Area Forecast Discussion

KEY MESSAGES

- Oppressive heat and humidity continues through early evening with heat indices peaking between 105 and 115 degrees.

- Multiple thunderstorm complexes will move across IL this afternoon/evening, presenting a conditional threat for severe weather. Damaging winds (60-70+ mph gusts) and heavy rains are a risk especially in eastern and southeastern IL this afternoon and early evening.

- A cold front will bring relief from the high heat and humidity on Tuesday, transitioning us back to near normal temperatures and humidity levels during mid week.

DISCUSSION

Issued at 218 PM CDT Mon Jul 27 2026

Outside of the storm risk today, it has been very hot and humid today. Heat indices have been around 100-105 so far but still have time to increase to about 115 degrees in some spots. The Extreme Heat Warning continues until tonight at 9 pm. If thunderstorms materialize and move in during the afternoon and early evening, it would cool temps/heat indices and would likely end the heat headlines earlier than 9 pm. Beyond today, temperatures become more seasonable and less humid with highs in the 80s and lows in the 60s through next week.

Today's forecast has been a doozy. The models haven't been of the greatest assistance recently with this round of convection. We have an Enhanced Risk (level 3 of 5) for areas near and north of a Henry to Marshall line and a Slight Risk (level 2 of 5) elsewhere for today. Primarily this risk is for damaging winds, potentially stronger than 70 mph, but there is also a 2% tornado risk for the area within the Enhanced Risk.

Our 12z sounding shows MUCAPE over 5500 J/kg, effective shear of ~50 knots, and PWATs of nearly 2 inches. A shortwave is pulsing through this afternoon across Illinois and Indiana, based off GFS 500 mb vorticity. The storms currently over the Chicago-Land area may clip eastern IL as it drops south into Indiana and they are evolving into a damaging wind threat as it transitions to a linear mode rather than supercellular. It is starting to trend towards areas west of I- 55 may not see any of the severe threat today. There may be some showers/storms that direction but it looks like the main show will remain east of I-55 this afternoon/evening. An additional wave may move behind it that will also drop south, potentially riding the IL/IN border, but could possibly be weaker in strength, based off the HRRR. The HRRR's 16z and 17z runs are picking up the active convection well. The storms should have exited by 10 pm tonight. Storms will be moving to the south-southeast at 30-40 mph this afternoon/evening.

The next chance for showers and thunderstorms comes in on Friday into Saturday as a shortwave moves through the area.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1233 PM CDT Mon Jul 27 2026

VFR conditions are expected through the 18z period. There is a chance for strong storms later this afternoon into the evening hours. Confidence in the location and timing of the storms is low so opted for the PROB30 group for 21-01z. Heavy rain will be accompanying any storms, so lowered visibility is possible if they move through. The winds will be veering throughout the period, becoming northerly by the morning.

ILX WATCHES / WARNINGS / ADVISORIES

Extreme Heat Warning until 9 PM CDT this evening for ILZ027>031-036>038-040>057-061>063-066>068-071>073.