textproduct: Central Illinois

KEY MESSAGES

- A conditional severe thunderstorm risk exists mid afternoon to early evening mainly near and east of Interstate 57, capable of localized damaging wind gusts and large hail.

- Comfortable and dry weather is anticipated Thursday through Saturday, followed by the return of summer-like heat early next week.

DISCUSSION

Issued at 1212 PM CDT Wed Aug 26 2026

A surface cold front currently stretches near the I-55 corridor in central IL, highlighted by and agitated cu field on visible satellite imagery. Recent radar and satellite trends suggest that convective initiation is already underway, a couple hours ahead of what the high-res models have been suggesting. Ahead of the front, surface dew points in the upper 60s to lower 70s support a moderately unstable environment, with RAP and HRRR soundings indicating MLCAPE values reaching near 2500 J/kg by mid- afternoon. 0-6 km bulk shear profiles of 35 kt, augmented by a subtle 500 mb speed max translating eastward across central Illinois, create an environment favorable for supercells, particularly over east central IL. The limiting factor in storm coverage will be a lack of forcing for ascent, which will be tied to weak low level convergence and lift ahead of the mid level wave. Suite of CAMs generally show little or no development (HRRR, HRW, NAM Nest), with a couple notable exceptions (RRFS/FV3) showing more robust coverage over east central IL. Given the CAPE/shear profiles, sufficient lapse rates, and DCAPE up to 1400 J/kg, damaging wind gusts and large hail are the primary severe hazards. SPC maintains a 2% tornado risk which would likely be tied to storm scale interactions. Any convection should track southeast around 30 mph, and exit into Indiana by 9-10 pm.

Surface high pressure builds southward from the Upper Midwest across the Great Lakes region Thursday and Friday. This will bring a nice stretch of weather to the region with comfortable temps and humidity for late Aug. Confidence remains good in dry conditions persisting through Saturday. Return southerly flow around the exiting surface high brings high temperatures into the mid-80s with overnight lows in the low 60s.

Model divergence increases regarding a shortwave trough traversing the northern stream Sunday into early next week. Deterministic GFS runs feature a deeper mid-level trough driving a cold front into Illinois. In contrast, deterministic ECMWF and ECMWF-AI runs maintain a flatter, displaced trough structure, keeping mainly dry conditions going. Consequently, the forecast relies heavily on NBM blend guidance, maintaining slight chance (20%) PoPs, primarily over the northern CWA.

Upper ridging builds across the Midwest early next week, bringing a return to heat and humidity as surface flow becomes southerly. Highs are projected to reach the upper 80 to mid 90s, with heat indices peaking in the upper 90s to around 100 Mon-Wed.

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/

Issued at 1212 PM CDT Wed Aug 26 2026

A cold front was nearing I-55 at midday, associated with SCT-BKN cu at 2500-3500 ft. Analysis and trends suggest that KCMI is the only terminal that has a decent chance of seeing TSRA this afternoon, so limited PROB30 to that site only. Otherwise winds veer to the WNW behind the front, becoming light this evening with mostly clear skies. A robust cu field is forecast to form late Thursday morning. This could initially be below 3000 feet, however confidence is too low for MVFR at this range.

ILX WATCHES/WARNINGS/ADVISORIES

None.