textproduct: Central Illinois

KEY MESSAGES

- Extreme Heat: An Extreme Heat Warning is in effect through Friday as high heat and humidity push peak heat index values to around 105F. Highs may exceed 100F (40% to 80% chance) south of a Springfield to Casey line through Friday, with heat indices lingering near 100F into Saturday.

- Isolated Severe Storm Potential: While dry conditions dominate, there is a low risk (under 20% chance) for isolated pop-up afternoon or evening thunderstorm Thursday and Friday. Any storm that develop could become severe with localized strong winds or hail.

DISCUSSION

Issued at 142 PM CDT Wed Sep 2 2026

Late Summer Heat Wave Upper-level ridging remains dominant across the Midwest, anchored by a robust 596 dam 500mb high centered over the mid- Mississippi and Tennessee Valleys. This ridge axis is projected to shift toward the southeastern CONUS by late week. Guidance continues to advertise 850mb thermal ridge temperatures of 23-26C over the region through Saturday, well above the 90th percentile, following this morning's 12Z sounding of 22C.

Surface temperatures will run well above normal through the weekend. Dew points are expected to persist in the upper 60s to lower 70s, though deep mixing into a drier column and a seasonal drop in evapotranspiration will help keep them somewhat in check. Even so, temperatures have a solid chance of reaching triple digits. NBM probabilities for hitting 100 degrees are highest (40-80%) today along and south of a Springfield to Casey line, lowering slightly to 30-60% across the southwestern CWA Thursday and Friday. Peak heat indices will generally range near 105 degrees through Friday and around 100 degrees Saturday. Due to the prolonged period of oppressive conditions, the Heat Advisory has been upgraded to an Extreme Heat Warning through Friday.

Convective Risk & Storm Potential Precipitation chances will stay mostly confined to northern Illinois and the Great Lakes near the jet stream through the remainder of the week. With surface temperatures approaching convective thresholds daily, an isolated pop-up afternoon or evening thunderstorm cannot be completely ruled out, though overall probability remains under 20% due to nebulous and weak synoptic forcing. Models like the RRFS continue to highlight this pop-up potential. Any storms that manage to develop on Thursday would be supported by MLCAPE of 2000-2500 J/kg and deep-layer shear of 25-30 kt. MLCAPE increases to 2500-3000 J/kg on Friday, but noticeably weaker shear points toward a more pulse-severe mode.

Weekend Pattern Shift Late this week into the weekend, a closed upper low near Hudson Bay will dig toward New England, deepening a longwave trough over the eastern U.S. This pushes the upper ridge axis west over the central Plains and places central Illinois in a northwest flow regime. Meanwhile, Canadian high pressure over Quebec will push a backdoor cold front south into Illinois, providing a focus for weekend rain chances (20-30%). Temperatures will gradually cool from the 90s ahead of the boundary down into the lower-to-mid 80s behind it. Because this transition unfolds slowly, a thermal gradient across central Illinois will persist into the first half of next week.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1204 PM CDT Wed Sep 2 2026

Central Illinois remains between high pressure over the southeast CONUS and a stationary front over the Upper Midwest resulting in a modest south/southwest breeze locally. SW winds around 10 kt during the daytime hours will drop off below 10 kt and back to southerly during the nighttime hours. VFR conditions are expected through the period. There is a low chance (around 15%) Thursday afternoon and evening for a few pop-up storms over the area.

CLIMATE

Issued at 440 PM CDT Mon Aug 31 2026

Record high temperatures this week:

Location Tue 9/1 Wed 9/2 Thu 9/3 Fri 9/4 ======== ======== ======== ======== ======== Bloomington... 102/1894 101/2011 102/1913 100/1925 Champaign..... 102/2011 101/2011 99/2011 100/1954 Charleston.... 99/2011 103/1953 100/1925 98/1925 Danville...... 101/1953 101/1953 99/1913 101/1954 Decatur....... 102/1953 102/1953 102/1913 103/1913 Jacksonville.. 103/1913 103/2011 101/2011 103/1899 Lincoln....... 102/1953 101/1953 100/1913 102/1954 Olney......... 105/1953 106/1953 105/1925 103/1954 Paris......... 101/1953 102/1953 101/1913 104/1954 Peoria........ 101/1913 100/1913 99/1913 98/2015 Springfield... 102/2011 101/2011 100/2011 100/1954

ILX WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 7 PM CDT Friday for ILZ027>031- 036>038-040>057-061>063-066>068-071>073.