First-ever flood forecast maps show homes and roads at risk

First-ever flood forecast maps show homes and roads at risk

The National Weather Service has launched the first flood forecasting system with accurate, real-time data showing locations at imminent risk of flooding.

An aerial view of floodwaters from the Mississippi River surrounding homes, trees and covering streets

Floodwaters from the Mississippi River surround homes and cover streets on May 3, 2023, in Rapides, Illinois.

ClimateWire | A new government forecasting system shows for the first time which roads, streets and properties are likely to be flooded by ongoing or upcoming rainstorms, providing unprecedented detail for preparations.

The National Weather Service project offers the first real-time forecast service that shows specific areas such as city blocks that are likely to see at least an inch of flooding over the next five days, with the areas shaded in blue on an online map.

A senior NWS scientist called the forecast system a “revolutionary advance” for providing detailed street-level flood forecasts in real time.

“We’re not just trying to say ‘there’s 10 feet of flooding.’ We’re trying to say ‘your street could flood,'” said Mark Glodimans, chief of the NWS Water Resources Services Branch. “With this five-day forecast, local authorities can “Emergency managers and the public are now making preparations to get ready.”

New NWS experimental flood inundation map
New NWS experimental flood inundation maps help communicate the timing and magnitude of high water events by showing the model’s inundated areas in blue. Emergency managers may use these services to pre-position resources, secure critical infrastructure, and recommend evacuations and evacuation routes. Credit: NOAA

Called the NWS Flood Map, the NWS system covers about 10 percent of U.S. households, and shows parts of eastern Texas, western Pennsylvania and upstate New York. By October 2025, flood forecasts will cover the entire country — including Alaska and Hawaii — with a five-day flood risk reported for all 3.4 million miles of rivers in the U.S., according to the National Weather Service.

The current warnings issued by the NWS — flood surge maps and flood watch watches — are much more limited than the flood maps released in September, Glodemans said. The warnings warn residents over a much wider area of ​​rising water levels in nearby rivers rather than predicting the exact locations facing high flood risks in the coming hours and days.

The new forecast system only forecasts flooding caused by riverine flooding and does not take into account coastal flooding, storm surge or sewer overflow.

“Saying there’s going to be that many feet (of water) above flood level doesn’t mean much” to most residents, said John Nelson Gammon, a Texas state climatologist. Communicating flood risks “in a pictorial way that people understand would be a huge advance.”

The new flood forecasts show how flooding will affect small areas “by putting water on the map,” Glodimans said. He added that local authorities and disaster responders will identify specific areas that require attention when heavy rains raise water levels to dangerous heights, helping them prepare for floods more quickly and efficiently.

The new online forecast also differs from flood maps created by the Federal Emergency Management Agency that show areas most at risk of flooding, Gleudemans said.

Climate scientists say global warming will lead to an increase in severe and frequent storms that inundate neighborhoods and key infrastructure such as roads, bridges and electricity grids. As sea temperatures rise, hurricanes are also increasing in strength and are likely to reach inland areas and drop inches of rain within hours.

The flood forecast combines ground elevation data with models that predict river level rise to identify areas on interactive maps that could be flooded after rain, Glodemans said. Lowlands begin to flood first when rivers overflow. A contour map showing elevation every 30 feet allows the NWS to perform a street-by-street flood projection for certain levels of precipitation.

But the new flood forecasts also have weaknesses and limitations.

National flood forecasts rely on estimates rather than observed data for local factors, affecting their accuracy, Glodemans said. Data showing factors such as river bed shapes and how much precipitation is absorbed by soil and plants are not available for “99% of the country,” Glodimans wrote in an email.

“Collecting this information is expensive and simply not possible for the 3.4 million miles of rivers covered” by the new flood forecast, Glodimans added.

The new forecast does not show the expected flood depth and ignores the chances of levee failure, says an NWS web page that answers frequently asked questions.

If a house or street is shaded blue on a flood map, “it does not necessarily mean you are or will experience flooding,” the web page says. But this means “there is a need for increased attention and awareness.”

“We know from weather forecasts that models can be wrong,” said Nelson Gammon, a climate scientist in Texas. “We’ll really need to see how it performs over a period of time to get a better idea of ​​its accuracy.”

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2023. E&E News provides essential news for energy and environmental professionals.

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