CHAMPAIGN, Ill. — June in Illinois started out particularly warm and ended with above-average rainfall from numerous storms, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.
Overall, the preliminary statewide average June temperature was 73.8 degrees, 1.6 degrees above the 1991-2020 average.
Drier soils in the northern two-thirds of the state helped elevate temperatures in the first half of June. Most of northern and central Illinois experienced temperatures that were between 5 and 15 degrees warmer than normal in the first half of June. However, temperatures in southern Illinois remained near to slightly cooler than normal.
The warm start to June broke 31 daily high maximum temperature records and 15 daily high minimum temperature records. The subsequent cooldown in the third week of June broke 12 daily low minimum temperature records across the state.
Overall, June average temperatures ranged from the low 70s in northern and central Illinois to the mid- to high-70s in southern Illinois, between 1 and 5 degrees warmer than normal across the state
Growing Degree Days
Following a slow start to the growing season from cooler April and May weather, June temperatures helped accumulate growing degree days. As of the start of July, growing degree day accumulation is ahead of normal in northern Illinois and slightly to much below normal in southern Illinois.
Precipitation
The first two-thirds of June were very dry across Illinois. In fact, the period between June 1 and 20 was the sixth and seventh driest on record in the northwest and northeast Illinois climate divisions, respectively.
As the large atmospheric ridge established over the Pacific Northwest, most of Illinois was on the edge of a stationary front that produced several rounds of heavy rain from the St. Louis Metro East to Chicagoland.
Due to the very wet last seven to 10 days of the month, June ended wetter than average in all, but the northwest and southwest climate divisions, and it was the sixth wettest on record in the east-central division.
Rain in the final week of June was particularly heavy along the Interstate 55 corridor between Bloomington-Normal and Chicago. Areas of McLean and Livingston counties observed 8 to 10 inches of rain in just four days, resulting in serious flooding in Bloomington, flooding on and the temporary shutdown of Interstates 55 and 74 and standing water in fields across central and northeast Illinois.
In both south Bloomington and Heyworth in McLean County over 10 inches of rain was observed in just three days between June 25 and 27. These totals exceeded the “100-year” rainfall event by 1.5 inches.
Overall, the preliminary statewide average total June precipitation was 5.33 inches, 0.68 inches above the 1991-2020 average.
With the dry, hot start to June, the U.S. Drought Monitor classified the drought in northeast Illinois as extreme in mid-June in response to very dry soils, low streamflow in major streams and increasingly concerning crop stress.
Persistent heavy rains in the Chicagoland area in late June were crucial to improving drought conditions and alleviating crop and water stress across the region, except in areas north of Interstate 88 and west of Interstate 39 where drought conditions have worsened.
Outlook
Longer-term outlooks for the entire month of July do not show much of a strong signal in either temperature or precipitation. Illinois appears to be on a boundary between warm, dry conditions in the north and northwest and cooler, wetter conditions in the south and southeast.