Why 'High Risk' Flood Forecasts Are Important | Weather.com (2024)

When a meteorologist mentions a "high risk" of flooding rain like we are seeing with Debby, chances are the flooding will be deadly and destructive, not simply a day of soaking rain.

One-third of all U.S. flood deaths from 2010-2022 occurred on days with a high risk forecast, according to research by Alex Lamers and Ashton Robinson-Cook, meteorologists at NOAA's Weather Prediction Center.

Put another way, 46% of "high risk" flood days had at least one flood fatality or injury, according to NOAA/WPC.

Furthermore, 80% of all flood-related damage occurs in and near these high risk areas. Lamers and Robinson-Cook found 62% of high risk days had at least $1 million in flood damage. In that same 13-year period, they found 186 high risk days accounted for $72.8 billion in total damage.

They also found high risk flood forecasts issued one or two days ahead of the event, rather than the day of the event, have produced even more damage and fatalities.

W​hat Is A High Risk Forecast?

Just as a high risk of severe thunderstorms in a severe thunderstorm outlook from NOAA's Storm Prediction Center indicates higher confidence of a major event such as a tornado outbreak, a high risk excessive rain outlook points out those scenarios that aren't your typical local flash flood event.

A more typical soaking rain may trigger minor flooding in areas that typically see runoff, such as urban, poor-drainage areas, roads and small streams. These are typically covered by "marginal" or "slight" risks in the WPC outlook.

When a high risk is issued, the WPC expects severe, widespread flash flooding, including areas that don't normally experience flash flooding.

Some of the areas that could flood during a high risk day include parts of your commute to work, a shopping center parking lot, the street outside your front door, your yard, even your basem*nt or home. You may see areas you've never seen flood before take on water if your area is covered by a "high risk" outlook.

W​here These High Risks Are Most Often Issued

High risks may be issued in advance of tropical storms and hurricanes with prolific rainfall such as Harvey and Florence. Because of this, since 2016, they've most often been issued near or along the Gulf Coast from Texas to Louisiana and Mississippi.

But there are other hot spots for high risks including much of the East from the New York City tri-state south, the middle Mississippi Valley, the southern Plains, and Southern California.

As was the case in March 2023,​ they can cover parts of California when a strong atmospheric river of moisture surges into the state, particularly over the state's coastal ranges, foothills and mountains.

-Late-May 2019 Kansas and Oklahoma flood

-Late-February 2019 Tennessee Valley flood

-Hurricane Michael's flooding rain in Virginia and North Carolina (October 2018)

-Hurricane Florence's record rain (September 2018)

-​Remnant of Hurricane Hilary in California and Nevada (August 2023)

W​hat You Should Do

What should you do if you're in a high risk for flooding rain?

F​irst, make sure you have multiple ways of receiving warnings from the National Weather Service, including via smartphone and NOAA weather radio, which can also wake you at night if a warning is issued.

Pay close attention to these NWS flash flood warnings and particularly flash flood emergencies, which are issued when life-threatening flooding is occurring.

G​iven the higher likelihood of flooding, avoid unnecessary travel on these high risk days.

If you must travel, never drive through flooded roads. Roughly two-thirds of all flash flood deaths occur in vehicles swept away or submerged in floodwater.

Keep off bridges over fast-moving water and stay away from fast-flowing creeks or rivers.

I​f you live near a hillside, area burned recently by a wildfire, or near a small creek or river, be prepared to evacuate or move to higher ground immediately if told to do so by local emergency management.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He completed a Bachelor's degree in physics at the University of Wisconsin-Madison, then a Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads, Facebook and Bluesky.