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Overstated Fargo flood predictions a head scratcher for National Weather Service

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Weather forecasters never offer guarantees, but the National Weather Service is taking a look at the reasons for an overstated spring flooding forecast for Fargo.

Forum Communications reports that NWS probability-based forecasts prompted volunteers to build 1.5 million new sandbags, none of which got wet in Fargo and only 100,000 of which were even placed.

“We’re kinda scratching our head,” said Steve Buan, a hydrologist with the weather service’s River Forecast Center in Chanhassen.

In mid-April, forecasters warned there was a 40 percent chance of the Red River topping 2009’s record 40.84-foot crest. Two weeks later, the flood fizzled into a non-event with a preliminary crest of 33.32 feet.

Why was the NWS forecast so far off? As March snow accumulated, Buan said, the weather service’s models didn’t know how soils would respond. The models predicted that between 60 and 75 percent of the snowpack would run off into streams, creating conditions similar to the 2006 flood. But the snow took an extra month to melt, and dry soils opened up like never before.

Models run by the U.S. Army Corps of Engineers showed “similar results,” said Alex Nelson, a hydrologic engineer for the corps.

Fargo Mayor Dennis Walaker said he believes the weather service is still living down the flood of 1997, when they incorrectly predicted levels in Grand Forks, and the city’s levees failed.

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