We're Heading Into The Peak Time For Flash Flooding In The US

Flash flooding has already produced significant impacts in the United States this year, but the height of when much of this threat typically peaks is still ahead from late spring through summer because of a variety of weather factors.

75% of flash flood reports happen from late April to mid-September. In the graph below, the thin bars illustrate the number of daily flood reports from 2007 through 2017, according to NOAA's Weather Prediction Center (WPC). While flash flooding can happen any time of year, reports rose rapidly starting in late April, reaching a summer peak before tailing off into the fall.

To be clear, by flash flooding, we mean only those short-fuse events triggered by heavy rain over a relatively small area, rather than longer-lived river flooding events taking place over days or weeks.

Here's w​hy this peak matters: The danger of flash flooding is sometimes overlooked when compared to the tornadoes, large hail and destructive winds that thunderstorms produce in spring and summer. But it should be taken just as seriously since it's one of the deadliest types of weather phenomena.

An average of 110 people were killed annually by flooding in the U.S. over the past 10 years ending with 2022, according to statistics compiled by the National Weather Service in La Crosse, Wisconsin. Already this year, flooding has contributed to 26 deaths in the U.S. through early May.

Vehicles are a major source of flooding deaths. Your local NWS office issues a flash flood warning when flash flooding is either already happening or expected to develop based on rainfall rates and radar trends.

Regardless of whether or not a warning is in effect at the time, avoid flooded areas.

Most importantly, never attempt to drive through a flooded stretch of road or around barriers that signal a road closure. Over half of deaths in floods happen in vehicles, according to NOAA statistics.

The reason this is so dangerous is that it may be hard to judge how deep the water is, or flooding might be hiding a washed-out section of road. Flooding can also be more difficult to spot at night.

(​IN DEPTH: More On How To Stay Safe In A Flash Flood Threat)

W​hy The Flash Flood Threat Peak Happens

More water vapor can exist in warmer air and that increases the rainfall potential for both individual thunderstorms and larger-scale weather systems.

2. The jet stream hits the brakes. A​nother factor is the jet stream moving slower heading into summer. With generally lighter steering winds aloft by summer, especially in the central and southern U.S., thunderstorms and clusters of storms move more slowly. The slower the movement, the greater the rainfall potential.

3​. Large thunderstorm complexes are more common. Despite the weakened jet stream, often times in late spring and summer individual thunderstorms will congeal into a large mass of storms known to meteorologists as a mesoscale convective system (MCS).

While some MCSs can move rapidly, producing widespread damaging winds, others can move slowly if winds aloft are weak, triggering major flash flooding, particularly in the overnight and morning hours in the nation's midsection.

4. The Southwest monsoon activates. Coupled with intense heating of the mountainous terrain, slow-moving thunderstorms can flare up over the high country of the Southwest and Rockies in summer, then spread over lower elevations.

If the atmospheric moisture is deep enough, the slow movement of these thunderstorms can dump torrential rain, flooding normally dry washes and arroyos and triggering flash flooding in urban areas like Phoenix and Las Vegas. Flash flooding can also happen where it's not raining as water filling washes and arroyos moves away from where the thunderstorm struck.

5​. Hurricanes and tropical storms return. Recent hurricanes like Harvey in 2017, Ida in 2021 and Ian in 2022 illustrate the serious flooding that moisture from tropical cyclones can produce.

But you don't need an intense landfalling hurricane to produce flooding rainfall; All that's required is a slow-moving tropical cyclone, whether it's a depression, storm, hurricane or remnant.

For example, a much weaker tropical cyclone, Allison in June 2001, technically wasn't still a tropical cyclone when it unleashed rain that turned flooding in Houston into a $5 billion disaster.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.