Chilling in Illinois

So far August has been cooler than normal. And that is likely to continue for a while. Here are the temperature departures for August so far. The statewide average temperature for Illinois is running about 2 degrees below the 1981-2010 normal.


As a cold front swept through Illinois yesterday and today, it has brought cooler and less humid air behind it. And according to the National Weather Service, those cooler temperatures will be around awhile. Over the next 7 days, highs will range from the low 70s in northern Illinois to the mid to upper 70s in central Illinois and the upper 70s and low 80s in southern Illinois. That’s about 6 degrees below normal for this time of year.

The cooler than normal weather is expected to continue with the 6-10 and 8-14 day forecasts showing an increased chance of below-normal temperatures in Illinois (image below, click to enlarge). And these cooler-than-normal conditions could last even longer, according to some models.

The likely impacts of this cooler weather include:

  • reduced heat stress in people, especially welcome as school starts around the state,
  • lower cooling bills,
  • reduced stress on crops, pasture, and lawns, even in areas struggling with dry weather,
  • livestock usually benefit from cooler temperatures in late summer with less stress, more weight gain, more milk production, etc.,
  • cooler temperatures in late summer can slow down crop development, which is usually more of a concern in northern Illinois than southern Illinois. You can track the impact of the cooler weather on corn using this U2U tool at the Midwestern Regional Climate Center,
  • average frost dates in Illinois range from early October in northern Illinois to late October in southern Illinois, in case you are wondering.


Temperature Roller Coaster Ride in Illinois this Summer

Here are the daily temperature departures for the summer of 2017 from the long-term daily normals for three sites in Illinois – Chicago, Champaign, and Carbondale.

Chicago experienced an early summer heat wave in the first two weeks of June with temperatures reaching up into the 90s, which was well above normal for that time of year. This was followed by cooler conditions in the second half of June. Periods of cooler and warmer than normal weather alternating throughout July, which is pretty common. August has been running cooler than normal so far. The lesson here is that early season heat waves do not necessarily set the tone for the rest of the summer (thank goodness). For Chicago, June was 3.5 degrees above normal, July was 0.6 degrees above normal, and August so far is 2.5 degrees below normal.


Champaign also experienced heat in the first half of June and a little more pronounced cooling than Chicago in the second half of June. July alternated between relatively warm and cool periods. August has been quite cool so far. For Champaign, June was 0.7 degrees above normal, July was 1.5 degrees above normal, and August so far is 2.9 degrees below normal.
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Cool, Dry August So Far in Illinois

So far, August has been about 4 degrees below normal and dry across the state.

Here are the precipitation maps for the Midwest for August – more rains across the northern and southern most parts of the Midwest. Meanwhile, the rains mostly missed the central Midwest, especially in northern Missouri, eastern Iowa, southwest Wisconsin, and northern Illinois. Click to enlarge.

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Big Change in Weather for August in Illinois

Summary: There is a big shift in the weather pattern coming to Illinois as a strong cold front moves through Illinois on Friday and Saturday. Behind it will be cooler, less humid air. In fact, cooler temperatures are expected to dominate in Illinois over the next few weeks.

For the weekend and next week, daytime highs will range from the 70s in northern Illinois, the 70s and low 80s  in central Illinois, and low 80s in southern Illinois. Overnight lows will range from the mid-50s to low 60s in northern and central Illinois, and in the low to mid-60s in southern Illinois.

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July – Extremely Wet or Dry in Illinois

There is an old statistics joke about “Stick one foot in the fire and one foot in a bucket of ice, and on average you are at a comfortable temperature”. That happened to precipitation in Illinois in July. Northern Illinois experienced heavy rains and widespread flooding. Meanwhile, parts of central and southern Illinois experienced dry weather that caused crop damage. However, on average the statewide precipitation was 4.81 inches, which is 0.73 inches above normal.

Three of the highest monthly totals for July in Illinois were Cary (McHenry County) with 13.34 inches, Gurnee (Lake County) with 13.21 inches, and Chadwick (Carroll County) with 13.15 inches.

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Forecast for August – Warmer than Normal

The NWS Climate Prediction Center just released their new outlooks (map below). For August, they indicated that Illinois has an increased chance of being warmer than normal (upper left panel). The same is true for the 3-month period of August-October (lower left panel). They have nothing to say about precipitation in Illinois – equal chances of above, below, or near-normal precipitation.

The NWS outlooks have pushed the increased chance of above-normal temperatures for 2017 in Illinois. They are doing well – January, February, March, April, and June have been above normal. Only May was slightly cooler than normal. So far in July, the statewide average temperature is 76 degrees, which is normal for this time of year.


Rainfall Extremes in Illinois in Last 30 Days

It is fairly typical of summer in Illinois to have areas with too much rain and other areas with not enough. However, the last 30 days have been extremely variable across the state. In the figure below of radar/rain-gage measured precipitation, we see amounts of 10 to 15 inches in Lake County (pink). In many other areas, we see shades of red which represent amounts between 4 and 10 inches. However, interlaced with those wet areas are areas with only 2 to 4 inches. And there are a few areas, especially around St. Louis that have had only 0.5 to 2 inches of rain (shades of green) in the last 30 days.

As you can imagine, areas in northern Illinois are struggling with flood conditions while areas in green are struggling with drought. There are reports of corn “firing” in Jersey and St. Clair Counties according to the Illinois Farm Bureau’s CropWatchers.


The recent NWS 7-day precipitation forecast doesn’t look promising with most of the rain expected to fall in northern Illinois (1 to 4 inches) where it is not needed. Meanwhile, the southern half of Illinois is expected to receive less than half an inch. We will see how this plays out.