Tuesday morning saw the first really widespread frost and/or freeze conditions over a good chunk of the Midwest. The graphic from the MRCC shows just how chilly it was. The only section of the Midwest to really escape the chill was most of Missouri. However, that will change by Friday morning as another batch of cold air rides down from Canada on a strong northwest flow aloft. This is all part of the pattern change that occurred last weekend.
Adding to the chill was a little Alberta clipper system that rapidly moved across about the southern 1/2 of the Midwest. The precipitation was in the form of light rain in some spots, and some very early season light snow in others. This link shows the location of the measurable snowfall during Tuesday. Amounts were generally in the 1 to 2 inch range before it quickly melted due to the warm ground and above freezing daytime temperatures.
For the Chicago metro area, the growing season is over. Overnight readings Tuesday morning were in the 20s most areas except for the core of the city. The metro area was on the north edge of the precipitation Tuesday. Amounts ranged from just trace amounts north to 1 to 2 tenths of an inch south. Areas to the south and southwest picked up from 1 to 2 inches of wet snow. NWS Chicago has produced a very nice article concerning the mini snow system.
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