textproduct: Central Illinois

KEY MESSAGES

- Multiple rounds of widespread rain will track across the region this afternoon through Friday morning. Broad rainfall totals of 0.25 to 0.75 inches are expected, though localized heavy rain bands could produce isolated amounts of 2 to 3 inches.

- Temperatures drop significantly heading into the weekend, with highs in the upper 60s Friday and Saturday, and overnight lows falling into the 40s.

- Crisp, quiet autumn weather will remain across the region through early next week before temperatures warm back into the 70s by mid-week.

DISCUSSION

Issued at 258 AM CDT Thu Oct 1 2026

An upper-level low remains centered over SE Colorado per early morning GOES water vapor imagery. Downstream, compact mid-level shortwaves continue traversing the Ozarks and Upper/Mid- Mississippi Valley, providing sufficient synoptic lift for showery precipitation over northern Missouri. Meanwhile, an elongated low-level jet (LLJ) axis stretching from the southern Plains to the Great Lakes is veering ahead of an approaching, slow-moving cold front, driving warm air advection and deep isentropic ascent to further bolster rain activity.

By midday, the southwest U.S. upper-level trough is forecast to open up and eject northeastward into the Mid-Mississippi Valley, positioning itself parallel to the sluggish surface frontal boundary. Strong upper-level dynamics, combined with a plume of rich tropical moisture and focused frontal forcing, will set the stage for multiple rounds of widespread rainfall beginning this afternoon and lasting through early Friday morning.

The latest 00z REFS LPMM QPF guidance indicates broad totals ranging from 0.25" to 0.75" east of the Illinois River Valley into southeast Illinois. Nevertheless, robust frontogenetical forcing and high moisture flux along the slow-moving boundary favor narrow mesoscale rain banding. Underneath these localized bands, rainfall accumulations of 2" to 3" remain entirely possible across the CWA.

Drier conditions will quickly overspread the region from northwest to southeast Friday morning as the frontal boundary exits and an expansive Continental Polar (cP) high pushes east across the Midwest and Great Lakes. Strong post-frontal cold air advection will drive a pronounced airmass change, replacing early-week warmth with seasonal fall conditions. Highs on Friday and Saturday will drop into the upper 60s, while overnight lows fall into the 40s. Ensemble models show high confidence that this expansive Canadian surface high will remain entrenched through early next week, prolonging a period of tranquil, crisp autumn weather.

A warming trend takes hold by mid-week as a potent mid-level trough digs from the Canadian Prairies toward the Great Lakes, shifting the surface high eastward. Southward veering surface winds ahead of an advancing cold front will subsequently boost temperatures back into the mid- to upper-70s.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1212 AM CDT Thu Oct 1 2026

Showers falling from a VFR cloud deck will be scattered across the central IL terminals tonight. A more solid area of -SHRA associated with MVFR ceiling and visibility is forecast to move in from the northwest this afternoon, and persist through this evening. There is a chance conditions will drop to IFR at times, but confidence is too low to include in TAFs. Winds will gradually veer from the south tonight, to the north by this evening as a cold front pushes through. Speeds will be highest at KDEC- KCMI, where 20-25 kt gusts will be common during the day.

ILX WATCHES/WARNINGS/ADVISORIES

None.