textproduct: Central Illinois

KEY MESSAGES

- Hot and mostly dry weather is forecast through mid week, with afternoon heat indices peaking above 100 degrees in west- central Illinois today and areawide Tuesday through Thursday.

- A Heat Advisory is posted from this afternoon through Thursday afternoon southwest of Marshall, Woodford, McLean, Champaign and Edgar counties.

DISCUSSION

Issued at 345 AM CDT Mon Aug 31 2026

Main weather story this week will be the late season heat wave, with afternoon heat indices reaching the upper 90s to around 100 today and Friday and 100-105 Tuesday through Thursday. We expanded the heat advisory ne to Stark, Peoria, Tazewell, Logan, DeWitt, Piatt, Douglas, Coles and Clark counties from this afternoon through Thu afternoon. We may eventually need to expand the heat advisory further ne over the remainder of CWA (Marshall, Woodford, McLean, Champaign, Edgar and Vermilion counties) from Tuesday through Thursday.

The 08Z/3 am surface analysis shows 1020-1022 mb high pressure over the Appalachians and mid Atlantic States. Meanwhile a frontal boundary extended from near the MN and Dakotas border into central Nebraska. A light SSE flow of warm and rather humid air mass was over IL. 3 am temps were in the upper 60s to mid 70s while dewpoints were in the low to mid 60s. Patches of mid/high clouds were in eastern IL and from Galesburg to Decatur ne and more widespread clouds in ne and northern IL where scattered showers were from Lacon to Hoopeston northeast and tracking ESE. A 30-45 kt sw low level jet extended from central KS ne over eastern Iowa, central/se WI and nw IL and helping support these showers. As sw low level jet wanes during the morning, these showers and isolated thunderstorms north of I-74 should diminish. Strong 500 mb high pressure ridge over the mid MS river valley to build northward with 500 mb heights rising to or above 594 decameters over CWA. 850 mb temps to rise to 22-24C this afternoon and warmest over western CWA. Skies to become partly to mostly sunny today over central/se IL and SSW winds 10-15 mph and gusts to around 20 mph to bring in the heat with highs 90-96F (warmest south of I-70 and from Springfield sw). Heat indices this afternoon peak in the upper 90s to around 100F, warmest readings near or just above 100F over western CWA and from highway 50 south in southeast IL.

Nocturnal low level jet to our north/nw tonight could spark some convection overnight into Tue morning but should stay along and north of I-80. GFS model is an outlying with convection getting as far south as I-72 late tonight and Tue morning. The strong mid/upper level ridge stays near IL Tue/Wed with temperatures getting a bit higher into the 90s over CWA and afternoon heat indices 100-105F (peaking around Wed). Will likely need to expand heat advisory further ne over rest of CWA during mid week. Mid/upper level ridge flattens on Thu and could see 20-30% chance of convection getting into northern/ne CWA by Thu morning. Highs Thu in the lower 90s ne CWA and mid to upper 90s sw CWA with afternoon heat indices 100-105F.

Forecast models continue to show 500 mb high pressure ridge retrograding westward into the Central Plains and MO valley late this week and this weekend with IL getting into a nw and possibly more unsettled flow. NBM has slight chances of convection over much of CWA overnight Thu night and Fri and again overnight Fri night and Sat. May see higher chances of convection during this time frame but too difficult to pin down in this pattern that far out. Does look like the heat wave eases slightly late this week into the weekend with more clouds expected and possible convection chances. Highs Fri in the lower 90s with afternoon heat indices in the upper 90s to near 100F. Highs of 85-90 on Sunday/Monday with dry weather expected as upper level heights rise again over IL with ridging from the Central Plains.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 626 AM CDT Mon Aug 31 2026

VFR conditions will prevail at all TAF sites next 24hrs. Scattered to broken mid clouds around 10-12kft will cover all sites today with the thicker clouds being at PIA and BMI. By evening, clouds will become more scattered or just clear. Winds will be south to southwest through the period with speeds less than 10kts now and again this evening. Daytime winds will be 10-15kts.

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 1 PM this afternoon to 7 PM CDT Thursday for ILZ027>029-036-037-040>044-047>056-061>063-066>068-071>073.