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The atmosphere over the American heartland is coiling like a spring under pressure — and meteorologists say it is about to snap. A prolonged, multi-day severe weather outbreak is taking shape across the Plains and upper Midwest, with NOAA's Storm Prediction Center warning of strong tornadoes, large to giant hail, and damaging winds beginning Sunday and potentially continuing through the middle of next week across some of the most tornado-prone geography on Earth.

What makes this outbreak particularly striking is its timing. May is statistically the most tornado-active month in the United States. Yet through the first two weeks of this May, the tornado map has been conspicuously, almost eerily blank — a product of an inverted weather pattern that kept the central and eastern US cooler than average while the West baked. Only eight confirmed tornadoes — all from supercell thunderstorms on May 6 in southern Mississippi — have marked the month so far.

That quiet, forecasters warn, is about to end.

Saturday: The Opening Salvo

The outbreak's opening act arrives Saturday, as scattered supercell thunderstorms fire up near a convergence of volatile atmospheric ingredients: a frontal boundary, a dryline, and an area of low pressure — a meteorological trifecta that can transform ordinary storm cells into rotating, destructive monsters.

Large to very large hail, wind gusts exceeding 75 mph, and a few strong tornadoes are all in play Saturday and Saturday night. The cities sitting inside the threat zone read like a roll call of the Midwest's major population centers: Des Moines, Lincoln, Omaha, Chicago, Indianapolis, and Sioux Falls are all in the crosshairs.

The most concentrated danger is mapped in dark orange and red on NOAA's outlook — a corridor where a full-scale outbreak of severe thunderstorms, complete with tornadoes, hail and damaging winds, is considered possible overnight into early Sunday.

Sunday: Rounds of Fury

Sunday raises the stakes. NOAA's Storm Prediction Center has specifically flagged the potential for strong tornadoes by Sunday evening across the Central Plains and upper Midwest — language that forecasters do not deploy lightly.

The severe weather Sunday may arrive in multiple waves rather than a single front, complicating the public safety response. The first round is expected to strike the upper Midwest in the morning hours. A second, potentially more dangerous round is forecast for the Plains through the upper Midwest during the afternoon and into the night — a sequence that could exhaust emergency management resources and keep residents in a state of prolonged alert for hours.

Farther south, Oklahoma sits in a comparatively lower-risk zone, though forecasters caution that any storms that do develop along the dryline in that region could organize into discrete supercells — capable of producing hail, destructive winds, and perhaps an isolated tornado.