textproduct: Central Illinois

KEY MESSAGES

- Dangerous Heat Wave Continues: Highs reaching the mid-90s to near 100 degrees will combine with lingering humidity to produce heat index values of 100 to 105+ degrees through Saturday. Highest probabilities (40-80%) for triple-digit heat focus south of I-74 through Wednesday.

- Isolated Storm Threat: A low probability (less than 20%) exists Thursday for an isolated pop-up afternoon or evening thunderstorm. While coverage will be sparse, strong instability could support an isolated severe storm with localized wind or hail impacts.

DISCUSSION

Issued at 1223 PM CDT Tue Sep 1 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 through Wednesday. This ridge axis is progged to shift toward the southeastern CONUS by late week. An 850mb thermal ridge continues to run well above the 90th percentile; with a 12Z morning sounding recording 23C, guidance continues to advertise 23-26C temperatures over the region through Saturday.

Surface temperatures will remain well above normal through the weekend. While dew points are expected to persist in the upper 60s to lower 70s, we are seeing a seasonal decline in low-level moisture contribution from evapotranspiration. This reduction, coupled with deep mixing into a drier column, will help keep dew points somewhat in check, though temperatures have a significant chance of reaching the 100-degree mark. NBM probabilities for hitting triple digits are decent across central Illinois south of I-74 through Saturday, with the highest probabilities (40-80%) occurring today and tomorrow. These probabilities lower slightly Thursday through Saturday (30-60%) for the southwestern half of the CWA. Daily heat indices will generally range from 100-105 degrees through Saturday. A Heat Advisory remains in effect until Thursday and will likely require an extension for at least portions of central Illinois.

Convective chances remain largely confined to northern Illinois and the Great Lakes, closer to the jet stream, through the remainder of the work week. However, an isolated afternoon/evening pop-up storm cannot be ruled out as surface temperatures approach convective thresholds daily. Given the lack of notable synoptic-scale forcing, the probability remains low (less than 20%), though some CAMs, such as the RRFS, highlight this potential. Should development occur, MLCAPE values of 2000-2500 J/kg and deep-layer shear of 25-30 kt Wednesday and Thursday would support stronger storm activity.

Slow Pattern starting this Weekend A closed upper low near Hudson Bay will dig toward New England by late week into the weekend, carving out a deep longwave trough over the eastern CONUS. This will shunt the upper ridge axis back west across the central Great Plains, transitioning central Illinois into a northwest flow regime. At the surface, Canadian high pressure building across Quebec will drive a backdoor cold front into Illinois, serving as the focus for precipitation chances over the weekend. Temperatures will moderate from the 90s ahead of the front to the lower-to-mid 80s in its wake. This transition appears slow, resulting in a temperature gradient from the low 90s in the southern counties to the lower/mid 80s in the northern CWA by early next week.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1158 AM CDT Tue Sep 1 2026

Central Illinois will remain between high pressure over the southeast CONUS and low pressure tracking slowly east across Ontario resulting in a modest SWerly breeze. Winds will pick up into the 8-12kt range during the daytime hours, and drop off below 10 kt during the nighttime hours. VFR conditions are expected through the period.

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 ======== ======== ======== ======== Bloomington 102/1894 101/2011 102/1913 Champaign 102/2011 101/2011 99/2011 Charleston 99/2011 103/1953 100/1925 Danville 101/1953 101/1953 99/1913 Decatur 102/1953 102/1953 102/1913 Jacksonville 103/1913 103/2011 101/2011 Lincoln 102/1953 101/1953 100/1913 Olney 105/1953 106/1953 105/1925 Paris 101/1953 102/1953 101/1913 Peoria 101/1913 100/1913 99/1913 Springfield 102/2011 101/2011 100/2011

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 7 PM CDT Thursday for ILZ027>029-036-037- 040>044-047>056-061>063-066>068-071>073. Heat Advisory until 7 PM CDT Thursday for ILZ030-031-038-045- 046-057.