textproduct: Central Illinois

KEY MESSAGES

- Air Quality: Smoke will thin this evening and tonight before a cold front brings slightly higher smoke levels once again Tuesday, keeping air quality in the moderate category.

- Severe Weather: Thunderstorms are expected tonight into Tuesday. While uncertainty remains regarding how far south the main line of storms will track, any severe storm could produce damaging winds and locally heavy rain.

- Late Week Outlook: Drier and slightly cooler conditions arrive Wednesday and Thursday, followed by a 20% to 30% chance of storms returning Friday.

UPDATE

Issued at 624 PM CDT Mon Jul 20 2026

Bow echo across Iowa has been pushing southeast the last few hours, and is approaching the Quad Cities this hour. Severe Thunderstorm Watch was recently issued for areas from about I-55 northwest. WoFS model guidance is in decent agreement with pushing this line into our northern CWA (Galesburg-Lacon), but is a little more sketchy on how far southeast it may make it. SPC mesoanalysis does have a fairly tight CAPE gradient over western Illinois at this hour, though it will work its way eastward with time. PoP's were updated earlier to increase rain chances over the northwest half of the forecast area for this evening.

DISCUSSION

Issued at 203 PM CDT Mon Jul 20 2026

Air Quality and Smoke: Smoke stemming from wildfire activity across Canada and the Upper Midwest continues to persist across portions of the Great Lakes and Midwest. Recent HRRR near-surface smoke guidance points toward a slight improvement in smoke concentrations as prevailing south/southwesterly flow overspreads the region, effectively shunting the plume northward. While EPA data indicates PM2.5 levels remain unhealthy for sensitive groups today, we anticipate a period of improvement this evening and overnight. As a cold front approaches tomorrow, smoke levels may see a slight increase behind the front, keeping AQI levels in the moderate category.

Severe Weather Outlook: Tonight and Tuesday: A cold front currently bisecting Minnesota is projected to push southeast across central Illinois overnight. Ahead of this feature, a decaying cluster of thunderstorms is tracking southeast, though southward propagation along the instability gradient--currently situated near our western CWA border--has kept the most robust activity just outside our region. We anticipate these storms will continue to weaken as they become outflow dominant.

Looking ahead, convective re-development is expected late this afternoon across the Upper Midwest, with these storms subsequently tracking southeast into central Illinois late this evening and overnight. A highly favorable parameter space for severe weather remains in place ahead of the front; latest RAP advertises MLCAPE values of 3000-4000 J/kg combined with 40-50 kt of deep-layer shear to our northwest, supportive of severe convective development upstream. The cold front continues to trend slightly slower, which implies that most of central Illinois will see storm arrival well after peak heating.

A modest southwesterly low-level jet will develop this evening, assisting in advecting instability into central Illinois. While mean steering flow generally drives storms from the west- southwest to east-southeast, Corfidi vectors suggest southward propagation into the region of better instability and LLJ forcing. However, confidence regarding the southernmost extent of this propagation remains low. Convective allowing models (CAMs) exhibit a significant spread; the HRRR suggests severe storms may skirt the I-74 corridor, whereas more robust solutions like the FV3 drive a solid line south to the I-70 corridor. We are currently discounting the NAMNest solution, which misses the local area entirely, as an outlier, but it further illustrates the uncertainty of the forecast.

The uncertainty in the evolution of tonights convection snowballs into Tuesday, leading to low confidence in the precise position of the effective cold front where storms are expected to redevelop. Strong instability is anticipated ahead of the front, and while redevelopment of storms near the I-70 corridor remains possible, the majority of models now favor this activity developing further south. Regardless of the exact location of midday redevelopment Tuesday, an attendant severe weather threat persists.

Extended Forecast: Wednesday through the Weekend: The remainder of the work week, beginning Wednesday, will be characterized by drier air and slightly below-normal temperatures. A strong Canadian high-pressure system will spread southeast across the Midwest, keeping highs generally in the low to mid 80s with no notable precipitation chances through Thursday. A signal for warm advection-driven storms exists for Thursday night off to our west, which may move into portions of central Illinois on Friday (20-30% chance). A warming trend will commence next weekend as the surface ridge axis shifts to our east, allowing for a return of warmer southerly flow.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1229 AM CDT Tue Jul 21 2026

VFR conditions will prevail at all TAF sites next 24hrs. No precip expected through the period either. Mid and high clouds are expected most of the night, but as the front drops south through the area, scattered CU is expected at all sites. Additional CU is expected behind the front during the morning and afternoon hours with high cirrus continuing. Clear skies are then expected during the evening hours. Winds will be southwest to start and then become more westerly through the overnight hours as the front moves south. Winds become west to northwest from morning through the end of the period once the front moves through. Speeds will be 10mph or less until after the front moves through and then all sites will see gusts of 20-25kts. Speeds lighten again during the evening hours.

ILX WATCHES/WARNINGS/ADVISORIES

None.