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Forecasting

Forecast Process

In this section, we will follow the forecast process from observations, through numerical weather prediction (NWP) models, into the hands of a weather forecaster, and then out to the emergency managers and the public.

The path through the forecast process from observations through NWP, then NHC divisions, followed by the local weather forecast office then to the emergency manager.

Observations are taken by a variety of monitoring systems, including satellites in the upper atmosphere and instruments closer to the surface. Aerial reconnaissance has become an important part of those observations, and these flights are coordinated by a group called CARCAH (Chief, Aerial Reconnaissance Coordination, All Hurricanes), which is based at the NHC.

The observations go into the Numerical Weather Prediction (NWP) system, which uses hurricane models to produce the forecasts that are then analyzed and refined by several groups at NHC, including:

  • TAFB (Tropical Analysis and Forecast Branch), which mainly produces tropical ocean forecasts, but provides additional services during hurricane season
  • HSU (Hurricane Specialists Unit), whose primary task is preparing the forecasts for dissemination
  • HLT ( Hurricane Liaison Team), which serves a communication function between NHC and public officials

Once the NHC forecast products are sent out, the affected WFOs use the information to tailor their local forecasts by including details specific to their warning areas so that emergency managers and the public can use the forecasts to prepare for the expected impacts.

We will begin our look at the forecast process by discussing the observations.

Observations

The NHC and NWS have several tools to observe tropical cyclones. When they are still far out in the ocean, satellites are the main observational tool, although ships and buoys also provide sparse observations. Once the system passes 55°W longitude and becomes a threat to land areas, reconnaissance aircraft are sent out to take measurements. Within about 280 mi (450 km) of the coast, coastal radars provide important precipitation and velocity information about the storm. When the system is over land, Automated Surface Observation Stations (ASOSs) and instrumented weather balloons (called radiosondes) provide additional measurements. We will look at several observational tools, beginning with those used for far offshore observation, then those closer to shore, and finally the on-shore observational platforms.

The path through the forecast process from observations through NWP, then NHC divisions, followed by the local weather forecast office then to the emergency manager.  The Observations are all highlighted.

Satellite

When tropical cyclones are still far from land, satellites help forecasters predict their formation, movement, and intensity; estimate their location; and analyze conditions in the surrounding environment. However, sometimes the bands and the eye are obscured by higher level clouds, making it difficult for forecasters to use regular satellite imagery to monitor the storm.

Two types of satellites provide useful data for tropical cyclone forecasting: geostationary and polar-orbiting satellites. A geostationary satellite orbits the earth over the equator, providing data over the same areas. Data from geostationary satellites have good temporal resolution, which allows forecasters to monitor systems almost continuously. However, they lack the spatial resolution that polar-orbiting satellites have. Polar-orbiting satellites pass over or near both poles, but track on different longitudes on each orbit. Data from polar-orbiting satellites are only available a couple times daily over a given location due to the orbits.


Data provided by the two types of satellites include:

  • Cloud cover
  • Cloud type
  • Cloud-top temperature
  • Cloud height
  • Land and sea surface temperature (when not obscured by clouds)
  • Atmospheric winds
  • Rain rate
  • Precipitation estimates

Interpreting Satellite Imagery

Shown below are the three most commonly used images from weather satellites: the visible, infrared, and so-called "water vapor" images (from left to right).

Full disk satellite images from GOES-13 showing Hurricane Ike on September 11, 2008.  Ike is centered off the coast of Louisiana.  From left to right the images are visible, infrared, and water vapor satellite images.  Because of the time of day, the visible image is lacking data on the western side of the globe that is in the image, but the other two images are not missing data.  The infrared image approximates the temperature of the surface which is emitting the energy toward the satellite.

Visible Imagery

The image below is a visible image from the Geostationary Operational Environmental Satellite (GOES). The view is similar to what a human eye would see from space, only in black and white. During daylight, clouds are white, land is gray, and water is black, but at night this imagery is not available because it is dark.

Full disk visible satellite image from GOES-13 showing Hurricane Ike on September 11, 2008.  Ike is centered off the coast of Louisiana.  The western portion of the graphic is black because the sun is not yet up on that portion of the earth.  The rest of the image appears in black and white like our eyes would see it.

Infrared (IR) Imagery

The image below is an infrared (IR) image from the GOES-East satellite taken at the same time as the visible image above. On the IR image, the gray shading indicates temperatures sensed by the satellite, with lighter shades being cooler and darker ones warmer. Since atmospheric temperature typically cools with increasing altitude, lighter shades generally emphasize features higher in the atmosphere, and darker shades indicate features lower in the atmosphere or sometimes at the Earth's surface. These images have slightly lower resolution than the visible imagery, but can be used at all times of day because the satellite receives infrared energy day and night.

Full disk infrared satellite image from GOES-13 showing Hurricane Ike on September 11, 2008.  Ike is centered off the coast of Louisiana.  Despite the sun not shining on the far western portion of this image, the satellite imagery is displayed because this image does not detect reflected light instead it detects infrared energy emitted by the surfaces.  Thus, this image approximates the temperature of the surface which is emitting the energy.

Enhanced Infrared Imagery

IR images are often colored to bring out details in cloud patterns. In the image shown below, cloud-top temperatures colder than -4°F (-20°C) are the colors, while the gray scale indicates temperatures warmer than -4°F (-20°C). Temperature data from IR images help in estimating cloud-top height, with taller clouds often meaning more active weather such as stronger thunderstorms.

Enhanced infrared satellite image from GOES-12 showing Hurricane Ike on September 11, 2008.  Ike is centered off the coast of Louisiana.  The image is colorized to make visualizing the higher clouds easier with yellow tones being the highest, then progressing lower: red, green, blue, white, grays, then black.  Around the eye of Hurricane Ike and in one rain band to the east and southeast of the eye, the cloud tops are red.  The eye is inconsistent in cloud top height with the southeast quadrant being higher (reds) than the other quadrants (greens).

Water Vapor Imagery

The image below is another type of infrared image, but it is a very specific wavelength that shows mid- and upper-level water vapor particularly well. Lighter shades represent more moisture while darker ones signify drier areas. Water vapor images are lower resolution than visible images, but can be used 24 hours a day to see mid- and upper-level water vapor.

Full disk water vapor satellite image from GOES-13 showing Hurricane Ike on September 11, 2008.  Ike is centered off the coast of Louisiana.  Despite the sun not shining on the far western portion of this image, the satellite imagery is displayed because this image does not detect reflected light instead it detects infrared energy emitted by the surfaces.

Ships and Buoys

NOAA's Oscar Dyson instrumented ship.

Ships and buoys provide additional information about surface winds, pressure, air and sea surface temperatures, and wave conditions in or near tropical cyclones. Although ships and buoys are sparse, they are the only routine source of measured waves data and are sometimes the only way to take direct measurements when a tropical cyclone is still at sea. As a result, they provide observational ground truth for satellite and radar measurements in the marine environment.

Instrumented buoy data including wind speed (kt; blue), wind gusts (kts; red), and air pressure (in Hg; green).

Reconnaissance Aircraft

The most direct method of measuring the winds in a tropical cyclone is to send reconnaissance aircraft into the storm. From these missions, forecasters gain an in-depth understanding of the environment in and around the system. However, these measurements are not continuous, sample only parts of the storm, and cannot be taken until the hurricane is relatively close to land. The typical flight pattern covers only key areas of the cyclone: the eye (if present), eyewall on multiple sides (if present), and some rainbands. Nonetheless, that information provides critical data needed to forecast the behavior of the storm.

The U.S. Air Force Reserve "Hurricane Hunters" specially equipped C-130 aircraft and the NOAA P-3 aircraft conduct most operational reconnaissance, while NOAA’s high altitude Gulfstream IV-SP surveys the larger scale environment.

Instrumented "Hurricane Hunter" C-130 flown by the U.S. Air Force Reserve 403rd Wing out of Keesler Air Force Base in Biloxi, Mississippi.

Instrumented "Hurricane Hunter" C-130 flown by the U.S. Air Force Reserve 403rd Wing out of
Keesler Air Force Base in Biloxi, Mississippi

Instrumented NOAA "Hurricane Hunter" Gulfstream IV (foreground) and P-3 Orion (background) aircraft.

Instrumented NOAA "Hurricane Hunter" Gulfstream IV (foreground) and P-3 Orion (background) aircraft

Instruments on or deployed by these aircraft relay information about the vertical and horizontal temperature, moisture, pressure, winds, and location of the hurricane's eye.

For more information on the Hurricane Hunters or the research aircraft, visit these sites:

Radiosondes & Dropsondes

A radiosonde is a small instrument package and radio transmitter that is launched via a weather balloon. Typically these are released twice daily at land-based stations spaced several hundred miles apart, but they can also be launched from ships. As the radiosonde rises through the atmosphere, its instruments measure:

  • Air temperature
  • Humidity
  • Pressure
  • Position (altitude and latitude/longitude)

The measurements are relayed back to the surface via a radio transmitter. Using the position information and time between each transmission, a computer program calculates wind speed and direction at each altitude.

4 men launch a radiosonde balloon

Launching a radiosonde balloon

Dropsondes are a variation of the radiosonde package. Instead of being carried aloft by a balloon, the dropsondes are parachute-dropped into the tropical cyclone from reconnaissance aircraft. These data provide important information about the vertical characteristics of the hurricane’s environment, which are critical for forecast models and, ultimately, the forecasters' decisions.

Left:  GPS Dropsonde. Right: Example of a dropsonde sounding during Hurricane Ike (September 11, 2008 – 1105 UTC)

Left: Dropsonde. Right: Example of a dropsonde's temperature measurements
during Hurricane Ike on September 11, 2008 at 1105 UTC (6 am EST)

Radar

Radars send out microwaves that reflect off the precipitation-sized particles and then "listen" for the reflected response. The returned signals are processed to provide detailed information on rain intensity, winds, and storm motions. The limitation is that a radar’s viewing distance is at most about 280 mi (450 km), which is usually when tropical cyclones are less than 36 hours from landfall, well after watches and warnings must be issued.

Radar radome in front of an ominous, yellowy, mammatus laden sky.

The two main products produced from radar data are reflectivity images (the typical radar image shown on television) and radial velocity images. The reflectivity image (below, left) shows the location and amount or intensity of precipitation and its motion (when the images are animated), while the radial velocity image (below, right) shows the wind speed and direction.

Left: Radar reflectivity image from NW Florida radar showing a tropical cyclone center heading to the west of the radar location.  Near the radar, one of the spiral rainbands is coming ashore and shows supercellular characteristics in some of its echoes.  
Right: Radial velocity image from NW Florida radar showing a tropical cyclone center heading to the west of the radar location.  Near the radar one of the spiral rainbands is coming ashore and shows a strong mesocyclone and Tornado Vortex Signature (TVS) one county south of the radar.

Left: Radar reflectivity image from the NW Florida radar, showing a tropical cyclone west of the radar location. Near the radar, one of the spiral rainbands is coming ashore and shows heavy precipitation (yellow and red colors) in some of its echoes.
Right: Radial velocity image from the same storm. Although it is not obvious to the layman, forecasters detected a tornado signature in one of the rainbands.

ASOS

An Automated Surface Observation Station (ASOS) is a land-based array of instruments used to automatically take standard weather observations around the clock. The ASOS observations only become useful when tropical cyclones are close to shore, have made landfall, or have moved inland. These systems measure:

  • Temperature
  • Humidity
  • Pressure
  • Wind speed
  • Wind direction
  • Precipitation amount
  • Precipitation type
  • Visibility
  • Cloud base height and amount
A picture of an automated surface observation station (ASOS).  From left to right, the instruments are: sonic anemometer, tipping rain gauge, hygrothermometer, precipitation identification sensor, power supply, laser beam ceilometer, freezing rain sensor, and forward scatter visibility sensor.

NWP

The path through the forecast process from observations through NWP, then NHC divisions, followed by the local weather forecast office then to the emergency manager.  The NWP piece of the process is highlighted.

The complex array of observations are used to "describe" the initial state of the atmosphere for numerical weather prediction models (sometimes called NWP or computer models or even just models). An important part of the weather forecaster’s prediction process, these models are a set of equations that describe the three-dimensional motion of the atmosphere, its heating and cooling, moisture content, and frictions. From these equations and the observations, models can predict future atmospheric conditions as far as 16 days ahead, although the accuracy is better in the earlier time periods.

Forecasters use several different models, which can result in different forecasts because each model has a slightly different way of representing atmospheric processes. In addition to the calculation differences among the models, there are other factors that can affect the quality of the model results. For example, we have only a few observations of the winds above the ocean surface where we measure hurricane strength, so the set of initial observations does not completely represent the state of the atmosphere. This can introduce errors into the models' results.

Because of these potential errors, forecasters must evaluate the strengths and weaknesses of each model’s output to produce the final or "consensus" forecast. This often means comparing a set of model outputs (called an "ensemble"). The image below shows the forecast tracks from several hurricane models in comparison with the actual track.

An ensemble track forecast of Hurricane Isabel showing 7 different model tracks for Isabel.  Each colored line shows a possible track from a different model.  The white line is the official track of Hurricane Isabel.  This map was made after Hurricane Isabel had dissipated, so the entire life cycle of Hurricane Isabel is shown which isn't usual for ensemble track forecasts.

Forecasters look at these ensemble plots to understand the probability of a given forecast. Because there is always some forecaster uncertainty, forecasts are often expressed as probabilities. In some instances, the final forecast is an educated consensus of the many different models’ tracks and intensities shown in the ensemble forecasts. By using multiple models to better understand the possible outcomes, the official consensus NHC track and intensity forecast improves the forecast accuracy over picking a single model's results as the official forecast.

Forecasters in the NHC have been continuously improving their forecast track accuracy. As of the publication of this module, the 5-year average track errors of the official 24-hour NHC forecasts are 53 nm (98 km), increasing to 97 nm (112 km) at 48 hours and 252 nm (290 km) at 120 hours.

The National Hurricane Center Official Annual Average Track Errors for Atlantic Basin Tropical Cyclones

It is much more difficult to forecast changes in tropical cyclone intensity than their tracks. Several methods are used. One approach compares the existing storm to historical storms. If the storm looks similar to previous storms, it will often experience similar intensity changes. Another approach is to apply a statistical hurricane intensity prediction scheme. This is similar to historical comparison, but applies current storm and atmospheric characteristics to statistically predict the most likely intensity change.

The final approach uses an ensemble of forecast models to gage the uncertainty in the intensity forecast. In this way, a forecaster can give a probability for the intensity of a given storm. However, official intensity forecasts from NHC have shown little improvement over the last 20 years as seen here by the dashed lines.

The National Hurricane Center Official Annual Average Intensity Errors for Atlantic Basin Tropical Cyclones.

Despite models being exceptionally useful tools for forecasters, they require a lot of computing power. This means they can only be run between 2 and 4 times a day, depending on the model. Thus, there is still a large void that needs to be filled by the forecaster to make predictions from the observations, especially in the short-term.

Tropical Cyclone Forecasts

Early in the tropical cyclone forecast process, NHC forecasters compare the forecast models’ results to the observations, which provide an indication of which model(s) has done the best job at representing past and present conditions. Forecaster experience plays an important role in reconciling differences between the various models in order to come up with their best forecast, which is then disseminated in a variety of products (discussed in the "Products" part of this section).

These forecasts are prepared at the National Hurricane Center (NHC), located on the campus of Florida International University in Miami, FL.

The outside of the National Hurricane Center building as viewed from the parking lot.  A short entrance-way is covered by a sky blue overhang supported by sandstone-colored pylons making a turret as the facade of the building.  At the front of this overhang is the NOAA symbol (silhouetted eagle with the sky above and the ocean below [abstracted]).  On top of this turret is a metal walkway which houses some of the meteorological equipment and communications dishes further over the building.

NHC is responsible for forecasting all tropical cyclone activity in the Atlantic and Eastern Pacific Basins around North America. Multiple units within the NHC are involved with the forecast process, as can be seen in this flow chart showing how forecasts get from observations to you. We’ll provide a brief overview of these units in the following pages.

The path through the forecast process from observations through NWP, then NHC divisions, followed by the local weather forecast office then to the emergency manager.  The NHC piece of the process is highlighted.

CARCAH

The Chief, Aerial Reconnaissance Coordination, All Hurricanes (CARCAH) is a remote unit of the Air Force’s 53rd Weather Reconnaissance Squadron. Their task is to coordinate tropical cyclone reconnaissance flights flown by the Hurricane Hunters. During reconnaissance flights, data are transmitted back to the CARCAH, checked for errors, and given to NHC forecasters for use in the forecasts.

Panoramic view of the Chief, Aerial Reconnaissance Coordination, All Hurricanes (CARCAH) facilities at the National Hurricane Center (NHC).  Standard, medium-gray, padded, cubicle-style office with five computers and six flat-panel monitors.

TAFB

The Tropical Analysis and Forecast Branch (TAFB) of the NHC provides forecasts for the tropical oceans. Its responsibilities include:

  • Marine high sea forecasts
  • Offshore waters marine forecasts
  • Tropical weather discussions
  • Tropical surface weather analyses

During hurricane season, TAFB forecasters provide supporting information such as:

  • Satellite-derived tropical cyclone position and intensity estimates
  • Radar location fixes for tropical cyclones
  • Tropical cyclone forecast support
  • News media support
  • General operational support
Panoramic view of the Tropical Forecast Analysis Branch (TAFB) facilities at the National Hurricane Center (NHC).  Open floor plan office with long wooden desks with black tops.  On the walls are many posters from presentations given by the NHC employees as well as maps of the tropical oceans and forecast areas.  31 computer screens sit on the desks in this large room with 12 computers.  Lots of open tables are out for spreading out papers and analyzing data by hand.

Hurricane Specialists Unit

The Hurricane Specialists Unit (HSU) is where the forecast finally comes together as a public product. This group maintains a continuous watch on tropical cyclones and areas of disturbed weather in the Atlantic and Eastern Pacific Oceans and analyzes all the information relayed to them from CARCAH and TAFB, the observations, and the model results.

The HSU provides text advisories and graphical forecast products and issues coastal tropical cyclone watches and warnings for the United States and its Caribbean territories. If the tropical cyclone looks like it will impact the U.S., the HSU coordinates with affected local NWS forecast offices and other national forecast centers. As a cooperative service, the HSU also provides watch and warning recommendations to Mexican, Central American, and Caribbean meteorological services.

Panoramic view of the Hurricane Specialists Unit (HSU) facilities at the National Hurricane Center (NHC).  Open floor plan office with long wooden desks with black tops.  On the walls are the NOAA and NWS emblems/logos and a very long map of the tropics covering from the African subcontinent to past the international date line.

Local WFO

After NHC issues a forecast, local NWS WFOs customize the forecast to take local conditions into account and describe current and anticipated storm effects in their area. This provides local officials, the public, and the media with important information to make timely and efficient decisions. Local forecasters also participate in coordination calls between NHC and other local WFOs to ensure forecast and warning consistency.

Aerial photo of a National Weather Service (NWS) local weather forecast office (WFO), specifically Lincoln, Illinois (ILX).  There are two identifiable structures a larger one story building in the background (tan brick exterior, gray shingled roof) and a smaller three story domed building in the foreground (tan brick with a white dome and a garage door).  The larger building is the forecast office, while the smaller building is a radiosonde station.

Some of the other WFO products include:

  • Warnings for severe thunderstorms and tornadoes
  • Watches and warnings for floods and flash floods
  • Local aviation forecasts
  • Marine warnings for coastal areas
  • Hydrologic forecasts
  • Local and zone public forecasts

Hurricane Liaison Team

The Hurricane Liaison Team (HLT) is a FEMA-sponsored team made up of federal, state, and local emergency managers; FEMA personnel; and NWS forecasters with extensive tropical cyclone operational experience. Their role is to serve as a bridge between the meteorologists producing the forecasts and the emergency managers dealing with a tropical cyclone threat.

View of a briefing by the Hurricane Liaison Team (HLT).

One of their main functions is to maintain open lines of communication about the progress and threat level of the storm with appropriate federal, state, and local officials. The HLT provides video-teleconference briefings from the NHC when a tropical cyclone is expected. During these calls, the NHC Director briefs all the threatened states, the federal government, and the White House. Since everyone then hears the same information, the various parties can share their issues, limitations, plans, and timelines. In addition, the team members are available to answer questions from emergency managers at any time during the storm. State and/or local officials, not the HLT or the NHC, make the final decisions concerning evacuations.

Emergency Manager

As the final decision maker in this process, you have a wide array of products at your disposal that have developed from the initial observations all the way to the final forecast. You have the daunting task of ordering evacuations, preparing the local community, and staging resources from other locations. In the next section, we will discuss the many products that can aid you in your decision-making process.

As an emergency manager, you are likely already connected to these different sources of information. If not, you can get many of the different forecast products and communications through these sources:

Products

The main products emergency managers use in tropical cyclone decisions are found in this section. Because numerous products are useful in different emergency management phases, the chart below organizes the main products and their use on a timeline. These products should generally be used in the phases listed across the bottom of the graphic (horizontal axis), but could be handy in any phase of your management of the situation. By following the graph from the "Situational Awareness" phase through the "Landfall" phase, we will look at each product more closely in the following pages.

Each rectangle is a product provided by either the NHC or the NWS. The vertical axis shows the specificity of the product from general (say, the Atlantic Basin) at the bottom to local (say, Morehead City, NC) at the top. The horizontal extent of each product box shows you the phases in which the product is most helpful. The "Future Situational Awareness" boxes represent the Tropical Weather Outlook and Hazardous Weather Outlook products that will keep you abreast of future developments even while a current storm may be occurring.

A bar chart of tropical cyclone products produced by the NHC and NWS ordered by location specificity (y-axis) and emergency management checkpoints (x-axis).  The timeline checkpoints are situational awareness, cyclone monitoring, staging, evacuation decision, evacuation, and landfall.

Tropical Weather Outlook

Before a tropical cyclone develops, the Tropical Weather Outlook gives a brief, non-technical description of significant areas of disturbed weather and the 48-hour potential for development in both the Atlantic and Eastern Pacific basins. Probabilities for tropical cyclone development are broken down in these products as "low" (< 30% chance), "medium" (30-50% chance), and "high" (> 50% chance). NHC forecasters issue these statements at 2 AM/ PM and 8 AM/PM EDT or 11 AM/PM and 5 AM/PM PDT (1 hour earlier during standard time).

This product’s header always begins with some codes that identify the product. The important information is in the last three lines of the header, which identify the product, its source, and valid time and date. It is always a good idea to check the date to make sure you have the most recent outlook.

The main body of the outlook begins with a discussion about the general location of the activity, with more specific details provided in the rest of the product.

Tropical Weather Outlook

NHC also issues a Graphical TWO (GTWO), a web-based graphic that shows the most recently available geostationary satellite imagery with disturbances of interest circled. Each disturbance is circled and numbered with a color code that represents the probability of tropical cyclone formation within 48 hours (color key provided at the bottom). The probability of development, in percent, is also shown below each circle. In the example below, there are three disturbances (highlighted with colored circles) being tracked by forecasters. Text descriptions for the disturbances (and active cyclones if any are present) also appear in a pop-up window when a user mouses over the system. If there been any tropical cyclones in this image, they would have been indicated on the graphic with a cyclone symbol (i.e., an "L" for tropical depressions, a tropical storm symbol, or a hurricane symbol).

The Graphical Tropical Weather Outlook from September 19, 2009

Hazardous Weather Outlook

The Hazardous Weather Outlook (HWO) is produced by the local NWS WFO for each county warning area and gives an outlook on the local conditions that you may have to cope with as an emergency manager. The HWO lists the Day 1 through 7 hazards for the specific area, and simple instructions are given about safety related to each day's hazards. Some NWS WFOs provide this product in both a text and graphical form; however not all WFOs use the graphical product yet.

The product starts with a header that includes some identifying codes, the name of the product, the issuing office’s name, and the time and date it is valid. Check to make sure that you have the most recently issued version. The discussion portion begins with a list of the affected areas/counties/parishes, followed by a day-by-day description of the potential hazards, sometimes including starting and ending times for those hazards. The last portion is a spotter activation statement for the area over the next 7 days.

Hazardous Weather Outlook

Tropical Cyclone Public Advisory

The Tropical Cyclone Public Advisory provides coastal tropical cyclone watch, warning, and forecast information for a general audience. It also gives:

  • Cyclone position in latitude and longitude
  • Distance from a well-known reference point
  • Current speed and direction
  • Maximum sustained winds
  • Estimated or measured minimum central pressure
  • Description of the predicted track and intensity over the next 24-48 hours

When warnings are in effect, the advisory will also include information on potential storm tides, rainfall, or tornadoes, as well as any pertinent weather observations. NHC issues these advisories every 6 hours at 5 AM/PM and 11 AM/PM EDT or 2 AM/PM and 8 AM/PM PDT (1 hour earlier during standard time) and every 2 or 3 hours when coastal watches or warnings are in effect. Special advisories are also sent out whenever watches and warnings are issued between the regularly scheduled advisories or when an unexpected, significant change has occurred in the tropical cyclone. When a special advisory is issued all of the associated advisory products are updated including the graphics (track forecast cone, wind probabilities, etc.).

This product’s header begins with codes, and the last three lines identify the product, its source, and valid time and date. It is always a good idea to check the date to make sure you have the most recent advisory.

The main advisory is broken into different sections that summarize the storm location, storm movement, and intensity; provide locations of watches and warnings; discuss storm details and the 48-hour outlook; and list hazards affecting land (storm surge, winds, rainfall, and tornadoes). This is then followed by information about when the next public advisory will be issued.

Tropical Cyclone Public Advisory

Tropical Cyclone Forecast/Advisory

The Tropical Cyclone Forecast/Advisory provides critical tropical cyclone watch, warning, track, and intensity forecast information for HURREVAC, including information on:

  • All current coastal watches and warnings
  • Cyclone position in latitude and longitude
  • Current speed and direction
  • Current maximum radial extent of 12-ft seas
  • Maximum radial extent of 34, 50, and 64 kt winds in each of the four quadrants around the storm
  • Forecasts of the track and intensity of the cyclone valid 12, 24, 36, 48, 72, 96, and 120 hours from the forecast’s initial time, with size information forecast out to 72 hours

NHC forecasters issue these advisories every 6 hours at 5 AM/PM and 11 AM/PM EDT or 2 AM/PM and 8 AM/PM PDT (1 hour earlier during standard time). Special advisories are sent out whenever watches and warnings are issued between the regularly scheduled advisories or when an unexpected, significant change has occurred in the tropical cyclone. When a special advisory is issued all of the associated advisory products are updated including the graphics (track forecast cone, wind probabilities, etc.).

This product’s header begins with codes, and the last three lines identify the product, its source, and valid time and date. It is always a good idea to check the date to make sure you are seeing the most recent advisory.

The main advisory is written in a specific order: watches and warnings, storm location, storm movement, minimum central pressure, current wind radial extents and 12-ft seas, and forecast maximum wind radial extents of 34, 50, and 64 kt winds. This is then followed by notification of when the next forecast/advisory will be issued.

Tropical Cyclone Forecast/Advisory

Tropical Cyclone Forecast Discussion

The Tropical Cyclone Forecast Discussion describes the rationale for the NHC forecaster’s analysis and forecast and is written mainly for other forecasters. As a result, it uses fairly technical language. Many emergency managers who are familiar with the terms find it useful to get a sense of the forecaster’s reasoning.

The Forecast Discussion typically includes comments about the observations that justify the analyzed intensity of the cyclone, a description of the environmental factors expected to influence the cyclone’s future track and intensity, and a discussion of the NWP models. It may also describe the forecaster’s degree of confidence in the official forecast, discuss possible alternate scenarios, and highlight unusual hazards. The product includes a table of forecast positions and intensities for the given cyclone out to 120 hours.

NHC issues these discussions every 6 hours at 5 AM/PM and 11 AM/PM EDT or 2 AM/PM and 8 AM/PM PDT (1 hour earlier during standard time).

This product’s header begins with codes, and the last three lines identify the product, its source, and valid time and date. It is always a good idea to check the date to make sure you are seeing the most recent outlook. The discussion section is followed by a table of forecast positions and intensity.

Tropical Cyclone Forecast Discussion

Track Forecast Cone

The Track Forecast Cone graphic shows coastal areas under a tropical storm watch (yellow), a tropical storm warning (blue), a hurricane watch (pink), or a hurricane warning (red), as shown in the example below for Hurricane Dean (2007). The current position of the tropical cyclone is indicated by an orange circle. The black dots show the official forecast track of the center at the indicated times. The letters inside the black dots indicate NHC’s forecast intensity for that time, with "M" denoting a major hurricane (category 3 or greater), "H" a hurricane (category 1 or 2), "S" a tropical storm, and "D" a tropical depression. This product is issued every six hours at 5 AM/PM and 11 AM/PM EDT or 2 AM/PM and 8 AM/PM PDT (1 hour earlier during standard time).

Track Forecast Cone graphic

Track Forecast Cone graphic without the official track line

Forecast uncertainty is conveyed by the track forecast "cone", the solid white and stippled white areas in the graphic. The solid area depicts the track forecast uncertainty for days 1-3 of the forecast, and the stippled area depicts the uncertainty on days 4-5. Historical data indicate that the entire 5-day path of the center of the tropical cyclone will remain within the cone about 60-70% of the time. To draw the cone, a set of imaginary circles are placed along the forecast track at the 12, 24, 36, 48, 72, 96, and 120 hour positions, where the size of each circle encloses 67% of the previous five years’ official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles. A variable cone graphic, with size based on forecaster confidence, is being developed and may be implemented in the future (check http://www.nhc.noaa.gov/aboutnhcgraphics.shtml for updates on NHC graphical products.)

In the example for Hurricane Dean, note that by the end of August 19, 2007, Dean was forecast to cross directly over Jamaica as a hurricane (the "H" noted in the center of the black dot over Jamaica), but the track also had a high probability of going anywhere within the cone, from the southern tip of Cuba to south of Jamaica. By 8 AM Tuesday August 21, 2007 (~3 days from issuance of this product), Dean was forecast to still be a hurricane over the Yucatan Peninsula. Where the forecast cone changes from solid white to stippled white is the transition from the 3-day cone to the 5-day cone. By 8 AM Thursday August 23, 2007, Dean was forecast to be a tropical storm over the mountains of Northeast Mexico. However, because of the larger uncertainty the farther out in time a forecast is made, the cone indicates the path has a 60-70% chance of being anywhere from south-central Texas to south-eastern Mexico. There was also a smaller chance (40-30%) that the storm track would be outside of the forecast cone, and emergency managers should always consider this possibility.

Another form of this graphic (shown below) displays a black line that depicts the forecast track, but because of the forecast uncertainty, your focus should not be on the line.

Track forecast cone with track line for Hurricane Dean on August 18, 2007 (Advisory 22).  Hurricane Dean is located at 15.7N and 68.6 W or south southeast of the Dominican Republic.  Parts of Hispañola are under Tropical Storm Warnings while the eastern peninsula is under a Hurricane Warning.  All of Jamaica is also covered by a Hurricane Warning.  Farther in the future, the southeast portion of Cuba is under a Tropical Storm Watch and The Cayman Islands are under a Hurricane Watch.

Track Forecast Cone graphic with the official track line

Wind Speed Probabilities

This product shows the probabilities of certain wind speeds impacting a specific location during the specified forecast period. There are three surface wind speed probability graphics:

  • Tropical storm-force winds and greater (>39 mph/34 kt)
  • 50 knot winds and greater (>58 mph)
  • Hurricane-force winds and greater (>74 mph/64 kt)

These products are issued every six hours at 5 AM/PM and 11 AM/PM EDT or 2 AM/PM and 8 AM/PM PDT (1 hour earlier during standard time).

In the following three graphics, Hurricane Ike is located over the central Gulf of Mexico (noted with a white dot). In the first graphic, the probabilities of locations within each color range having tropical storm-force winds are depicted. For instance, Lake Pontchartrain, LA (indicated by "X") has a 30-40% chance of seeing tropical storm force winds or greater in the next 120 hours or 5 days. The next two graphics are similar except they show chances of 50 knot wind speeds or greater and hurricane-force winds or greater. It is important to note that these graphics also include wind speeds greater than just the speed listed on them.

Tropical storm wind speed probabilities graphic from the National Hurricane Center for Advisory #42 of Hurricane Ike on September 11, 2008

Tropical storm-force wind probabilities for Advisory #42 of Hurricane Ike (2008)

50 knot wind speed probabilities graphic from the National Hurricane Center for Advisory #42 of Hurricane Ike on September 11, 2008

58 mph/50 kt wind probabilities for Advisory #42 of Hurricane Ike (2008)

64 knot wind speed probabilities graphic from the National Hurricane Center for Advisory #42 of Hurricane Ike on September 11, 2008

Hurricane-force wind probabilities for Advisory #42 of Hurricane Ike (2008)

Storm Surge Probabilities

These products provide users with storm surge information to help with local preparedness decisions. Two types of graphics are available: a storm surge probability product and an exceedance height product. These products are issued whenever a U.S. hurricane watch or warning is active on a tropical cyclone, typically up to 48 hours before impacts will start along the coast. The data in these graphics come from an ensemble of Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model runs using the NHC official advisory and accounts for track, size, and intensity errors based on historical errors.

The storm surge probability product (Hurricane Ike example, below) gives the chance of exceeding a certain storm surge height at individual locations, expressed as a percentage and available in 1-foot increments between 2 and 25 ft. In the graphic, different colors show the probability of 8 foot and greater storm surge heights. So, for example, there is a 90-100% chance of exceeding 8 foot storm surge height in the purple area southeast of Beaumont, TX. A large portion of Galveston Bay has a 60-70% chance (orange) of exceeding 8 foot storm surge heights as well. There are many other storm surge heights that emergency managers can look at, so be sure to select the proper height for your specific situation.

Storm surge probabilities graphic with a greater than 8 foot threshold of storm surge plotted from the NHC Advisory for Hurricane Ike on September 12, 2008

Storm surge probabilities graphic with a greater than 8-foot threshold of storm surge plotted from NHC Advisory #45 for Hurricane Ike on September 12, 2008. The highest storm surge probability is east of Port Arthur, TX at between 90 and 100%. Concentric rings of probability surround this one area with decreasing percentages by 10% intervals.

This exceedance height graphic (shown below) shows storm surge heights by probability of exceedance, which is a different way of visualizing storm surge risk. The product gives the height that has a certain probability of being exceeded at individual locations, expressed in feet and available in 10% increments between 10 and 90%. In the 10% probability of exceedance graphic below, there is a 10% chance of more than 21-23 feet of storm surge (orange colors) over a significant portion of Galveston Bay.

Storm surge probabilities graphic with a ten percent chance of exceedance plotted from the NHC Advisory for Hurricane Ike on September 12, 2008

Storm surge probabilities graphic with a 10% chance of exceedance plotted from NHC Advisory #45 for Hurricane Ike on September 12, 2008. The highest storm surge height is in the northwest corner of Galveston Bay at between 27 and 29 feet above the normal tide line. All the way to Port Arthur, Texas, there is a 10% chance of 23-25 feet of storm surge above the normal tide line. Concentric rings of lower storm surge heights surround the area, decreasing by two foot increments and extending south to Brownsville, TX, and east to the Alabama/Mississippi border.

Surface Wind Field

This product shows what areas are likely being affected by certain wind speeds in at the current time. There are two surface wind speed colors on this graphic: one for tropical-storm force winds (orange) and one for hurricane-force winds (dark red). The quasi-circular rings around the tropical cyclone center are produced from the Tropical Cyclone Forecast/Advisory maximum radial extent of winds. This product is issued every six hours at 5 AM/PM and 11 AM/PM EDT or 2 AM/PM and 8 AM/PM PDT (1 hour earlier during standard time).

In the graphic below, Hurricane Ike is in the middle of the Gulf of Mexico (white dot), and the black dashed line indicates the historical path of the storm. In addition to the surface wind speed colors, current watches and warnings are also indicated (red and blue). Currently, no locations in the U.S. are experiencing tropical storm-force winds, but the mouth of the Mississippi will likely have them in the near-future based on the extent of wind circles.

Surface wind field map from the National Hurricane Center for Hurricane Ike Advisory #42

NWS Area Forecast Discussion

Each NWS WFO produces an Area Forecast Discussion similar to the Tropical Cyclone Forecast Discussion but for their county warning area. Written for forecasters and emergency managers, this text product contains more detailed information about the local topography, hydrology, and environment than the NHC discussion product.

This product starts with a header containing the product identification number, the issuing office abbreviation, and a three letter code "AFD," which stands for Area Forecast Discussion. After the header, a discussion of the forecast from the NWS forecaster’s perspective is provided, along with links to terms found in the NWS glossary. Five sections are usually included in this discussion: short term (day 1), long term (days 2 through 7), aviation, marine, and watches and warnings.

NWS Area Forecast Discussion

Hurricane Local Statement

Local NWS WFOs produce Hurricane Local Statements to keep their decision makers, public, and media current on present and anticipated storm effects in their area. These contain essential, detailed hurricane or tropical storm information, expanding on the storm’s potential effects on the local area and on any actions declared by local emergency managers. An experimental graphical version was tested by the NWS in the 2009 Hurricane Season and could be available in the future.

This product’s header begins with codes that identify the product. The last three lines identify the product, its source, and valid time and date. It is always a good idea to check the date to make sure you have the most recent local statement.

The statement begins by summarizing the key point of the forecast discussion that follows. After the discussion, the counties/parishes/areas affected are summarized, followed by the most recent information since the last update and a more detailed description of the affected areas. Next are the watches and warnings, followed by precautionary and preparedness actions statements based on the threat posed by each hazard. A sub-section for each hazard (which may include storm surge and storm tides, winds, inland flooding, tornadoes, and local marine impacts) lists the potential impacts of that threat. The last section tells when the next update for this product will be issued.

Hurricane Local Statement

Watches/Warnings

Watches and warnings are key tools to inform the public and others about dangerous conditions that may occur in the next 48 hours.

Hurricane Watches and Warnings

Hurricane watches are issued by the NHC when hurricane-force winds (74 mph/64 kt or greater) are possible within a specified area and announced 48 hours in advance of the onset of tropical storm-force winds. Hurricane warnings are issued when hurricane-force winds are expected within a specified area and are announced 36 hours before the onset of tropical storm-force winds.

Note the two main differences between a watch and a warning: the time that will elapse before conditions occur (48 hours vs 36 hours) and forecaster’s confidence (possible vs. expected) that the conditions will occur.

As shown in the graphics below, hurricane watches and warnings are issued for areas extending beyond the forecast cone to account for forecast track, intensity and size uncertainties. Not every location under a hurricane watch or warning will experience hurricane-force winds. Only coastal watches and warnings are displayed on this product as of the publication date of this module.

Track forecast cone with track line for Hurricane Ike on September 11, 2008 (Advisory 42)

Hurricane Warning

Track forecast cone with track line for Hurricane Ike on September 10, 2008 (Advisory 39A)

Hurricane Watch

Tropical Storm Watches and Warnings

Tropical storm watches are issued by the NHC when tropical storm-force winds (39-73 mph/34-63 kt) are possible within a specified area within 48 hours, and tropical storm warnings are issued when tropical storm-force winds are expected within a specified area within 36 hours.

Note the two differences between a watch and a warning: time before conditions occur and forecaster confidence (possible vs. expected) in the conditions occurring.

Similar to hurricane watches and warnings, tropical storm watches and warnings cover a larger area than the forecast cone to account for uncertainties. Not every location under a tropical storm watch or warning will experience tropical storm- force winds. Only coastal watches and warnings are displayed on this product as of the publication date of this module.

Track forecast cone with track line for Tropical Storm Erin on August 15, 2007 and tropical storm warning area

Tropical Storm Warning

Track forecast cone with track line for Tropical Depression #5 on August 14, 2007 (Advisory 1) and tropical storm watch area

Tropical Storm Watch

Questions

Question 2

Based on the graph below, if your location is 100 nm from where the 24-hour forecast indicates a hurricane will make landfall, you can expect no impacts. (True or False)
The National Hurricane Center Official Annual Average Track Errors for Atlantic Basin Tropical Cyclones

The correct answer is False.

It is true that at 24 hours from landfall, the forecast accuracy is about 50 miles, so it is unlikely the center of the hurricane will strike your area. This does not mean, however, that you will necessarily escape all impacts. You may still experience hurricane or tropical storm force winds, high waves, flooding from heavy rain, and even perhaps a tornado.

Question 5

Numerical models are a complement to human forecasters, and not a stand-alone way to generate a public forecast. (True or False)

The correct answer is True.

Numerical models are an extremely valuable tool for forecasters, but they are used in conjunction with observations, historical information, and human expertise.

Question 7

A tropical storm watch means that winds of 74 mph or greater are possible in the next 48 hours. (True or False)

The correct answer is False.

A tropical storm force watch is issued when tropical storm force winds (39-73 mph/ 34-63 kt) are possible in the next 48 hours.

Question 8

A hurricane watch is usually issued before a hurricane warning. (True or False)

The correct answer is True.

Hurricane watches are issued when hurricane force winds are possible in the next 48 hours, while hurricane warnings are issued when those winds are expected in the next 36 hours.

Question 9

The surface wind speed probabilities products shows probabilities for wind gusts of the specified speed. (True or False)

The correct answer is False.

Surface wind speed probability graphics show probabilities of sustained (1-minute-averaged) wind speeds, NOT wind gusts.