textproduct: Central Illinois

KEY MESSAGES

- Summer heat and humidity return Tuesday and Wednesday. Heat indices are forecast to exceed 100 degrees Wednesday afternoon, mainly south of Interstate 72.

- Rain and thunderstorms move in Wednesday and Wednesday night, bringing a 0.25 to 0.75-inch average rainfall. There is a 10% chance for localized rain totals over 1.0 to 1.5

DISCUSSION

Issued at 135 PM CDT Mon Sep 7 2026

Today through Tonight A welcome break from the recent heat wave continues the rest of today under the influence of a surface ridge centered over the Great Lakes. This high is maintaining a modest easterly breeze across central Illinois, drawing in a much drier airmass with dewpoints in the upper 50s to lower 60s. High temperatures this afternoon will top out in the low to mid 80s under mostly sunny skies, followed by a quiet and comfortable night with lows in the lower 60s.

Tuesday and Wednesday (Return of Summer Heat) The respite from the heat will be short-lived as summer-like conditions return. An upper-level low currently over the Pacific Northwest is projected to track eastward into the Canadian Prairies by Tuesday, flattening the strong central U.S. upper ridge and pushing its warm eastern periphery back across the Midwest. An associated 850mb thermal ridge (with temperatures climbing to 22-24C, reminiscent of the recent heat wave) will shift over central Illinois Tuesday, then progress into southern Illinois by Wednesday. Highs will surge back into the 90s across most of central Illinois on Tuesday.

Fortunately, southerly flow will not be established long enough by Tuesday afternoon for notable moisture return; deep boundary layer mixing will keep dewpoints relatively low (low to mid 60s), holding heat indices in check in the low to mid 90s.

Moisture returns in earnest on Wednesday as southwesterly flow strengthens ahead of an approaching cold front. This flow will transport a much muggier airmass into the region, boosting dewpoints into the low to mid 70s. Combined with temperatures in the low to mid 90s, heat indices are expected to exceed 100 degrees in several locations Wednesday afternoon, particularly south of the I-72 corridor.

Wednesday through Thursday (Cold Front & Storm Potential) The cold front is modeled to cross central Illinois mid-to- late Wednesday morning. Due to the timing near the diurnal instability minimum, instability will be weak (GFS MLCAPE values at or below 500 J/kg), and initial convection is expected to be in a decaying phase as it enters the area. However, the frontal boundary is projected to slow and stall as it moves south across the I-72 and I-70 corridors Wednesday evening into Wednesday night. A second round of convection is expected to develop along a modest low-level jet to our west and track eastward along the stalling boundary overnight. Total rainfall through Thursday is forecast to average 0.25 to 0.50 inches north of I-72/Danville, and 0.50 to 0.75 inches to the south. The 90th percentile guidance suggests localized training of storms could produce isolated streaks of 1.0 to 1.5 inches.

Friday through Sunday High pressure will build across the Upper Midwest behind the front on Thursday, gradually shifting east across the Great Lakes by Friday. While dry weather will prevail further north under the influence of this ridge, some precipitation chances will linger across central and southern Illinois in the vicinity of the stalled frontal boundary. The upper-level pattern is modeled to flatten further over the weekend, with low-amplitude waves in the zonal flow bringing periodic chances for showers and storms. Due to low predictability and model spread, broad- brushed 20-30% PoPs remain in place from Friday through early next week.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 610 PM CDT Mon Sep 7 2026

High pressure centered over the Great Lakes will spread into the Northeast US the next 24 hours. Winds will veer from easterly to southerly by tomorrow ahead of an approaching cold front. High clouds will increase in coverage through the period, but flight conditions will remain VFR.

ILX WATCHES/WARNINGS/ADVISORIES

None.