We are entering the peak season for a frequently underestimated danger in the United States: flash flooding. While this hazard may not receive as much attention as tornadoes and hurricanes, it should be taken seriously, especially as heavy rainfall becomes more common in late spring and summer.
What Is a Flash Flood? Flash floods are sudden events triggered by intense rainfall over a small area, such as a thunderstorm dumping several inches of rain in just a few hours. This is different from prolonged river flooding that occurs over days or weeks.
Peak Season: While flash floods can occur at any time, about 75% of reports between 2007 and 2017 happened from late April to mid-September, according to NOAA’s Weather Prediction Center.
Factors Contributing to Flash Flooding: Several key factors explain the peak in flash flood reports during this time:
1. Abundance of Warm, Humid Air: Warmer air can hold more water vapor, leading to increased rainfall potential in thunderstorms and larger weather systems.
2. Slower Jet Stream: In summer, the jet stream weakens and shifts north, causing lighter winds that slow down thunderstorms and increase rainfall.
3. Thunderstorm Complexes: Mesoscale convective systems can move slowly, leading to major flash flooding, especially overnight in the central U.S.
4. Southwest Monsoon: From July through September, the monsoon increases the risk of flooding in regions like Phoenix and Las Vegas.
5. Hurricane Season: Tropical cyclones can bring heavy rainfall and flooding, as seen in past hurricanes like Harvey and Ida.
Why It Matters and How to Stay Safe: Flooding is a significant cause of storm-related deaths, with an average of 103 fatalities per year. Driving through flooded roads is dangerous and has contributed to a significant number of flooding deaths. It’s crucial to heed flash flood warnings issued by your local National Weather Service office and never underestimate the power of floodwaters. Remember the mantra: “Turn around. Don’t drown.”
If a flash flood emergency is issued, it is crucial to refrain from traveling, especially at night. This signal indicates a severe situation, prompting the need to avoid travel and seek higher ground if located in a flood-prone area. Chris Dolce, a senior digital meteorologist at weather.com for almost 15 years, started his career with The Weather Channel in the early 2000s.
