche Center issued an avalanche hazard rating of “considerable” on the morning of April 1. After the additional snow that day, the hazard rating remained the same on April 2. Unfortunately, a snowmobiler was killed on April 2 in an avalanche in the mountains just south of the Teton Range.

In looking at the radar loop, you may have noticed that the Salt Lake City (SLC) radar data were not available. Since you couldn’t see precipitation moving into the region from the southwest, you may have underestimated your initial analysis of the amount of precipitation that fell in the mountains just before the forecast period began. By using the satellite loop in combination with the radar data, you could have surmised that more precipitation was occurring over the mountains upstream of the forecast area than was being shown on the radar loop.

Case 5: Cascade Mountains

Links for the case: Data Viewer, Forecast Guide, Data Form

You’ve been asked to make an avalanche weather forecast for Mt. Baker Ski Resort, which is in the northern Cascade Mountains of Washington. All you know is that:

  • Mt. Baker Ski Resort is in near latitude 49°N, longitude 122°W, with a ridge-top elevation of approximately 5,000 feet (1500 m)
  • The forecast elevation is 4,500 feet (1400 m)
  • You’ll be forecasting from 1200Z April 2 to 1200Z April 3
Topo map of western Washington with contours every 2500 feet; the location of Mt. Baker Ski Resort is noted

Here’s the process to follow:

Step 1: Open the Data Viewer and examine the following data.

  • Satellite imagery: 0000Z to 1500Z on April 2
  • Composite radar reflectivity: 0000Z to 1500Z on April 2
  • GFS model output: From 1200Z April 2 through 1200Z April 3. Six-hourly forecast charts beginning with the initial analysis at 1200Z April 2 and going through 1200Z April 3
    • 300-mb charts (near jet-stream level)
      • Geopotential heights (m): Black lines with contour intervals of 120 m
      • Isotachs (kts): Color-filled contours beginning at 50 kts (57 mph, 26 m/s) with contour intervals of 20 kts (23 mph, 10 m/s)
    • 500-mb, 700-mb, 850-mb charts
      • Geopotential heights (m): Black lines with contour intervals of 60 m
      • Winds (kts): Standard wind barbs in black (pennant = 50 kts or 26 m/s; long staff = 10 kts or 5 m/s; short staff = 5 kts or 3 m/s)
      • Temperatures (°C): Red lines, dashed for < 0°C, solid otherwise with contour intervals of 5°C (9°F)
      • Relative humidity (%): Color-filled contours beginning at 50% with contour intervals of 10%
Table of the most useful upper-air maps to use for various elevations
    • QPF charts: 6-hour accumulated liquid precipitation; Color-filled contours beginning at 0.05 inch (1.3 mm) with contour intervals of 0.05 inch (1.3 mm)

Step 2: Forecast the following avalanche weather variables, using the Data Form to keep track of your findings. (Note that you can enter data in the form but not save it. It’s for use at one session only.)

  • Total 24-hour water amount (add up each 6-hour period)
  • Minimum and maximum temperature range (convert to °F)
  • From your temperature forecast, estimate the new snow density using the temperature-density table; divide the estimated total water by the estimated density to calculate a 24-hr snowfall accumulation

    Table showing how to estimate snow density from air temperature

  • Average wind speed and direction for the 24-hr period
  • Average sky condition for 24 hours

Step 3: When you’ve finished examining the data, access the Questions tab. We’ll walk you through the avalanche weather forecast process to determine the impact of expected weather conditions on avalanche potential.

Step 4: When done, access the Synopsis tab and review the case.

Links for the case: Data Viewer, Forecast Guide, Data Form

1. What’s the estimated total accumulated water amount for the 24-hr forecast period? (Choose the best answer.)

The correct answer is A.

The total GFS QPF is approximately 0.97 in (25 mm), with QPF during the four 6-hour time periods of 0.06 in (1.5 mm), 0.13 inches (3 mm), 0.40 inches (10 mm), and 0.38 inches (10 mm). Since the winds were strong and, during the latter half of the forecast period, aligned perpendicularly to the Cascade Mountains, it’d be reasonable to increase the forecast amount to over 1.0 inches (25 mm). Actual instrument data recorded 1.25 inches (32 mm).

2. What’s the minimum and maximum temperature range for the 24-hr forecast period? (Choose the best answer.)

The correct answer is B.

The elevation of the Mt. Baker Ski Resort is closest to the 850-mb pressure surface. On the GFS 850-mb forecast charts, the temperature rises for the first half of the forecast period and falls after that. The temperatures get close to freezing (-2 or -1°C) between 1800Z April 2 and 0000Z April 3. Then they fall to -5 or -6°C by 1200Z April 3. Actual instrument data had a maximum of 29°F (-2 °C) the afternoon of April 2 and a minimum of 22°F (-6 °C) at 1200Z on April 3.

3. What’s your estimate of new snow density based on temperature-density table? (Choose the best answer.)

The correct answer is A.

The forecast temperatures at 850 mb imply a fairly dense snow (at least 0.09). But this might be an overestimate since temperatures in the snow-producing portion of the clouds were probably colder. Actual data from the snow study plot at 4500 feet (1372 m) showed an average snow density of 0.07.

4. What is the forecast for the average wind speed for the 24-hr forecast period? (Choose the best answer.)

The correct answer is B.

This is tricky because much higher winds just offshore are circulating around the low pressure system coming onshore. The wind speeds on the 850-mb charts go from about 15 knots (17 mph or 8 m/s) at 1200Z April 2 to 25 knots (29 mph or 13 m/s) at 1200Z April 3. The average appears to be in the low-20 knot range (mid-20 mph range or around 12 m/s). Actual instrument data from that day had average wind speeds around 25 mph (11 m/s) and gusts to 75 mph (34 m/s).

5. What is the average wind direction for the 24-hour forecast period? (Choose the best answer.)

The correct answer is C.

Winds are southerly to southeasterly during the first half of the forecast period and shift to the southwest and west-southwest during the latter half. Actual instrument data from that day had the average wind direction from the southeast in the morning and south-southwest for the rest of day and overnight.

6. What is the average sky condition for the 24-hr forecast period? (Choose the best answer.)

The correct answer is A.

High relative humidity at 850, 700, and 500 mb persists throughout the forecast period, virtually guaranteeing overcast conditions. The satellite data show a well-developed, comma-shaped cloud coming onshore with heavy cloudiness covering northwestern Washington. The observations had cloudy skies throughout the forecast period with a few breaks after 0600Z April 3.

7. Is enough new snow, water, or wind loading forecasted in the next 24 hours to increase avalanche potential? (Choose the best answer.)

The correct answer is A.

To calculate the 24-hour total new snowfall, divide the estimated total water by the estimated new snow density (1.10 to 1.30 water / 0.07 density). This yields a snowfall forecast of 12 to 16 inches (30 to 40 cm). The actual 24-hour total snowfall at 4,500 feet (1400 m) at Mt. Baker was 17 inches (43 cm). Since all of the forecast values exceed critical thresholds, avalanche potential is expected to increase. (To review, the forecast is for more than 12 inches or 30 cm of snow in 24 hours, more than 1 inch or 25 mm of water in 24 hours, and average wind speeds greater than 20 mph or 9 m/s.)

8. Was the ridge-top wind direction consistent over the 24-hr forecast period? (Choose the best answer.)

The correct answer is B.

It shifted from southeast to southwest.

9. Based on your ridge-top wind direction forecast, which slope aspects have the best chance of being wind loaded? (Choose the best answer.)

The correct answer is C.

The southerly component to the winds would load the north-facing slopes. Later on, the westerly component would load the east-facing slopes.

10. Based on all of this information, what do you expect avalanche potential to be for the next day? (Choose the best answer.)

The correct answer is A.

The avalanche potential is expected to increase or stay the same. Access the synopsis to review the case.

Links for the case: Data Viewer, Forecast Guide, Data Form

The avalanche hazard rating issued by the Northwest Avalanche Center for these elevations on April 2 was considerable and forecast to slightly increase the next morning due to expected new snow and wind.

Earlier in the week (from March 28 to 30), one to two feet (30 to 60 cm) of snow fell in the Cascade Mountains, accompanied by strong winds. A two-day break between storms allowed the snow to settle and stabilize. Crusts formed on south-facing slopes. In the overnight hours, during the break in the weather, some surface hoar was reported to have formed on some wind-protected, north-facing slopes at around 5,000 feet (1500 m).

On April 3, two young snowboarders were buried in a small avalanche just outside a ski area to the south of Mt. Baker in the North Cascades. It happened at 5,200 feet (1585 m) on a northeast aspect whose slope angle ranged from 37 to 41 degrees. Thankfully both snowboarders survived unhurt. But later in the day, skiers triggered a second avalanche in the area. The crown depth averaged almost three feet (1 m) and released on the surface hoar that had formed between the storms.

Summary and Resources

Summary

Basic information:

  • An avalanche is a mass of snow that moves rapidly down a steep mountain slope
  • The main weather parameters that impact avalanche formation are precipitation, wind, air temperature, and cloud cover

Avalanche weather forecasts:

  • Contain detailed, area-specific weather information for mountainous areas that indicate the likely impact of current and upcoming weather on avalanche potential
  • Are created by weather forecasters for avalanche forecasters who integrate information about the weather, snowpack, and terrain to create a comprehensive avalanche hazard forecast

Avalanche climate:

  • The average winter weather patterns that cause certain kinds of avalanche conditions to develop
  • Three primary types: maritime (lie within coastal mountain ranges), continental (located in higher elevation, interior mountain ranges), and intermountain (transitional zones, usually found between coastal and interior mountain ranges)

Slab avalanches:

  • Occur when a cohesive layer of snow slides down a slope
  • Can consist of snow from a single storm or from multiple layers of snow from several storms
  • Cause the most fatalities and do the most property damage

Parts of a slab:

  • A fracture line at the upper limit on the slope
  • Flanks or continuations of the fracture lines down both sides of the slab
  • A stauchwall (bottom or lower limit of the slab); is often obliterated by the avalanche
  • A bed surface upon which the avalanche slides; is usually smooth and planar

Soft slab avalanches:

  • Form when winds are relatively light and/or the snow has relatively low density
  • Break up easily and become more powdery as they run downhill

Hard slab avalanches:

  • Form when winds are relatively strong and/or the snow is of higher density
  • Maintain large blocks of snow as they descend to the bottom of the slope

Avalanche path: A fixed area within which avalanches travel

  • Starting zone: Uppermost part of the avalanche path
  • Track: Area within which a particular avalanche travels
  • Runout zone: Where debris accumulates

Most common slope angle for avalanche formation: 30 to 45 degrees

Direct-action avalanches occur during a storm or just after it’s ended whereas delayed-action avalanches occur more than 24 hours after a storm has stopped.

Basic components of a slab avalanche:

  • Slab:
    • A consolidated mass of snow put into motion as a unit when the avalanche releases
    • Usually forms when wind transports new snow from a windward to leeward slope
    • Usually forms during storms but can develop between storms given sufficient wind
  • Weak layer:
    • Unconsolidated or poorly bonded snow that can easily collapse under stress
    • Three main types: Surface hoar, depth hoar, near-surface facets
    • Generally forms between storm periods with clear skies and cold temperatures
    • Most avalanches occur when a weak layer fails, putting the slab above it in motion
    • Can remain in the pack and cause delayed-action avalanches weeks or even months after forming
  • Sliding surface under a weak layer:
    • A hard or slick snow surface that provides a relatively smooth surface upon which an avalanche can move
    • Three main types of crusts: Ice, rain, melt-freeze
    • Problems arise when a good sliding surface is buried by new snowfall

Unstable snowpack:

  • Has a slab on a weak layer that’s on top of a good sliding surface
  • Contains weak and strong snow layers

Good bonding between layers is likely if:

  • The snow surface is relatively warm (air temperatures near or above freezing) just before new snow falls; then new snow of almost any temperature or crystal type will bond with it
  • There’s a crust on the surface and new snow occurs at relatively warm temperatures
  • Weak snow and warmer temperatures precede a warmer snowfall or rain event

Poor bonding between layers is likely if:

  • The new snow surface is relatively cold and consists of a weak snow type
  • The new snow surface consists of preserved, colder snow crystals, such as stellars and dendrites
  • The new snow surface is a crust and air temperatures remain cold as new snow begins falling

Avalanche weather forecast process:

  • Pre-forecast preparation: Gather basic information about your area (its avalanche climate, any areas prone to avalanches, the weather history from the start of the season, snowpack observations about the snow surface and stability of the pack, and reports of recent avalanche activity)
  • Assessing current weather: Evaluate the weather over the last 24 hours or so
  • Forecasting future weather: Make a weather forecast for the important avalanche weather parameters (precipitation, wind, temperature, and cloud cover)
  • Making the avalanche weather forecast: Evaluate the impact of the current and forecasted weather on your area’s avalanche potential and determine if it is increasing, decreasing, or remaining the same

For information on the avalanche weather forecast process, access the Avalanche Weather Forecast Guide and the Avalanche Weather Forecast Data Form (instructions)

High-risk situations that make an area particularly prone to avalanches:

  • Heavy, dense snowfall of 6 inches (15 cm) or more produces avalanches, especially on steeper terrain with slopes greater than 40 degrees
  • 12 inches (30 cm) of new snow produce sluffing on steeper terrain at a minimum
  • 12 inches (30 cm) or more of new snow (especially with higher densities) may overload an already weak snowpack and produce deeper slab releases
  • New slabs created with 6 inches (15 cm) or more of new snow and winds from a consistent direction for 6 hours or more averaging 20 to 60 mph (~17 to 52 kts or 10 to 30 m/s) pose high risks
  • Rain on snow almost always produces avalanching, with heavier rain producing larger avalanches
  • Storms beginning with cold temperatures and low-density snow and ending with warm temperatures and higher-density snow often initiate direct-action avalanches
  • Rapidly warming temperatures during the day increase snowpack settlement rates and can cause avalanches without any new snow loading
  • Warm surface temperatures (at or above freezing) combined with intense solar radiation produce wet snow avalanches
  • Water percolates deeper into the snowpack if continuous, above-freezing temperatures last for more than 24 hours; the longer the situation continues, the deeper the avalanches may be (even reaching the ground)
  • One day of clear, cold, calm weather followed by a significant snowfall can bury a layer of surface hoar, posing risks to the future stability of the pack
  • Longer spells of dry, relatively cold weather followed by a significant snowfall often will bury near-surface faceted snow or depth hoar and may pose a risk if more loading occurs

Resources

Module resources:

Avalanche Weather Forecast Guide

Avalanche Weather Forecast Data Form and Instructions

Books:

Snow Sense, by Jill Fredston and Doug Fesler, 1999

Staying Alive in Avalanche Terrain, by Bruce Tremper 2001

The Avalanche Handbook, by David McClung & Peter Schaerer, 1993

The Avalanche Book, by Betsy Armstrong & Knox Williams, 1986

Avalanche Safety for Skiers & Climbers, Tony Daffern, 1983

The Snow Booklet, A Guide to the Science, Climatology, and Measurement of Snow in the United States, Colorado Climate Center, by Nolan Doessken & Arthur Judson, 1996

Mountain Meteorology, by C. David Whiteman, 2000

The International Classification for Seasonal Snow on the Ground, Colbeck, S.C. et al., Wallingford, Oxfordshire, International Association of Scientific Hydrology, International Commission on Snow and Ice, 1990

Guardian Angel TTPs supplied by AFWA

Organization websites:

Colorado Avalanche Information Center, http://avalanche.state.co.us/index.php

Portal to all major U.S. forecast centers, http://avalanche.org

American Avalanche Association, http://www.americanavalancheassociation.org/

USFS National Avalanche Center site, http://www.fsavalanche.org/

Canadian Avalanche Centre, http://avalanche.ca/cac/

Northwest Weather and Avalanche Center, http://www.nwac.us/

SnowPit Technologies, http://www.snowpit.com/index.htm

COMET modules:

Snowpack and Its Assessment, http://meted.ucar.edu/afwa/snowpack/

Microwave Remote Sensing: Land and Ocean Surface Applications, http://www.meted.ucar.edu/npoess/microwave_topics/land_ocean/main.htm

Snowmelt Processes, http://www.meted.ucar.edu/hydro/basic/Snowmelt/