Fall Frost in Illinois


Fall frost near my office a few years ago, Jim Angel 2013.

Summary: the median dates for fall frost in Illinois range from early October in northern Illinois, to mid-October for central Illinois, and late October for southern Illinois.
Frost is the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. If a frost period is severe enough to end the growing season or delay its beginning, it is referred to as a “killing frost”.
Frost in both spring and fall can be a concern to farmers, landscapers, and gardeners. However, we usually do not directly measure frost at weather stations in Illinois. Sometimes observers may note the presence of frost in their comments on the forms. To get around the lack of direct observations, we use a temperature threshold of 32° for frost and 28° for a hard freeze.
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September So Far – Warm and Rainy

So far September has been much warmer and somewhat wetter than normal for Illinois. The statewide average temperature for the month is 72.2 degrees, 4.3 degrees above normal. The statewide rainfall is 3.10 inches, 31% above normal for the month to date. Warm and rainy also applies to much of the rest of the Midwest.


Below are maps of the rainfall across the Midwest for September. The left panel is the actual rainfall, the right panel is the departure from normal. Rains have been widespread across the Midwest in September and generally near to above normal across much of MN, IA, MO, WI, IL, and IN. Meanwhile, MI, OH, and KY were mostly below normal. On closer examination, central and southern Illinois were near to above normal while northern Illinois has been a little drier.
The heaviest rains of 5 or more inches have occurred in two blue blobs (left panel): one blog from Kansas through southwestern Illinois; and another blob from NE and SD, eastward through IA, MN, and WI. The heavy rains in IA, MN, and WI may cause minor to moderate flooding in the next several days along the Mississippi River above St. Louis.


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Lake Michigan Warmer than Average in 2016

Lake Michigan has been warmer than average throughout 2016, according to the NOAA Coastwatch site.
Here are what the current surface water temperatures look like across the Great Lakes. Temperatures on the southern end of Lake Michigan are in the 70s and in the low 70s or upper 60s in much of the rest of the lake. There are some spots along the Wisconsin shore that are in the low 60s and 50s.
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