Understanding the Basics of Flash Flooding and its Dangers

This is the time of the year when the threat of flash flooding tends to ramp up across the U.S. There are a number of reasons why this is when you are most likely to be caught up in a flash flood warning. Here is a look at this force of Mother Nature and why the late spring and summer are the most common times of the year to experience its wrath.

Understanding the Risk of Flash Flooding

Flash flooding does not generally garner the headlines that tornadoes and hurricanes do, however, this does not mean that it should not be taken seriously. Floods of this magnitude are generally acute events that happen when torrential rain falls over a small zone in a short amount of time. This differs from a longer-duration river flooding event that takes time to develop.

The peak of flash flooding concerns is typically in the late spring months and through the summer. According to the Weather Prediction Center (WPC) out of the National Oceanic and Atmospheric Administration (NOAA), about 75% of all flash flood reports happen between late April through the middle of September.

NOAA also warns that flooding is the biggest storm-related killer in the U.S. The agency said that there was an average of 103 deaths blamed on flooding between the years 2014 and 2023. This statistic is more than twice the number of deaths blamed on tornadoes in this country.

Reasons Why This is the Most Dangerous Time of the Year for Flash Flooding

There are a variety of reasons that make the latter part of the spring season and summer the peak time for flash flooding events. The most obvious reason is that this is when temperatures and humidity levels begin to climb in tandem. The moisture-rich air is more likely to produce heavy bands of rain. Regardless of whether the rain falls in smaller storm cells or as the result of larger weather systems, the increased rainfall potential naturally increases the threat of flash flooding.

The position of the jet stream also influences the odds of flash flooding. This is the point in the year when the jet stream tends to lift to the north. The movement causes thunderstorms to move at a slower pace, amplifying the amount of rain that may train over one specific area for a longer period of time.

The late spring and summer months are also when mesoscale convective systems (MCS) are the most prevalent. These large complexes of storms tend to move at a slower speed, raising the risk of flash flooding across the nation's heartland.

Although flash flooding is most generally associated with the central and eastern portions of the country, it can also happen in the Desert Southwest. The annual North American monsoon season is often the culprit for heavy rain in this corner of the country. This season of rainy activity picks up in July and lingers into October. The slowly-moving monsoon storms send copious amounts of moisture across dry arroyos, raising the threat of urban flooding in cities such as Phoenix and Las Vegas.

Lastly, the start of the Atlantic hurricane season also serves as a conduit for heavy rain. Just last year, Hurricane Helene and its torrential rainfall were responsible for 95 deaths at the hands of flooding. Most of the deaths happened well away from the point of landfall when the heaviest bands of rain hit western North Carolina.