textproduct: Central Illinois

KEY MESSAGES

- Isolated locally heavy rain will occur across parts of central IL this afternoon and again Monday afternoon and evening.

- High heat and humidity will build across the region Tuesday through Thursday, driving afternoon heat index values around 100 degrees.

- A severe thunderstorm risk accompanies a cold frontal passage on Thursday.

UPDATE

Issued at 911 PM CDT Sun Jun 7 2026

Storms are finally pushing north of the CWA but scattered light showers are still around the CWA...based on radar trends. Clouds through the night will hamper much cooling to temps, so will make an adjustment to overnight lows in the update. Pop, wx and chances of thunder also need to be adjusted to better reflect reality on current radar. Still expecting showers and storms tomorrow, with chances of scattered showers in the AM, followed by both during the afternoon and early evening...as the low pressure area in MO moves east and northeast into parts of IL.

Evening update will be out shortly.

DISCUSSION

Issued at 138 PM CDT Sun Jun 7 2026

An unseasonably moist airmass remains anchored over central and southeast Illinois. 12z ILX sounding indicated precipitable water values (PWAT) at 1.95", just shy of a record for the date. The moist environment and long skinny CAPE profiles promote highly efficient warm-rain processes. While overall forcing is weak, it has been aided by convergence bands lifting north, east of a baggy mid level low over the central Plains. Convective elements have been slow moving due to winds <20 kt through the depth of the profile, but generally have been tracking north and mainly limited to the northern CWA and points north today. Short range CAMS indicate this trend towards higher coverage will continue near and north of I-74 into early this evening, before coverage diminishes with the loss of diurnal heating.

On Monday, a similar setup remains in place, though the upper wave moves overhead, promoting a higher coverage of showers and storms from late morning into the evening. Guidance shows PWATs rising over 2" with light winds aloft, promoting efficient slow moving convective elements. The 12z HREF LPMM 24-hr rainfall through 00z Tue shows isolated areas of 2-5" (most likely worst case scenario) over portions of the central to northern CWA. If this were to occur over urban areas or isolated areas that saw heavy rain last night and this morning, flash flooding will be a concern. Though overall antecedent dry conditions and isolated nature of the projected heavy rain preclude widespread flash flood concerns and watch issuance at this time.

The upper low pushes east as subtropical ridging expands across the Ohio Valley Tuesday and Wednesday, leading to a hot and humid period. The ECMWF Extreme Forecast Index (EFI) highlights this period with values between 0.7 and 0.8 for maximum temperatures, signaling a climatologically unusual heat event for early June. The National Blend of Models (NBM) projects afternoon highs in the low to mid-90s. Combining these temperatures with dewpoints in the lower 70s yields heat indices over 100 degrees. LREF probs indicate a 70 to 80 percent probability of heat index values exceeding 100 degrees, highest on Wednesday afternoon. Southwest winds will increase Wed amid a tightening MSLP gradient, with diurnal mixing supporting wind gusts over 25 mph.

Attention then shifts to Thursday as an upper-level trough moves across the upper Midwest, dragging a cold front into the region. While some timing differences are noted among the ensemble suite, the better dynamics appear to overlap favorably with diurnal heating, increasing the severe thunderstorm risk. SPC's 15% risk area (equivalent to Slight Risk) for Thursday highlights the area north of I-70. This is supported by a number of AI/ML composite severe products. Heat will also be a concern to the southeast of convective cloud cover/rain cooled outflows, where heat indices will likely reach around 100 degrees.

Behind the front for Friday and Saturday, high pressure builds southeast from Canada, delivering a more seasonable and less humid airmass. Looking further ahead, the CPC 6-10 and 8-14 day outlook indicate a trend for near to below-normal temperatures.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 649 PM CDT Sun Jun 7 2026

Going to keep VFR conditions at all TAF sites through the night and tomorrow. However, outflow boundary south of I-74 is still kicking off some showers/storms and could affect CMI and BMI for another 1-2hrs. With the storms will have vis at 4sm, but cigs will be VFR. Lower clouds are forecast to roll over each site late tonight with cigs around 1.5kft, which will continue into the afternoon. Also looks like there might be a break in the precip until the next wave arrives tomorrow morning, which will continue into the afternoon. This next wave will be the focus for more showers and storms and have a PROB30 group for all sites for this. Winds will be northeast at BMI and PIA this evening, then become southeast after midnight...and continue through end of the period. Could be some gusts with the winds tomorrow, but unsure and not adding at this time.

ILX WATCHES/WARNINGS/ADVISORIES

None.