Monthly Archives: September 2014

Illinois and Global Temperatures for June-August

Based on the latest updates from the National Climatic Data Center, this summer in Illinois was the 29th coolest. Daytime highs were much cooler than average while the nighttime lows were near-average.

The average maximum temperature in Illinois was 82.3 degrees Fahrenheit, 2.6 degrees below the 1900-2000 base period and the 14th coolest on record.


The average minimum temperature in Illinois was 62.3 degrees F, 0.3 degrees above the 1900-2000 based period.



In contrast to the cool summer in the central US, most of the globe was warmer than average this summer (June-August), according to the National Climatic Data Center. In fact, even in the US, temperatures were significantly warmer in the West than the East.


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When Is Fall? Astronomical, Meteorological, Climatological?


The Quad – University of Illinois in fall color.

No doubt today (September 22) will be announced as the “first day of fall” because of the fall or autumnal equinox. However, that concept refers to the date when we get equal amounts of daylight and dark. ¬†I don’t think it was ever intended that this astronomical event would be the start of fall. In fact, this equinox would be the start of spring in the southern hemisphere. So to be fair to everyone we should call it the September equinox and leave fall out of it. ¬†ūüėČ

Climatologists and meteorologists prefer to use calendar months to define the four seasons in the US. For example, fall would start September 1 and end on November 30. Not only is this more convenient, because¬†you can use monthly data, but it lines up better with the typical or average temperature pattern for Illinois. Unfortunately, the meteorologists would describe this three-month period as “meteorological fall”. However, I would argue it is “climatological fall” since we are looking at long-term average to determine the season.

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NWS Outlook – Wet October, Warmer Fall for Illinois

There is an increased chance of above-average precipitation for the northern half of Illinois in October and an increased chance of above-average temperatures in October-November-December for all of Illinois, according to the latest outlooks from the NOAA Climate Prediction Center.

October Precipitation

The first map shows the precipitation outlook for October. Wetter-than-average conditions are expected in much of the northern Corn Belt as well. The rest of Illinois is in equal chances (EC) of above, below, and near-average precipitation. All of Illinois has equal chances of above, below, and near-average temperatures. Another way to think of “EC” is that the odds are evenly divided among the three categories.


Click to enlarge.

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Cool, Wet Start to September in Illinois

No surprise Рthe first half of September was very wet across much of Illinois. And it is raining across central Illinois as I write this.  Temperatures for the first half of September are 3 to 4 degrees below average in Illinois.

The first map show the accumulated precipitation for the Midwest. The largest precipitation totals occurred in western Illinois, southern Iowa, western and northern Missouri, as well as parts of eastern Nebraska and eastern Kansas. A second area of heavy precipitation occurred in northern Wisconsin and northern Michigan. For agriculture, having this much rain at the end of the growing season is probably not welcome.

The second map shows the temperature departures from average across the Midwest. For Illinois, the coolest departures from average occurred in western Illinois – no surprise due to all the rain and cloudiness there. Temperatures were cooler-than-average across the Corn Belt but the departures became increasingly stronger when moving westward and northward.


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Frost in Illinois – Not Quite Yet

While cold air moved in behind the low-pressure system that passed through the Midwest this week, it looks like Illinois will likely dodge the freezing temperatures for now. But someday they will come. Here are the median dates for when we see the temperatures drop to 32 or below in fall, as well as the earliest date in 1 out of 10 years.

In general, we hit 32 degrees in early October for northern Illinois and mid October for central and southern Illinois. In 1 year in 10, we have seen 32 degrees in the last week of September (second map) or the first few days of October. See more maps and discussion of frost on the State Climatologist web site.


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