Mild May Wraps Up an Otherwise Warm Spring in Illinois

The preliminary statewide average May temperature was 63.0 degrees, 0.2 degrees below the 1991–2020 average and the 58th warmest on record going back to 1895. The preliminary statewide average total May precipitation was 3.75 inches, 1.02 inches below the 1991–2020 average and the 55th driest on record statewide.    

Data are provisional and may change slightly over time

The first four months of 2025 collectively were just under 1 degree warmer than normal statewide. May temperatures likewise varied within 1 degree of normal statewide, with periods of much warmer and much cooler weather during the month (Figure 1). Daily temperatures and temperature departures from normal in Charleston are shown in Figure 1, illustrating the typical temperature variability in May. Temperatures reached into the upper 80s to low 90s statewide around mid-month before dropping to 10-15 degrees below normal a week later.     

Figure 1. Daily May average temperature departures in Charleston.

May average temperatures ranged from the mid-50s in northern Illinois to the high 60s in southern Illinois, within 1 degree of normal virtually statewide (Figure 2). Several stations saw their first 90+ degree temperatures last month, including a daily record breaking 94 degrees at Chicago’s O’Hare Airport on May 15. Meanwhile, the state was largely spared a late spring freeze in May, with only a handful of stations experiencing temperatures below 40 degrees.

Figure 2. Maps of (left) May average temperature and (right) May average temperature departures from normal.

Overall, the preliminary statewide average May temperature was 63.0 degrees, 0.2 degrees below the 1991–2020 average and the 58th warmest on record.

May capped off a very warm climatological spring season. March temperatures were 4 to 8 degrees above normal and April temperatures were 1 to 3 degrees above normal (Figure 3). Despite the milder May temperatures, the spring season was still a top 10 warmest on record in many places in the Midwest, including in St. Louis, Peoria, and Paducah. In recent years, early spring warmth has pushed spring phenology ahead of normal, increasing the risk of frost or freeze damage to tender perennials and horticultural crops. However, our colder winter this year helped extend dormancy a bit longer, resulting in a timely spring phenology and (hopefully) good fruit and berry crops this year.

Figure 3. Maps of monthly average temperature departures from normal in March, April, and May 2025.

Drier May Leading into Summer

May is climatologically the third wettest month in Illinois, and the wettest in parts of southern Illinois. While last month’s precipitation was typically variable across Illinois, May was overall drier in most places. Specifically, total May precipitation ranged from around 2 inches in the Chicagoland area to nearly 8 inches far southern Illinois. Most areas south of Interstate 70 were near to slightly wetter than normal, while most of northern and central Illinois were 1 to 3 inches drier than normal (Figure 4).

Figure 4. Maps show (left) total May precipitation and (right) May precipitation departures from normal.

Despite somewhat lackluster May precipitation totals, last month was very active on the severe weather front. The NOAA Storm Prediction Center listed 23 tornado reports, 92 severe wind reports, and 61 severe hail reports in Illinois in May. Among these include an EF-4 tornado in Williamson County, and 2-inch hail in Morgan and Sangamon Counties. Illinois set a new statewide tornado record in 2024, but we are ahead of where we were this time last year on statewide tornadoes, with 105 reports total. In fact, Illinois is tied with Missouri with the highest number of tornado reports to date in 2025.

Overall, climatological spring precipitation ranged from around 6 inches in far northern Illinois to over 20 inches in southern Illinois. The northern half of the state was 1 to 2 inches drier than normal, while southern Illinois was 2 to 8 inches wetter than normal (Figure 5). Last season was the 4th wettest on record in Salem and the 5th wettest spring in Centralia.

Figure 5. Maps show (left) total spring precipitation and (right) spring precipitation departures from normal.

June & Summer Outlooks

June is the start of climatological summer and is an exciting month for many reasons. Warm weather is here and schools are out. The Climate Prediction Center’s June outlook shows best chances of summer starting a bit on the warm side, with best chance of near normal precipitation and maybe a continued wetter trend in southern Illinois (Figure 6).

Figure 6. Climate Prediction Center (left) temperature and (right) precipitation outlooks for June.

The summer season outlooks (June–August) also show higher chances of above normal temperatures this summer. For precipitation, Illinois is squeezed between a band of expected drier than normal conditions to the west and wetter conditions to the east. That setup often, but not always, corresponds with active stormy weather in the summer.

Figure 7. Climate Prediction Center (left) temperature and (right) precipitation outlooks for June through August.

Rivers Have No Effect on Tornado Tracks

The Kansas City NWS office posted this image showing that rivers have no measurable effect on tornado tracks.

tornadoes-vs-rivers
Click to enlarge.

The case for St. Louis reminded me that one of the worst tornado disasters in US history occurred when a tornado tracked through St. Louis, jumped the Mississippi River, and continued doing damage in East St. Louis. That was May 27, 1896. Besides busting the myth about rivers, it busted the myth that tornadoes do not hit major cities. At the time, St. Louis had a population close to 500,000.

The St. Louis Public Library has a great collection of photos and newspaper articles on the event. A total of 255 people were killed in both Illinois and Missouri. It was estimated to be an F-4 tornado on the Fujita scale, based on the damage seen in photographs.

Nine days after the event, a book was published based on newspaper accounts with lots of photos. It has recently been reprinted by Southern Illinois University Press and called “The Great Cyclone at St Louis and East St Louis, May 27, 1896”. Besides the incredible amount of detail on the storm’s damage, you are treated to some vivid and lurid prose (which was the newspaper style of the day).

Example of the damage from the 1896 event.

"St Louis Jefferson-Allen Damage" by http://www.photolib.noaa.gov/historic/nws/wea00256.htm "Photo by Strauss". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:St_Louis_Jefferson-Allen_Damage.jpg#/media/File:St_Louis_Jefferson-Allen_Damage.jpg
Click to enlarge. “St Louis Jefferson-Allen Damage” by http://www.photolib.noaa.gov/historic/nws/wea00256.htm “Photo by Strauss”. Licensed under Public Domain via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:St_Louis_Jefferson-Allen_Damage.jpg#/media/File:St_Louis_Jefferson-Allen_Damage.jpg

April EF-4 Tornadoes Rare in Illinois

According to the National Weather Service report on the April 9, 2015, tornadoes, the tornado that tracked through Fairdale was rated EF-4 on the Enhanced Fujita scale. This would be the first EF-4 tornado in April since 1981 for Illinois and only the 33rd in Illinois history, regardless of month, since reliable records began in 1950.

According to the NWS Storm Prediction Center database, only 33 F/EF-4 tornadoes have struck Illinois since 1950. Even more rare, only 8 have hit Illinois in April. Of those eight in April, six struck in the 1960s, one in 1981, and one in 2015:

Here are all the F/EF-4 tracks for Illinois from 1950-2014, from the MRCC tornado tracker tool. Unfortunately, we cannot screen out just the April events at this time. While this is a screenshot of the tool, the actual tool allows you to zoom in and out. If you point to a track then you get a pop up window with the details of that track. Check out the tornado tracker tool – it’s free.

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On this Date in 1953, First Documented Tornado Hook Echo Captured on Radar

Today is the 62nd anniversary of the first documented case of a tornado detected by radar. Illinois State Water Survey staff, at Willard Airport in Champaign, IL, captured the historic event on film on April 9, 1953. This discovery helped lead to the first national weather radar network in the United States.

Radar hook echo
First recorded radar hook echo associated with a tornado, April 9, 1953, near Champaign, IL. The radar was located to the south of Champaign-Urbana at Willard Airport. The tornado was located north of Champaign-Urbana. Photo by Illinois State Water Survey, Prairie Research Institute, University of Illinois. 

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