textproduct: Central Illinois

KEY MESSAGES

- Heavy Rain & Localized Flash Flooding: Repeated rainfall along a stalled front presents a localized flash flood threat early this morning through midday today north of the boundary. Localized pockets of rainfall totaling 3 to 7 inches are possible, primarily across central Illinois along and north of I-72.

- Isolated Strong Storms: Isolated strong thunderstorms may produce localized damaging winds this afternoon and evening near the I-72 corridor, with a low-end threat continuing Thursday and Friday.

- Dangerous Heat & Humidity: Sweltering conditions will continue south of I-72 today, where high humidity will drive heat indices to 100-104F.

- Unsettled Pattern & Temperature Contrast: Daily shower and storm chances persist into early next week alongside a sharp contrast between 90s south and 80s north, before cooler air starts to filter in late this weekend into early next week.

DISCUSSION

Issued at 1258 AM CDT Wed Sep 16 2026

SHORT TERM... (Today through Thursday) A stalled surface boundary currently positioned between the I-72 and I-70 corridors stretching west-southwest into central Missouri remains the main driver for active weather across the region. Elevated convection continues early this morning north of the boundary, sustained by broad southwesterly flow aloft tapping into 1500 J/kg MUCAPE (rooted between 800-900 mb). While the severe threat remains low early this morning in this elevated regime, training convection parallel to the surface front presents a localized heavy rainfall hazard. HREF LPMM guidance highlights a broad 1-3 inch swath of rainfall across central Illinois north of the front, with localized high-end pockets between 3-7 inches. With 1-hr Flash Flood Guidance (FFG) around 2.0 inches and 3-hr FFG between 2.75-3.00 inches, an isolated Flash Flood Warning or two remains possible through midday today.

By this afternoon, the frontal boundary is expected to inch northward, becoming oriented along or near the I-72 corridor. Strong diurnal heating within the warm sector will yield MLCAPE values peaking around 2500 J/kg. Deep-layer shear remains somewhat marginal (0-6km shear vectors of 20-30 kt, peaking near 30 kt along the northern edge of the instability axis), but parameter space will support isolated strong pulse convection this afternoon and evening.

A sharp thermal gradient will span the area today: temperatures will climb well into the 90s south of I-72, where low-to-mid 70s dewpoints will produce peak heat indices between 100-104F. A few isolated spots may touch Heat Advisory criteria (105F), but low confidence in widespread coverage precludes heat headline issuance at this time. Further north, temperatures will trail off through the 80s, with upper 70s clipping northern counties (Knox through Marshall).

LONG TERM... (Thursday Night through Next Wednesday)

A strong Canadian high tracking from Manitoba (Thursday) to Quebec (Saturday) will initially push the surface boundary south across central Illinois before southerly return flow nudges it back northward late in the week. The persisting frontal boundary over the region will maintain daily precipitation chances alongside a sharp north-to-south temperature contrast (mid 90s south, low 80s north). Strong diurnal destabilization combined with modest deep-layer shear will keep a low-end threat for a few stronger storms in place Thursday and Friday.

A broader synoptic pattern change arrives late weekend as the upper-level ridge centered over the southern/southeastern CONUS breaks down. An upper low tracking across the northern Great Plains into the Upper Midwest will sweep another round of showers and thunderstorms through the region on Sunday. Cooler air filtering in behind this system will finally push 90-degree temperatures south of the area through the first half of next week while precip chances gradually become more muted in the drier air mass..

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/

Issued at 558 AM CDT Wed Sep 16 2026

Multiple waves of showers and thunderstorms currently stretch from central Illinois back across northern MO into northeastern KS. These will continue to shift east-northeast across portions of central Illinois this morning, but should diminish in coverage through the late morning and afternoon. Expect primarily MVFR ceilings this morning, though conditions will be variable, bouncing between IFR and VFR at times over short periods. Winds will generally favor an easterly direction today, but may also be variable at times due to convective outflow. Another round of showers and storms is expected to drop southeast into central Illinois late tonight into Thursday morning.

ILX WATCHES/WARNINGS/ADVISORIES

None.