textproduct: Central Illinois

KEY MESSAGES

- A level 1 risk of severe weather continues through this evening, with potential for localized damaging winds.

- The storms tonight will produce areas of heavy rain, with potential for localized flooding north of I-72 where training storms are more likely.

- The unseasonably hot weather will continue in areas south of I-72 into the weekend.

DISCUSSION

Issued at 206 PM CDT Tue Sep 15 2026

Primary boundary stretches from just north of Quincy to near La Salle this hour, while satellite continues to show an area of ACCAS just south of it. Temperatures south of the boundary are widely in the mid 90s, with a few mesonet sites south of I-72 even warmer. The cap has eroded and mesoanalysis indicates about 2500 J/kg of SBCAPE near this boundary, along with about 30 knots of 0-6km shear. So far the convection has remained fairly scattered and mostly thunder free, though an uptick in convection is expected toward mid afternoon where cumulus clouds have recently been blossoming. A couple main periods of storms are expected, one through about sunset, then an uptick in development from the west mid to late evening and past midnight. Besides the threat of some isolated damaging wind gusts, high precipitable water values over 2 inches (near the top of climatology for mid September) will produce areas of heavy rain. LPMM guidance from the HREF suggests potential for 2-3 inches in swaths of heavy rain, especially this evening when more in the way of cell training is expected. Flash flood guidance is 2-2.5 inches in an hour across much of central Illinois, so there could be some localized flooding if the rain comes down too fast.

By Wednesday morning, the front is expected to be somewhere between I-72 and I-70, where it should loiter for a day or so as it becomes parallel to the upper flow. Rain chances will continue into Thursday north of the boundary, with the heavy rain threat gradually shifting into northern Illinois. By late week, the front should be back north of the area, before a shortwave arrives to start the weekend. The boundary is expected to stall once again over central Illinois. As an upper low closes off over the northern Plains and upper Mississippi Valley early next week, any push of this front out of the area will be slow. Thus, rain chances will linger over portions of central Illinois, though they will be highest Saturday night into Sunday evening.

Temperature-wise, a significant range in highs is expected over the forecast area through the weekend. Highs in the 90s will continue south of the boundary. North of there, 80s will be more common, though with lower clouds and rain on Wednesday, areas north of Havana and Bloomington are likely to remain in the 70s.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1223 PM CDT Tue Sep 15 2026

Scattered showers/storms will increase this afternoon along a boundary, primarily impacting KPIA/KBMI through 00z. Storms further south will be more widely scattered, and will only mention a PROB30 for the KSPI-KCMI area vs. the TEMPO further north. A more widespread area of showers and thunderstorms will move in mid to late evening and persist much of the night. Accompanying this latter area of rain will be deteriorating ceilings, with 50-80% chances of IFR conditions from KSPI-KBMI before 12z. Slow improvement in the ceilings is expected from south to north Wednesday morning, though chances of ceilings below 3000 feet remain high (70-90%) at KPIA/KBMI through midday.

Southwest winds will become northerly toward 00z at KBMI/KPIA by 00z as a front settles into the area. The boundary should be south of KSPI-KCMI by 06z.

ILX WATCHES/WARNINGS/ADVISORIES

None.