textproduct: Central Illinois

KEY MESSAGES

- Persistent heat and humidity will continue to build through Saturday. Heat indices peak around 100F today and tomorrow and 100-105F Saturday.

- Isolated to widely scattered thunderstorms (20-40% coverage) are possible today and tomorrow, mainly during the afternoon and evening hours. Locally heavy rain is the primary concern.

- A cold front brings a potential for strong to severe thunderstorms Saturday afternoon and evening.

DISCUSSION

Issued at 126 PM CDT Thu Jul 16 2026

Synoptic Overview... This afternoon, surface analysis reveals a split high-pressure regime with centers located over the Great Lakes and the lower Mississippi Valley. A neutral point lies between these two features across central Illinois. Aloft, a robust 590 dam high is anchored over the Desert Southwest, establishing a persistent northwest flow downstream across the Great Lakes region.

Heat and Humidity... Near-term concerns focus on persistent heat and humidity. An 850mb thermal ridge, currently observed near 25C over the northern Great Plains, is progged to drift eastward across the Upper Midwest on Friday before settling over central Illinois on Saturday, where temperatures aloft will ease slightly to around 20C. Daytime highs will remain near 90F today and tomorrow then into the low 90s Saturday under the thermal ridge axis. Given dew points in the lower to mid-70s, heat indices are expected to peak between 100-105F Saturday afternoon.

Convective Outlook... For today and Friday, moderate instability with MLCAPE values around 1500 J/kg will be present. In the absence of large-scale forcing and with weak deep-layer shear (<15kt), the severe threat remains low. However, isolated to widely scattered thunderstorms will be possible during peak heating under weakly or uncapped conditions. Slow storm motions of less than 15 mph present a risk for localized heavy rain; HREF LPMM guidance suggests potential for rainfall totals in excess of 2 inches. In addition, non-supercell tornado parameter values around 1 are currently observed within the area of ongoing developing convection south of I-72 suggesting a few brief funnel clouds may be possible.

Saturday, a robust shortwave trough is expected to dig across the Great Lakes, driving an attendant surface cold front south across central Illinois. This boundary will interact with strong instability, with MLCAPE progged to exceed 3000 J/kg. While deep-layer shear remains marginal (20-25kt), the timing of the front appears favorable for peak diurnal heating, potentially supporting stronger storm development Saturday afternoon and evening.

Extended Forecast... Surface high pressure builds over the Great Lakes on Sunday behind the departing cold front, providing a brief window of near-normal conditions with highs in the mid-80s. The frontal boundary is expected to lift north across the region on Monday. While placement uncertainty exists at this range, a strengthening low-level jet could drive a nocturnal MCS along the boundary Monday evening and overnight. By mid-week, a stronger Canadian high will build south across the northern Great Plains and Upper Midwest, ushering in drier, slightly below-normal conditions for the latter half of the week.

Wildfire Smoke... Canadian and Upper Midwest wildfire activity continues to produce a considerable smoke plume across the Great Lakes into New England. Current HRRR near-surface smoke guidance indicates the southern edge of this plume may graze the I-74 corridor tonight before shifting north on Friday. While confidence in significant local impacts remains low, we will monitor trends closely to determine if current Air Quality Alerts need to be expanded southward. A secondary, more favorable window for smoke advection into central Illinois appears possible Saturday behind the passing cold front, though guidance suggests this may be in a lower concentration than is currently being observed to our north today.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1212 PM CDT Thu Jul 16 2026

High pressure over the region is resulting in light and variable or calm winds today and tonight. A few isolated to widely scattered pop-up showers and storms are possible this afternoon into the evening, with the best chances along and south of the I-72 corridor. Winds will set up out of the west by mid to late Friday morning.

ILX WATCHES/WARNINGS/ADVISORIES

None.