Low pressure in the upper atmosphere has been stuck swirling over the Great Basin during the Thanksgiving holiday, trapped to the south of a corresponding area of high pressure aloft in an atmospheric logjam known to meteorologists as a "Rex block".
As a result, the weather had been rather stagnant and unchangeable, with periods of freezing rain, sleet and snow persisting in the Plains, and locally heavy rain soaking the warm side of Winter Storm Cara.
Now, that blocked-up jet stream pattern is finally giving way.
The upper-level low mentioned above will finally pivot east into the Midwest early in the week ahead. As it does so, moisture in the atmosphere will be lifted, and cold air in place will yield a swath of snow from late Sunday into Tuesday night from the High Plains to the Corn Belt, Upper Mississippi Valley and northern Great Lakes.
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