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.

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 /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1217 PM CDT Sun Jun 7 2026

TSRA will be scattered around the central IL terminals into early this evening with around 10-30% coverage. Outside of that conditions will primarily be VFR until an MVFR cloud deck moves in from the south late tonight (08-10z). Confidence is high in these ceilings through much of Monday morning, with around 20-30% probs of IFR ceilings - so left that mention out for now. Another round of scattered TSRA looks to develop by late morning. Any storms today and Monday have to potential to briefly reduce to IFR visibility in +RA. Winds will generally be southeast under 10 kt through tonight, with some increase in speed expected toward midday Monday.

ILX WATCHES/WARNINGS/ADVISORIES

None.