textproduct: Central Illinois

KEY MESSAGES

- A Marginal Risk (level 1 of 5) for severe weather exists across parts of central Illinois both today and Sunday. The primary hazard will be isolated strong wind gusts.

- Much cooler conditions will be on tap next week as high temperatures dip into the 60s everywhere north of the I-70 corridor Monday and Tuesday.

DISCUSSION

Issued at 348 AM CDT Sat Sep 19 2026

08z/2am radar imagery shows a large cluster of convection along the Illinois/Wisconsin border. The storms are occurring ahead of a weak area of low pressure analyzed over northern Iowa and are being fed by a 30-40kt 850mb jet streak oriented from the Central Plains to the southern Great Lakes. The convection will continue to track E/SE across northern Illinois and will remain largely north of the KILX CWA this morning. Have featured 30-60 PoPs along/north of I-74 through midday in case some of the storms are able to drop/develop that far south as the main cluster pushes into northern Indiana.

A prominent outflow boundary is currently evident from near Waukegan to Moline...and this feature will likely serve as a focus for additional convective development this afternoon as it sags further southward. Most CAMs key in on locations along/east of a Champaign to Shelbyville line after 20z/3pm. While the atmosphere will be moderately unstable with SBCAPEs greater than 3000J/kg, weak 0-6km wind shear of less than 30kt will inhibit widespread storm organization. As a result, am only expecting isolated marginally severe wind gusts of 50-60mph with the strongest cells.

As the nocturnal low-level jet shifts a bit further southward tonight, showers and thunderstorms will become more widespread across the N/NW CWA after dark. Based on the latest model guidance, have opted to carry 60-80 PoPs along/north of a Rushville to Normal line. Further southeast, locations along and south of the I-70 corridor will remain dry through the entire night.

With a short-wave trough interacting with a stationary frontal boundary draped across the region, showers and thunderstorms will continue into Sunday. While instability will be slightly lower than today, an uptick in wind shear associated with the wave will lead to a somewhat better chance for strong to potentially severe storms during peak heating. NAM SBCAPEs reach 2000-2500J/kg while 0-6km bulk shear increases to around 30k by afternoon. Given these parameters and a consistent reflectivity signal noted by the RRFS, think some of the storms late Sunday afternoon and evening will be capable of marginally severe wind gusts/hail. The latest SPC Day 2 Convective Outlook highlights areas along/south of a Rushville to Bloomington line for a Marginal Risk (level 1 of 5) for severe weather. This seems reasonable, although it may need to be nudged northward a bit if RRFS trends continue.

Wet weather will continue on Monday and even into Tuesday as an upper low evident on latest water vapor imagery over Idaho gradually traverses the region. Once the low passes to the east, synoptic subsidence will lead to a return to dry weather for the remainder of next week. The big story will be the cooler temperatures as highs drop into the 60s north of the I-70 corridor Monday and Tuesday, then only slowly rise back into the 70s as the week progresses. The coolest period continues to be Tuesday night when lows reach the lower to middle 50s. A few favored low-lying spots may even dip into the upper 40s as the 00z NBM indicates a 30-50% chance of temps below 50 degrees by 12z Wednesday.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 554 AM CDT Sat Sep 19 2026

Convection is tracking across northern Illinois early this morning and may graze the I-74 corridor through midday. Based on radar trends and the latest HRRR/RRFS, have maintained PROB30 groups for thunder and MVFR ceilings at KPIA/KBMI/KCMI from 14z through 18z. Further south, other than perhaps a stray light shower, think KSPI/KDEC will remain dry. An outflow boundary is evident on 1045z/545am radar from near KGBG to KC75. While the HRRR wind forecast suggests it may drop southward past the I-72 corridor, think this is overdone. As a result, have only mentioned a wind switch to NE at the I-74 sites...while S/SW winds persist further south. After the morning convection passes, attention turns to additional showers/storms expected to develop across Iowa into north-central Illinois tonight. The activity should be a bit further south than what it presently is, so have opted to include predominant showers/thunder at KPIA after 06z, but have kept the other sites dry through 12z Sun.

ILX WATCHES/WARNINGS/ADVISORIES

None.