Severe weather outbreak with possible tornadoes over north-central U.S. Monday

WEATHER NEWS: Severe weather outbreak with possible tornadoes over north-central U.S. Monday

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Sunday featured six-inch hailstones bombarding central Nebraska, but violent storms on Memorial Day may be far more widespread in the Plains and Upper Midwest.

The National Weather Service’s Storm Prediction Center has declared up to a Level 4 out of 5 risk for dangerous storms between northeastern Kansas and Minnesota on Monday.

“Large to giant hail, 60-80 mph gusts, and tornadoes are probable, including the possibility for a couple strong/long-tracked tornadoes,” the center wrote.

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Monday’s severe weather setup is a “rare” one, according to the National Weather Service office serving the area around Minneapolis. “We ask that people pay extra attention to the forecast, and make sure they can seek shelter if warnings are issued,” it said.

While violent storms are anticipated in some areas, there are some off-ramps that could mean others are spared.

The Level 4 risk of severe weather encompasses much of western Minnesota and eastern parts of the Dakotas east of the James River Valley and along Interstate 29. Nearly 700,000 people reside within that risk area, including residents of Watertown and Brookings, S.D., and Willmar and Fergus Falls, Minn.

A Level 3 risk stretches from northern Minnesota along the Canadian border all the way south into northeastern Kansas.

Minneapolis and Sioux Falls, S.D., are all included in the Level 3 category, as are areas just west of the Kansas City area. A Level 2 risk reaches toward Wichita.

An intensifying surface low-pressure zone from western Minnesota to northeastern Kansas will shift north and east with time, swirling thunderstorms north and east into the Upper Midwest.

A cold front and dry line — or the boundary between warm, humid Gulf of Mexico air to the east and cooler, drier air to the west — will trail south of the low. They will be the impetus, or triggering mechanism, for severe thunderstorms.

Simultaneously, the jet stream will be racing overhead, with the resulting change in wind speed and/or direction with altitude set to foster rotation within storms.

The combination of ingredients in place is favorable for rotating thunderstorms, or supercells, with tornadoes. However, that’s predicated on discrete supercells forming and remaining isolated from neighbors.

While available instability and spin could support long-lived, significant tornadoes, it is uncertain whether they’ll materialize. If thunderstorms develop too quickly or become too widespread, they’ll interfere and reduce each other’s severity.

Still, large hail, destructive straight-line winds and tornadoes are all possible in storms that form. They’ll race off to the north-northeast at highway speeds, offering little time to seek shelter as they approach.

Some models show the potential for a few supercells as far south as the Kansas City area, which could bring a localized tornado threat.

There is also the chance that warm air doesn’t make it as far north into Minnesota as some models suggest, which would mitigate tornado threat some.

It’s crucial for people in the risk area to remain abreast of changing weather. They should have a way to be notified of watches and especially warnings, and make sure to never be more than a stone’s throw from a storm shelter as Memorial Day is observed.

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