INDIANAPOLIS, INDIANA — The I-70 corridor is where the temperature gradient will sit today, September 20, with areas south of I-70 still experiencing heat index values in the 90s this afternoon, while areas north will be much cooler, according to a modeled heat index map from BAMWeather, valid 4:00 p.m. ET. After today, forecasters say we can say goodbye to those higher temperatures through the rest of the month, making this the last real hot afternoon of September.
Chicago and Milwaukee Enjoy Much Cooler Air North of the Divide
If you’re in Chicago, today’s heat index is forecast at a comfortable 66 degrees, with Milwaukee running even cooler at 64 degrees. Both cities sit well within the zone forecasters circled as much cooler today, meaning residents across this part of the Midwest should expect a genuinely pleasant, jacket-optional afternoon rather than the sticky warmth still gripping communities just a few hours south.
Detroit, Toledo and Cleveland Also Stay Comfortably Cool
Across the Great Lakes, Detroit is forecast for a heat index of 66 degrees, with Toledoslightly cooler at 64 degrees and Clevelandat 72 degrees. If you’re anywhere in this stretch, from Detroit through Toledo and into Cleveland, today should feel like a genuine relief after recent stretches of heat, with all three cities comfortably inside the circled cooler zone on today’s map.
Pittsburgh Sits at the Northern Edge, Still Comfortably Cool
Farther east, Pittsburgh checks in at 75 degrees, the warmest reading among the northern cities but still solidly inside today’s much cooler zone. If you’re in Pittsburgh, expect a mild, comfortable afternoon compared to the muggy 90s forecast just to your south, even though your numbers trend a touch higher than Chicago or Cleveland.
Peoria and Springfield Sit Right on the Temperature Gradient
Peoria, Illinois, checks in at 72 degreestoday, while Springfield, Illinois, sits noticeably warmer at 86 degrees, illustrating just how sharp today’s temperature gradient really is over a relatively short distance. If you’re in Peoria, you’re comfortably on the cool side of the line, but if you’re in Springfield, you’re already starting to feel the warmer, more humid air pushing up from the south.
Indianapolis, Dayton and Columbus Bake in the 90s South of the Line
Just south of the I-70 corridor, Indianapolisis forecast for a heat index of 91 degrees, matching Dayton at the same 91 degrees, while Columbus comes in slightly cooler but still warm at 86 degrees. If you’re anywhere across this stretch of central Indiana and Ohio, today is your last real hot, muggy afternoon before the cooler pattern forecasters mentioned takes hold for the rest of the month.
Cincinnati Tops the Ohio Valley at a Sweltering 97 Degrees
Cincinnati posts one of the highest heat index readings anywhere on today’s map at a sweltering 97 degrees. If you’re in Cincinnati, today is shaping up to be a genuinely uncomfortable, summer-like afternoon, standing in sharp contrast to communities just two hours north in Dayton and Columbus, let alone the much cooler conditions in Chicagoor Detroit.
St. Louis Matches Cincinnati’s Intense Heat Index Today
Farther west, St. Louis also checks in at 97 degrees, tying Cincinnati for one of the hottest readings on the entire map. If you’re in St. Louis, expect today to feel like peak summer rather than late September, making it worth taking the same heat precautions, staying hydrated, limiting strenuous outdoor activity, that you’d use during a July heat wave.
Evansville and Louisville See the Hottest Readings in the Region
The absolute hottest spots on today’s map belong to Evansville, Indiana, and Louisville, Kentucky, both forecast for a heat index of 99 degrees. If you’re in either city, today represents the peak of this stretch of warmth, with Lexington close behind at 95 degreesand Charleston, West Virginia, checking in at 91 degrees as the warm air extends into the southern Ohio Valley.
Why This Sharp North-South Divide Is Happening Right Along I-70
Forecasters specifically identified the I-70 corridor as the dividing line because it marks the boundary between the cooler air already established across the Midwest and Great Lakes and the lingering warm, humid airmass still in place farther south. That kind of sharp gradient, comfortable 60s just a couple hours north of sweltering 90s and even a 97-degree reading, is a strong signal that a real pattern change is actively pushing through the region today rather than easing in gradually.
Goodbye to the Heat for the Rest of the Month
The most important takeaway for residents anywhere south of I-70 today, whether that’s Cincinnati, St. Louis, Louisville or Evansville, is that this is very likely your last hot, humid afternoon of the month. Forecasters say we can say goodbye to these higher temperatures through the rest of September, meaning the cooler air currently sitting comfortably over Chicago, Milwaukeeand Detroit is set to spread south and take hold across the entire region in the days ahead.
