textproduct: Central Illinois

KEY MESSAGES

- High temperatures and humidity will bring heat index values between 100 and 105 degrees Saturday afternoon. There is a 30% to 50% chance for heat index values to exceed 105 degrees west of Interstate 55.

- Thunderstorms will bring a risk of localized heavy rainfall exceeding 2 inches this afternoon. On Saturday afternoon and evening, a passing cold front brings a threat for severe storms.

- Wildfire smoke will move into the region from the north after midnight Saturday night. Expect smoke to impact most of central Illinois by Sunday afternoon.

DISCUSSION

Issued at 120 PM CDT Fri Jul 17 2026

Synoptic Overview... Surface analysis this afternoon depicts high pressure centered over the lower Mississippi Valley, with a ridge axis extending northeast into the Appalachians. Aloft, a robust 590 dam high remains anchored over the Desert Southwest, maintaining a persistent northwest flow pattern downstream across the Great Lakes region.

Heat and Humidity... Near-term concerns continue to focus on a period of building heat and humidity. An 850mb thermal ridge of 20-25C will shift from the northern Great Plains into central Illinois by Saturday. Following daytime highs near 90F today, temperatures are expected to reach the low 90s on Saturday. With dew points remaining in the low to mid-70s, peak heat indices are projected between 100-105F Saturday afternoon. HREF probabilities for exceeding 105F heat indices are 30-50% along and west of I-55, with lower probabilities to the east. At this juncture, confidence in reaching Heat Advisory criteria is insufficient for headlines, though trends will be closely monitored should heat potential increase.

Convective Outlook... For this afternoon and evening, moderate instability with MLCAPE values near 2000 J/kg is present. In the absence of strong large-scale forcing and given weak deep-layer shear (<15kt), the severe threat remains low. Isolated to widely scattered thunderstorms are possible during peak heating. Slow storm motions (less than 15 mph) present a risk for localized heavy rain, with HREF LPMM guidance highlighting potential pockets of rainfall exceeding 2 inches.

On Saturday, a robust shortwave trough is expected to dig across the Great Lakes, driving a cold front south through northern and central Illinois. This boundary will interact with strong instability, with MLCAPE progged to exceed 3000 J/kg. While deep-layer shear remains marginal (25kt), the frontal timing appears favorable for peak diurnal heating, supporting the potential for stronger storm development Saturday afternoon and evening. Guidance has slowed slightly regarding frontal arrival, and the severe threat appears more focused north of Springfield to Paris line a result.

Extended Forecast... The cold front will settle along the Ohio Valley Sunday, as high pressure builds across the Great Lakes in its wake. This transition will provide a brief window of near-normal conditions, with highs in the mid-80s and slightly lower humidity levels as dew points dip into the upper 60s to lower 70s. The boundary is expected to lift back north across the region on Monday, serving as a focus for additional storms. While placement uncertainty remains, a strengthening low-level jet Monday evening should aid in increasing storm coverage, with shear and instability parameters appearing supportive of severe weather potential. A trailing cold front will push across central Illinois Monday night into Tuesday morning; while diurnal timing is less favorable for much of the area, we will monitor for potential redevelopment along the front in southeastern counties on Tuesday. By mid-week, a stronger Canadian high-pressure system 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 significant smoke plume across the Great Lakes into the mid-Atlantic coast. Current HRRR near-surface smoke guidance indicates the southern edge of this plume will shift north away from the local area today. Behind Saturday's cold front, the environment will become more favorable for smoke advection into central Illinois. REFS near-surface smoke guidance suggests smoke may begin to spread across the region from the north after midnight Saturday night, potentially covering most of central Illinois by late Sunday morning or early afternoon, albeit at concentrations much lower than we are currently observing to our north today.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1222 PM CDT Fri Jul 17 2026

Isolated to widely scattered showers and storms are expected this afternoon and evening, though low coverage and confidence of impacts on station precludes more than a prob30. Light southwest winds less than 10 kt today and tonight will veer to westerly Saturday and increase into the 10-12 kt range. Outside of precip, VFR conditions are expected through the period.

ILX WATCHES/WARNINGS/ADVISORIES

None.