Outlook for Summer in Illinois – Warm, Humid, Chance of Thunderstorms

Okay, I’m just having a little fun with the headline – every summer in Illinois is warm and humid with a chance of thunderstorms. I wouldn’t have it any other way.

The resort to humor occurred because there was not much exciting to report from the latest NWS monthly and seasonal forecasts released yesterday. For June, Illinois has equal chances of above, below, and near-normal temperature and precipitation. For summer (June-August), Illinois has a slightly increased chance of above-normal temperatures, and equal chances on above, below, and near-normal precipitation. Or to put it another way, there are no strong indications of anything out of the ordinary for this summer.


Summer Temperature and Humidity

Here is how I think summer will likely play out – plenty of humidity, daytime highs near-normal, and nighttime lows above-normal. Right now we are coming off a very wet April and May, resulting in an abundance of soil moisture that will be recycled into the air this summer as higher humidity.

These wetter conditions will also impact summer temperatures. In wetter summers, more of the sun’s energy is used to drive evaporation from the ground and transpiration from the crops and vegetation. That’s less energy being used to heat up the land surface. As a result, we get lower daytime temperatures and more humidity. The reverse is true as well – we usually see 100-degree heat only when the ground is dry (think drought) and more of the sun’s energy goes towards heating up the ground.

However, the higher humidity levels do not allow the temperatures to drop as much at night. You can feel the difference if you have ever camped out West where the dry desert air allows the temperatures to drop quite a bit at night. One forecasting tool for predicting the night-time temperature is to look at the dew-point temperature. If the dewpoint temperature is 60 degrees, you have a chance of getting that cool at night. If your dewpoint temperature is running 75 degrees, it means the nighttime temperature will likely be much warmer.

Okay, why the long explanation? We can actually see this pattern playing out in the historical summer temperatures in Illinois. Our spring and summer precipitation in Illinois has increased by about 15% over the last century. As a result of the wetter conditions, our daytime high temperatures in summer have not increased after the spike in the 1930s (first graph). However, our nighttime temperatures have been increased by almost 2 degrees in the last century (second graph). In addition, 2010, 2016, and 2011 hold the top three positions with the warmest summer nighttime temperatures.

Summer High Temperature
Summer Low Temperature

Summer Rainfall

Summer rainfall in Illinois is notoriously hard to predict. Much of our rain in summer comes from thunderstorms driven by local conditions or short-term weather patterns. As a result, some parts of the state can be struggling with drought while other parts are struggling with too much rain. Even in the 2012 drought, we had a few spots in Illinois that were wet.

Having said that, we have seen that summer rainfall has increased historically by about 15% with a lot of variability from one year to the next. The last three summers have been well above normal on rainfall. So that tips the scales towards a wetter summer with a smaller chance of widespread drought at the state level. Your mileage may vary.


Cool, Wet Start to May in Illinois

May is starting out cool across Illinois. Temperatures are running 2 to 4 degrees below normal in parts of western and southern Illinois, to over 9 degrees below normal in the Chicago area (first map). The statewide average temperature for May so far is 53.4 degrees, 5.2 degrees below normal. This comes on the heels of a very warm April.

Parts of Illinois have been wet as well. Rainfall totals of 2 to 4 inches were common in the southern half of Illinois while the northern half has been drier with many areas at 1 to 2 inches and a few areas in northwestern IL with less than an inch of rain (second map).



Here are the potential rainfall totals over the next 7 days according to one long-range NWS model. As a rule of thumb, normal 7-day rainfall in May in Illinois is just over an inch. Amounts of less than an inch are forecasted over much of northern and central Illinois, making it below normal. Amounts of 1 to 2 inches are possible in parts of southern Illinois, making it near to above normal. Most of that rain is expected in the next two days as a system moves through the Midwest.


The map is courtesy of the College of DuPage’s Next Generation Weather Lab.


Rainfall Over the Last 2 Weeks in Illinois


Here are the amounts for the last week, as of this morning. The areas in purple in far southern Illinois have received between 10 and 15 inches. Areas in shades of red have received between 5 and 10 inches. The areas in orange and yellow have received between 2 and 5 inches. Areas in green in northwest IL have received between  0.5 and 2 inches.



Here are the rainfall totals for the past two weeks. Same color scheme as before. Notice how the areas with 10 to 15 inches have expanded across much of southern IL south of Interstate 64 with a few areas just to the north.


NWS Cooperative Observer Network rainfall totals in Illinois for the period of April 26 to May 5, 2017, ranked from high to low. Totals that have exceeded the expected 10-day, 100-year rainfall amounts for that area are in red. Continue reading

Wet April and 2nd Warmest January-April on Record for Illinois

Precipitation: The statewide average precipitation for April in Illinois was 7.01 inches, 3.23 inches above normal and the 2nd wettest April on record. Here are the top five wettest Aprils. Notice a pattern? Three out of the five have been since 2011.

  1. April 2011 – 7.62 inches
  2. April 2017 – 7.01 inches
  3. April 1957 – 6.99 inches
  4. April 2013 – 6.93 inches
  5. April 1927 – 6.87 inches

The largest monthly total for April in Illinois was Carbondale with 14.41 inches. Several other sites in southern Illinois had similar amounts including Bush (Williamson County) with 13.63 inches, West Frankfort (Franklin County) with 13.35 inches, Kaskaskia (Randolph County) with 13.34 inches, and Murphysboro (Jackson County) with 13.02 inches.

Temperature: The statewide average temperature for April was 56.4 degrees, 3.8 degrees above average and the 11th warmest April on record. The warmest reading for April was 89 degrees at Kaskaskia on April 20.  The coldest reading for the month was 19 degrees at Morrison on April 10.

Statewide records of temperature and precipitation go back to 1895.


Here are the temperature departures for 2017. As you can see, every month in 2017 has been well above normal. So far this January-April is the 2nd warmest such period on record with a statewide average temperature of 43.2 degrees, 5.4 degrees above normal. Only 2012 was warmer at 44.5 degrees, 6.7 degrees above normal. [corrected from an earlier version]


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Heavy Rains Over the Last 7 Days in Illinois

As predicted, heavy rains have fallen across much of Illinois in the last 7 days. The first round came on Wednesday and Thursday of last week, followed by another round starting Friday night and tapering off Sunday night.

The largest total for the last 7 days, as of Monday morning, was 12.03 inches at Carbondale. Five other stations reported amounts in the 8-inch range: Kaskaskia with 8.96 inches, Steeleville with 8.66 inches, Murphysboro with 8.60 inches, Chester with 8.23 inches, and Flora with 8.15 inches. A more extensive list is at the bottom of this post.

Here is the radar/rain gage product for the last 7 days. As confirmed by the rainfall reports, the heaviest rains fell in southern Illinois. The area around Carbondale received between 10 and 15 inches. Areas in the shades of red were between 5 and 10 inches. Meanwhile, the northern half of Illinois got off a little easier with amounts of 2 to 4 inches.


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Widespread, heavy rains possible over next week in Illinois

As of April 25, the statewide average precipitation for Illinois is 2.8 inches, which is 94% of normal. However, we have several opportunities for widespread rains this week and into the weekend, according to the NWS precipitation forecast.

The first round of rain on Wednesday and Thursday has potential rainfall amounts of 1 to 2 inches across most of Illinois, along with the chance for severe weather. Continue reading

April Showers Bring May Flowers?

The first part of April has been warmer and wetter than average across much of Illinois and the Midwest. As these maps show, there is a wide band of 1 to 3 inches of rain stretching from Kansas, through Missouri, Indiana, Michigan, and Ohio (click to enlarge) in April. These rains have helped ease concerns of any developing drought after the dry winter. In fact, the concerns have reversed and center around delays in field work from wet conditions. Continue reading