textproduct: Central Illinois

KEY MESSAGES

- There is a Marginal Risk (5-15% chance) of excessive rainfall along and south of a Danville to Beardstown line Thursday through Friday.

- A major pattern change will bring a return to typical summertime heat and humidity by early next week. The latest NBM indicates an 80-100% chance of high temperatures climbing above 90 degrees by next Monday and Tuesday.

DISCUSSION

Issued at 250 PM CDT Tue Jun 23 2026

*** Scattered Thunderstorms Wednesday ***

A cold front currently extending from the Dakotas to Colorado will sink southeastward over the next 24 hours. The pre-frontal airmass across central Illinois will initially be dry/subsident: however, southwesterly flow ahead of the boundary will lead to dewpoints climbing into the middle 60s and precipitable water values increasing to 1.25-1.50 by Wednesday afternoon. Due to the lack of boundary layer moisture, instability will remain weak...with SBCAPEs less than 1000J/kg across much of central Illinois. The exception will be in a narrow corridor closer to the front along/northwest of the Illinois River where dewpoints may reach the upper 60s and SBCAPEs will climb into the 1000-2000J/kg range. Deep-layer shear will also be maximized within this corridor as evidenced by NAM 0-6km shear values of around 40kt. As a result, think scattered showers and thunderstorms will develop Wednesday afternoon...mainly west of the I-57 corridor. While SPC has not yet highlighted a severe risk for any part of central Illinois, a few of the cells along/west of the Illinois River may contain gusty winds and small hail during the late afternoon/early evening. The scattered showers/thunder will spread further east across the remainder of central Illinois Wednesday night, then will depart into Indiana by Thursday morning.

*** Heavy Rainfall Risk Thursday into Friday ***

The mid-week cold front will become parallel with the prevailing zonal flow pattern and will gradually stall across south-central Illinois on Thursday. As a series of short-wave troughs ejecting eastward from the Plains interacts with the stalled boundary, periods of showers and thunderstorms will develop across mainly the southern two-thirds of the KILX CWA Thursday afternoon through Friday night. With a 40-50kt 850mb jet streak progged to develop from Texas northeastward into southern Illinois and the Ohio River Valley, precipitable water values will surge into the 2.00-2.50 range along/south of the I-72 corridor Thursday night. As a result, showers and thunderstorms will become widespread and heavy rainfall will be likely. Once the initial wave passes, a second disturbance will bring another round of rain Friday night into Saturday morning. The exact location of the second round is still somewhat in question and may be slightly further south than the initial Thursday night rainfall: however, all indications strongly suggest the highest amounts of 2 to 3.50 inches will focus along/south of a Paris to Pittsfield line. Rainfall of this magnitude occurring on already saturated soils/water-logged fields will lead to steep rises on area streams and may pose a localized flash flood risk. As a result, a Flood Watch will need to be carefully considered over the next couple of forecast cycles. Further north away from the strongest forcing and deepest moisture, rainfall will be considerably lighter...with locations along/north of a Macomb to Bloomington line seeing less than 1 inch.

*** Summertime Heat and Humidity Early Next Week ***

A strong jet stream over the Northern Pacific will nudge a deep upper low over the Gulf of Alaska southeastward later this week. Models have been consistently showing this feature digging west of the Rockies, resulting in downstream ridging over the Midwest by early next week. As upper heights rise, the persistent frontal boundary from Missouri into the Ohio River Valley will gradually get shifted northward over the weekend...resulting in a continued risk for a few showers and thunderstorms, but also an increase in heat and humidity. High temperatures will reach the upper 80s/lower 90s by Sunday, then as the ridge becomes more prominent across the area, the 12z NBM indicates an 80-100% chance of temps climbing above 90 degrees by next Monday and Tuesday. Given dewpoints in the middle to upper 70s, corresponding heat index readings will exceed 100 degrees. As a result, the latest Probabilistic Heat Risk shows a 60-90% chance of reaching the Major category across nearly all of central and southeast Illinois for both days.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 1156 PM CDT Tue Jun 23 2026

A couple periods of scattered thunderstorms are possible over the upcoming 24 hours. The first looks to be around 19Z-01Z, potentially producing isolated MVFR conditions in storms. The second looks to be somewhat more compact and only reaching KPIA and KBMI from around 03Z-06Z as it moves in from the NW, also potentially producing isolated MVFR conditions. Have included PROB30 groups in TAFs for these features. Otherwise, VFR conditions are expected, with scattered diurnal cloud cover around 5000 ft AGL expected from around 15Z-01Z. Winds light and variable, becoming SW 9-12 kts after 15Z, then decreasing after 01Z.

ILX WATCHES/WARNINGS/ADVISORIES

None.