Severe Storms, and a Possible Derecho, Threaten the Northern U.S. Plains

New York Times

3 days ago

Severe thunderstorms were expected Monday over parts of the Northern Plains as forecasters warned of hurricane-force winds associated with a likely derecho — a fast-moving, long-lasting line of violent storms capable of causing widespread damage.
The most intense weather was forecast for central and eastern South Dakota and southwestern Minnesota, where the Storm Prediction Center issued a Level 4 out of 5 risk for severe weather through Tuesday.
Damaging straight-line winds were expected to be the primary threat, with gusts potentially exceeding 75 miles per hour, strong enough to cause serious structural damage.
'It's not just a routine severe thunderstorm. It's much more intense than that and potentially more widespread,' said John Hart, a meteorologist at the Storm Prediction Center. He warned of potential damage to 'roofs, trees and significant structural problems.'
Forecasters also said that tornadoes could develop in the region along with pingpong ball-sized hail on Monday in some areas.
The severe weather is being driven by a surge of hot, humid air across the region, where heat advisories and extreme heat warnings are in place. The heat index — which takes into account both heat and humidity to measure what conditions actually feel like to the human body — is expected to reach 110 degrees Fahrenheit in some areas on Monday, which could help trigger thunderstorms.
Storms had already begun over parts of Montana early Monday and were expected to quickly organize into a powerful storm complex as they raced eastward across the Dakotas.
'By midafternoon we expect significant severe thunderstorms to have developed in the South Dakota region,' Mr. Hart said. 'Whatever forms will rapidly spread eastward across the state and into parts of Minnesota during the evening, producing the potential at least for widespread significant wind damage, some hail and a few tornadoes.'
Forecasters said the potential for a derecho exists, but that classification would come only after the weather event, depending on the wind strength and the size of the area affected.
Flash flooding was also a concern, with the Weather Prediction Center placing central and eastern South Dakota, southern Minnesota and northern Iowa under a Level 2 out of 4 risk for excessive rainfall.
Although the storms were expected to move quickly, certain areas could still experience two inches of rain or more. The greatest risk of flash floods was in eastern South Dakota and southwestern Minnesota, where the ground was already saturated from heavy rain on Sunday night.
As the storms move east Monday night into Iowa and Wisconsin, the threat of wind damage will continue, although forecasters expect the system to begin weakening as the night progresses.
'The storms will weaken after midnight,' Mr. Hart said. 'Then as we go into Tuesday, the threat appears to diminish.'
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