textproduct: Central Illinois

KEY MESSAGES

- Temperature Spread Sunday: A stalling cold front creates a sharp temperature split Sunday, ranging from the mid 70s north of the Illinois River to near 90 south of I-70. Light showers or sprinkles (20-30% chance) remain possible along the boundary.

- Midweek Storm Potential: Multiple rounds of thunderstorms arrive Tuesday into Wednesday. There is potential for localized heavy rainfall exceeding 1-2 inches, especially north of Beardstown to Bloomington, along with a potential for severe weather.

- Late-Week Uncertainty: Unsettled conditions persist Thursday and Friday with periodic rain chances, though high uncertainty remains regarding exact frontal placement and daily temperature trends.

DISCUSSION

Issued at 207 AM CDT Sat Sep 12 2026

This Weekend... This morning a closed upper low is pushing east across the mid Mississippi Valley just south of Illinois. It is expected to open up as it drifts eastward through the lower Ohio Valley this morning. A few isolated light showers or storms will be possible with this feature east of I-57 this morning, but most of central Illinois is expected to stay dry today.

To the northwest, a cold front is dropping across portions of the Upper Midwest and central Great Plains this morning. Ahead of the front, a thermal ridge, with 850mb temperatures reaching around 18C, will push into central Illinois. This will allow temperatures to rebound this afternoon, with widespread highs into the upper 80s, and a few spots touching 90.

As the front moves into and eventually stalls over the area on Sunday, a sharp temperature gradient will form across the region. Highs north of the Illinois River are projected to top out in the mid 70s, whereas areas south of I-70 will warm to around 90.

A prominent 700/750mb warm nose will keep deep surface based convection capped Sunday, however, some light precipitation still appears possible. Forcing is quite nebulous Sunday, but weak isentropic ascent eventually develops along the 310-315K surfaces resulting in mid level saturation that could produce a few light showers or sprinkles. NBM mean 24-hour QPF for Sunday generally shows up to a third of an inch in the west to 0.15 inches in the east.

Monday through Wednesday... An active, unsettled pattern is expected during the first half of next week. A high-amplitude upper wave is forecast to move from the Pacific Northwest across the Canadian Rockies Sunday night into Monday. A deep surface low will develop in response and track from the Canadian Prairies toward Hudson Bay Monday into Tuesday.

An associated warm front will advance across the Great Plains Sunday night, triggering nocturnal convection. Some of this decaying activity could reach parts of west central Illinois as the warm front shifts northward through the area on Monday.

A better chance for widespread showers and storms arrives Tuesday into Wednesday as the trailing cold front moves into and stalls over the state. This period will need to be monitored for severe potential, as the upper-level jet extends into the Great Lakes, increasing deep-layer shear along the stalled front. Heavy rain will also be a concern with repeated rounds of storms tracking east along the front. Through Wednesday night, NBM mean QPF shows one inch along and north of a Beardstown to Bloomington line with the two inch contour clipping our far northwest forecast area near Galesburg.

Thursday and Friday... Unsettled conditions look to continue late next week as the baroclinic zone remains draped across the Midwest, providing a focus for daily rain chances. Guidance diverges on the precise location of the boundary by Thursday and especially Friday. Although specific timing and positioning details remain uncertain, daily chances for showers and thunderstorms will persist near the front. Temperatures will depend heavily on where the front settles, but current guidance favors highs from the 70s north of the front to the upper 80s or low 90s south.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 555 AM CDT Sat Sep 12 2026

An upper level disturbance over far southern Illinois will bring some mid and high clouds to portions of east central (DEC/CMI) IL early this morning. Light south winds will be in place early this morning ahead of an approaching weak cold front. Some patchy fog is possible for an hour or two after 18Z. Winds will turn west today then northwest with passage of front this evening, but should remain light through the period, at or below 10 kt.

ILX WATCHES/WARNINGS/ADVISORIES

None.