textproduct: Central Illinois

KEY MESSAGES

- Repeating rounds of showers and storms could cause flash flooding south of a Shelbyville to Paris line through this evening. A Flood Watch is in effect for these areas.

- Additional thunderstorms will develop along an advancing cold front on Tuesday, with a slight risk (level 2 of 5) for severe storms east of Interstate 55. Damaging winds, large hail, and heavy rainfall would be the most likely hazards.

DISCUSSION

Issued at 223 PM CDT Mon May 18 2026

A line of showers and thunderstorms has stalled out south of the I- 72 corridor this afternoon. Although wind shear has increased over this area since this morning, instability has significantly waned and has prevented storms from largely becoming organized. Despite this, cannot completely rule out a strong updraft or two over the next few hours in far southeast Illinois where RAP mesoanalysis shows SBCAPE of 1000+ J/kg. If any strong storms do occur, damaging winds would be possible. The main threat through the evening will be heavy rainfall with PWATS up around 1.5 to 2 inches and hourly rainfall rates up around 1 inch in the heaviest of storms. A Flood Watch has been issued for areas south of a Shelbyville to Paris line through this evening.

Further west, additional thunderstorms have developed over the central Plains ahead of a cold front and will track east through tonight, approaching western Illinois sometime after midnight. These storms should weaken as they enter the area due to lack of instability, though they could still produce gusty subsevere winds as they track through the western half of the state. Most of the shower and storm activity should subside by or before daybreak.

The cold front will approach the I-55 corridor around midday Tuesday and be the focus for additional storm development by afternoon. However, one limiting factor will be how long cloud cover from the overnight storms lingers, which would ultimately limit our destabilization potential. CAMS suggest destabilization will be most likely in east-central and southeast Illinois by mid to late afternoon in a moderately sheared environment. However, this appears to be highly conditional at this point. If storms do develop, damaging winds, large hail, and heavy rainfall will be the primary concerns.

A drier and cooler period shapes up for midweek as high pressure ridging slides through the Upper Midwest. This will be relatively short-lived as upper troughing moves into the central US and shortwave energy sends several rounds of precipitation through Illinois this weekend into early next week. While the severe weather risk appears low through this period, better moisture/instability building by late weekend/early next week will support thunderstorms at times.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1237 PM CDT Mon May 18 2026

Precipitation will continue to slowly spread east along a remnant outflow boundary through the remainder of this afternoon. Although most of the thunderstorm activity should remain south of the airfields, can't completely rule out a low chance for lightning at KSPI and KDEC through about 00Z. After a lull this evening, another wave of storms will approach from the west late tonight into the early morning hours of Tuesday. PROB30 groups remain in place for this potential at all locations. South-southwest winds will be breezy through Tuesday, with gusts between 20-25 kts.

ILX WATCHES/WARNINGS/ADVISORIES

Flood Watch until 10 PM CDT this evening for ILZ054>057- 061>063-066>068-071>073.