textproduct: Central Illinois

KEY MESSAGES

- Unseasonable Heat & Temperature Contrast: Areas south of the front will see heat indices near 100 F through Saturday. This is primarily near and south of Interstate 72.

- Thunderstorms & Microburst Risk: Scattered showers and storms are forecast through Sunday. On Saturday, stronger storms could produce localized damaging wind gusts up to 60 mph and quarter-sized hail.

- Significantly Cooler Next Week: High confidence that the hot air will be pushed south by Monday. Expect comfortable, dry conditions with highs in the upper 60s to mid-70s areawide Tuesday through Friday.

DISCUSSION

Issued at 1231 PM CDT Fri Sep 18 2026

An anomalous 594-mb heat ridge continues to hold firm over the central U.S. early this afternoon. Positioned along the northern periphery of this heat dome, a weak surface low rests near Sioux City, Iowa, with an associated warm front stretching southeastward through Columbia, Missouri toward Terre Haute, Indiana.

As weak shortwave energy ejects out of the Desert Southwest, it will ride over the frontal boundary, offering sufficient synoptic lift to drive scattered shower activity today. These shortwaves will also push the frontal system slightly north from tonight into Saturday.

Near the warm front, pooling surface dewpoints in the lower 70s will help support MLCAPE around 2000 J/kg, keeping the potential for an isolated thunderstorm in place. Poor mid-level lapse rates will limit any severe threat today. However, as the front drifts northward on Saturday, increasing bulk shear (2535 kts) may compensate for the weak lapse rates, allowing for occasionally organized storms capable of producing localized severe microburst winds and quarter-sized hail. Brief heavy downpours resulting in 1"/hr rates are a low (10-20%) probability, but could accompany the more organized convection. This potential is best highlighted by the HREF LPMM QPF, which hints at localized amounts up to 1.5".

Beyond the scattered showers and isolated storm potential through Saturday, the primary forecast difficulty centers on the sharp temperature contrast across the region. With the boundary draped across central Illinois, locations to the north will be capped in the 80s, whereas areas to the south will climb back into the 90s with heat indices approaching 100 F. Outflow from passing precipitation could lead to sharp local variations in temperatures through Saturday.

By Sunday, model guidance and ensemble suites exhibit high confidence that a closed upper-level trough will track east across the Plains and into the Mid-Mississippi Valley by Monday. This system will dismantle the heat ridge and push the frontal boundary south and east of the forecast area. In its wake, expansive high pressure will settle in under a blocking split- flow regime aloft, bringing a period of drier and significantly cooler conditions. Daytime highs from Tuesday through Friday are expected to remain in the upper 60s to mid 70s.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1157 AM CDT Fri Sep 18 2026

Broken MVFR to IFR ceilings persist early this afternoon across KPIA, KBMI, and KCMI, positioned just north of a surface front situated south of Interstate 72. However, satellite imagery and recent high-resolution guidance indicate rapid clearing to VFR conditions over the coming hours as boundary layer mixing expands.

Ceilings are expected to deteriorate back to MVFR overnight along the frontal boundary, particularly near the Interstate 74 terminals (KPIA/KBMI/KCMI). Furthermore, a low (2030%) risk of scattered thunderstorms exists along the front Saturday morning. To account for this potential, a PROB30 group has been introduced for TSRA across the I-74 corridor sites.

For the remainder of the TAF period, surface winds will gradually shift from easterly to southerly as the frontal system slowly lifts across the region.

ILX WATCHES/WARNINGS/ADVISORIES

None.