textproduct: Central Illinois

KEY MESSAGES

- A marginal risk for severe thunderstorms exists on Friday, with isolated damaging wind gusts serving as the primary risk.

- An unsettled weather pattern will bring daily chances of showers and thunderstorms Friday through the middle of next week.

- Humidity builds over central and southeast Illinois this weekend and lingers into next week, driving peak heat index values into the low to mid 90s.

DISCUSSION

Issued at 133 AM CDT Thu Jun 4 2026

High pressure slides eastward today, allowing a return to southerly boundary-layer flow and gradually increasing dewpoints through the 50s. Afternoon RH should drop to around 35-40%, and with winds gusting 20-25 mph at times there will be a low risk for fire spread, as well as localized blowing dust where farming activity is in progress. High temperatures will again peak in the mid-80s.

A frontal boundary moves toward central and southeast Illinois on Friday. Instability climbs substantially as low- level moisture deepens, pushing surface dewpoints into the upper 60s. The HREF indicates a 40-50% probability of surface-based CAPE exceeding 2000 J/kg across the forecast area by Friday afternoon. However, guidance reveals a weak kinematic environment, with 0-6 km bulk shear remaining under 20 knots. Due to this combination of moderate instability and low shear, the SPC has outlined a Marginal Risk (level 1/5) of severe weather north of a Rushville to Bloomington line. The primary threat will be isolated damaging wind gusts stemming from pulse or multicell storm collapses. Outside of convective activity, a tightening pressure gradient will cause southwesterly wind gusts to exceed 25 mph at times.

A long-duration, unsettled weather pattern will dominate the region this weekend through the middle of next week. GEFS and EPS ensemble means strongly favor a blocked, slow- moving synoptic pattern through the period. A cutoff upper-level low or slow-moving trough is forecast to develop and meander across the Midwest, while a weak frontal boundary wavers across the area. Due to weak forcing, confidence in timing and rainfall amounts is low, though confidence in a prolonged warm/humid and frequently wet pattern is high. Weak shear throughout this period will minimize organized severe weather, though localized heavy rain totals and isolated gusty thunderstorm winds will be possible.

NBM guidance maintains afternoon temperatures in the upper 80s to lower 90s this weekend and early next week, while dewpoints climb into the upper 60s to lower 70s. This humid environment will drive daily peak heat index values into the low-mid 90s through Tuesday, and around 100 degrees on Wednesday west of I-55. The ECMWF Extreme Forecast Index (EFI) highlights this airmass as climatologically unusual for early June, particularly concerning the elevated moisture levels. Furthermore, the Climate Prediction Center 6-10 day outlook indicates a strong probability (60-70% chance) of above- normal temperatures across the region through the end of next week.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 430 AM CDT Thu Jun 4 2026

VFR conditions continue through the period, with increasing cirrus cloud coverage. Winds will initially be from the SSE, then turn to SW by mid-morning. Sustained wind speeds will reach 10-15 kts by 18z, with occasional gusts to 20 kts. Wind speeds then remain around those thresholds through the rest of the period.

ILX WATCHES/WARNINGS/ADVISORIES

None.