Influences
Risk from flooding depends on a number of factors:
- Speed of the storm
- Size of the storm
- Persistent rain bands
- Interactions with other weather systems, such as cold fronts
- Terrain
- Ground saturation
Rainfall is generally greatest with slow moving storms (less than 10 mph or 16 kph). To estimate the total rainfall in inches, one rule of thumb is to divide 100 by the forward speed of the hurricane in miles per hour (100/forward speed = estimated inches of rain). Your local Weather Forecast Office (WFO) may have a more accurate estimation method for your area.
So, while some tropical storm hazards like tornadoes and high winds generally become less of a threat as a storm moves farther inland (although there are exceptions), heavy rains frequently continue and may even intensify as tropical cyclones merge with other storm systems. In mountainous regions, the remaining storm can be forced up the terrain. As the air rises, it cools, and even more moisture condenses into rain drops, adding to the precipitation amounts. And as the rain falls, mountain streams quickly fill, and flash floods become the major threat along with mudslides on the steep, saturated slopes.
House damaged by debris flow from the Peek’s Creek flood
Hurricane Ivan (2004) is another example of a storm that produced extensive inland flooding, especially as it encountered higher terrain. As the storm interacted with the topography of the Appalachian Mountains, Ivan produced extensive and dangerous flooding in many areas of the Southeast, with some locations in North Carolina receiving more rainfall than fell on the coast as the storm made landfall. Western North Carolina had been saturated just a week and a half earlier from the remnants of Hurricane Frances. The resulting floods, mudslides, and landslides caused major damage and debris that swept down Peek’s Creek, NC, carried off 15 houses and killed 5 people.
