textproduct: Central Illinois

KEY MESSAGES

- Isolated strong to severe thunderstorms south of Interstate 70 this afternoon carry a risk of localized damaging wind gusts.

- Localized heavy rainfall tonight will yield pockets of 1 to 2 inch totals near and south of I-70, leading to potential ponding in low-lying areas.

DISCUSSION

Issued at 1222 PM CDT Wed Sep 9 2026

Regional radar mosaic indicates a band of showers along a confluence zone dissipating as it tracks east near I-55 early this afternoon. An outflow boundary well ahead of this activity was tracking toward SE IL, and appears to be the focus for isolated thunderstorm development this afternoon. Thermodynamic analysis indicates MLCAPE values building to around 1000-1500 J/kg across southern portions of the forecast area, primarily near and south of Interstate 70. Weak 06 km bulk shear around 20-25 knots will tend to limit storm organization, but steep low- level lapse rates driving DCAPE near 1000 J/kg will lead to pulsey downburst winds in any thunderstorms that manage to develop mid- late afternoon. This activity should quickly fade early evening with diminishing instability.

Moisture transport ramps up again overnight, ahead of a shortwave diving southeast from the Plains. Precipitable water (PWAT) values will climb to 1.82.0 inches along an elevated boundary stretched west-east over central IL, well above the 90th percentile for early September. Hi-res model guidance depict clustered convective development tracking eastward late tonight and Thursday morning, mainly south of the I-72 corridor. Probabilities from the HREF reveal a 70 percent chance of rainfall exceeding 1.0 inch south of the I-70 corridor, with localized maxima up to 2.0 inches across southeast Illinois by Thursday morning. Pavement ponding and localized nuisance urban flooding will be the primary impacts.

Showers are expected to dissipate over the south half of the area Thursday morning. Light winds will prevail as the frontal zone slowly sinks south, leading to another day of moderate humidity levels, especially south of I-72. Dry and mild weather will dominate Friday as high pressure builds through the eastern Great Lakes.

Ensemble guidance generally agrees on the next front reaching central IL this weekend. Upper level support doesn't look great and we appear to be near the tail end of the front, so we maintain low chance PoPs for Saturday and Saturday night. Temperatures will average a bit above normal Sat-Sun with highs in the mid- upper 80s. A building upper- level ridge across the central CONUS early next week will foster a further warming trend for Monday and Tuesday, pushing high temperatures into the lower 90s. Furthermore, the Climate Prediction Center (CPC) 6-10 day outlook indicates a 40 to 70 percent probability of above-normal temperatures across central Illinois through September 18th.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1222 PM CDT Wed Sep 9 2026

An area of -SHRA was dissipating over central IL at midday, and will pose no aviation impacts. The next round of -SHRA is forecast to develop late tonight, impacting the KSPI-KDEC-KCMI corridor through Thursday morning with MVFR ceilings, and at times MVFR visibility. There is around a 30% chance of IFR ceilings at these terminals, but kept this out of the forecast for now and will monitor trends. Farther north at KPIA-KBMI, MVFR ceilings are likely 09-18z. These sites will be near the north edge of the -SHRA, so went with VCSH for now. WNW winds near/under 10 kt this afternoon will go light and variable tonight and Thursday morning as the frontal zone moves in.

ILX WATCHES/WARNINGS/ADVISORIES

None.