textproduct: Central Illinois

KEY MESSAGES

- The heart of the hot airmass will shift south of central Illinois today: however, heat index readings will still climb to near 100 degrees along and south of a Jacksonville...to Effingham...to Robinson line this afternoon.

- There is a Marginal Risk (level 1 of 5) for severe weather both Saturday and Sunday as a nearly stationary frontal boundary wavers across the region.

DISCUSSION

Issued at 314 AM CDT Fri Sep 18 2026

06z/1am surface analysis shows a stationary front extending from just north of Saint Louis to near Cincinnati. Winds have shifted to northeasterly across much of the KILX CWA: however, drier air is only slowly filtering into the area...as surface dewpoints are still in the 70s except across the far north around Galesburg and Lacon where readings have dropped into the upper 60s.

There will be two primary forcing mechanisms in the region today...the frontal boundary to the south and a short-wave trough tracking from the Dakotas into the Great Lakes. With central Illinois between these two features, it appears rain chances will remain low. Several CAMs indicate a few showers/thunderstorms associated with the short-wave may graze the northern half of the CWA along/north of I-72 this morning through early this afternoon. Meanwhile models such as the HRRR/RRFS have consistently shown isolated convection firing along the front from central/southern Missouri into southeastern Illinois this afternoon. Overall areal coverage remains low on all models, so have only mentioned 20 PoPs in most areas. The main weather story today will be the cooler conditions. A large north-south temperature gradient will exist...with highs ranging from the upper 70s far north around Galesburg to the middle 90s far south near Flora and Lawrenceville.

Another short-wave trough tracking along the northern periphery of the prevailing upper ridge centered over the Southern Plains will trigger renewed clusters of convection across Iowa into southern Wisconsin/northern Illinois late tonight into Saturday morning. While most of this activity will stay north of central Illinois, a few showers/thunderstorms may spread as far south as I-72. Will need to keep an eye on the severe weather potential for Saturday afternoon/evening as convection tries to re-develop along existing outflow from the morning activity. While surface dewpoints in the 70s will support moderate instability, deep layer shear will remain weak. The 00z HREF focuses the highest probability (40-60% chance) of SBCAPEs greater than 1000J/kg and 0-6km shear of more than 30kt along the I-74 corridor. While SPC indicates a Marginal Risk (level 1 of 5) for severe weather across all of central/southeast Illinois on Saturday, think the I-74 corridor will see the highest risk for a few storms with marginally severe wind gusts during peak heating.

With the primary baroclinic zone wavering across the region and additional short-wave troughs gradually flattening the upper ridge, rain chances will increase further south across all of central Illinois Sunday into Monday. Instability will be less on Sunday: however, a severe weather risk may materialize during the afternoon/evening mainly south of the I-72 corridor due to an uptick in shear as the NAM shows 0-6km values reaching 30-40kt. The front will sink well south of central Illinois by Monday, but additional short-wave energy will keep showers in the forecast. After that, high pressure will build in from the Northern Plains and bring cooler and less humid conditions through the end of next week. The coolest period will be Tuesday and Wednesday night when lows drop into the lower to middle 50s. The 00z NBM even hints at cooler readings...with a 30-50% chance of lows below 50 degrees everywhere north of I-70.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

Issued at 1219 AM CDT Fri Sep 18 2026

A cold front will slowly push south across central Illinois overnight with a modest northeast breeze and IFR stratus overspreading the terminals in its wake. Ceilings should lift/scatter back to VFR Friday afternoon. The front will lift back north with winds veering to east Friday afternoon then southeast Friday evening. A few spotty showers or storms cannot be ruled out, but low coverage/confidence precludes any mention in the TAF at this time.

ILX WATCHES/WARNINGS/ADVISORIES

None.