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.

Carbondale was warm in the first half of June, and cool in the second half. However, these warm and cool periods were not as pronounced as in Chicago and Champaign. Days in July were more often warmer than normal, while late July and all of August have been much cooler than normal so far. For Carbondale, June was 0.5 degrees below normal, July was 0.6 degrees above normal, and August so far is 6.2 degrees below normal.

Why are there different responses across the state from the periods of warmer and cooler than normal periods? It is, in part, due to climatology. In the early June heat, temperatures reached into the 90s across the state. However, those temperatures are much more unusual in northern Illinois than southern Illinois. Therefore, the daily departures from normal were much larger in Chicago than Carbondale.

Also, it was wetter in southern Illinois during the first half of June, and likely more cloudiness, which would lead to slightly lower temperatures during that period.

The reverse was true in August. High temperatures were in the 70s and low 80s across the state, which was much more unusual in southern Illinois than northern Illinois.

Temperature Maps

Temperature departures from normal for the first half of June. The heat in northern Illinois was an extension of the heatwave in the upper Midwest and High Plains. The deeper the orange/red, the warmer the temperature departure.


Temperature departures from normal for the first half of August. The deeper the green, the cooler the departure from normal.


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.


Dryness Across Illinois

While heavy rains have fallen along the edges of Illinois, most of the state has received below normal rainfall over the last 30 days. Current soil moisture conditions and the NWS forecasts are reviewed below. In general, areas that missed out on rains in the last 30 days could be in for a rough ride in July.

Here is the map with the percent of normal rainfall for the last 30-days: areas in green and blue are above normal while areas in beige and brown are below normal. Several areas in central and southern Illinois have had less than 50 percent of their normal 4 or so inches.


As a result of the below-normal rainfall, crops and vegetation have had to rely more heavily on soil moisture to grow. This is causing a rapid depletion of soil moisture in the top 8 inches. As those reserves are used up, roots will have to go deeper to tap into soil moisture at lower levels. Corn and soybeans have much deeper root systems than lawn grass and can do fine even after the grass has turned brown.

On a side note, my grass never turned brown in 2015 or 2016 thanks to plenty of rainfall and moderate temperatures. It has been a different story this summer.

Here is what the USDA said about soil moisture in Illinois this week. Statewide topsoil is 3% “very short” and 25% “short. Subsoil moisture is 2% “very short” and 18 “short”.


And here is what our Water Survey soil moisture network says about soil moisture.

4-Inch Soil Moisture: values of 0.3 or greater mean soil moisture is in good shape, values in the 0.20 to 0.29 range are on the dry side, values in the teens are near the wilting point.



8-Inch Soil Moisture. For the most part, a little more soil moisture in this layer than in the 4-inch layer, which makes sense.


20-Inch Soil Moisture. Looks to be in great shape for now. The sites near Havana and Savanna are on sandier soils that do not hold water well (hence the low numbers) and usually require irrigation for crop production.


How Does the Rainfall Forecast Look?

Here is the 5-day precipitation forecast, according to the NWS, with amounts in Illinois ranging from 0.5 to 1.5 inches. These are the potential amounts – your mileage may vary. The 7-day precipitation forecast shows much larger totals but I take less stock in precipitation amounts that far out.

The NWS 6-10 day forecast does show that Illinois has an increased chance of above-normal rainfall (green) during that time …

However, the NWS 8-14 day forecast shows Illinois with an increased chance of having below-normal rainfall (shaded in brown). The good news is that the indications are that Illinois may dodge the really hot weather with highs staying below 95 degrees over the next 14 days.