textproduct: Central Illinois

KEY MESSAGES

- Typical summertime heat and humidity will prevail today through Saturday with heat index values peaking 95 to 100 this afternoon and at or above 100 degrees each afternoon from Wednesday through Saturday.

- There is a low chance (20-30%) of showers and thunderstorms Thursday afternoon over central and southeast IL. Showers and thunderstorms are more likely on Friday especially during the afternoon. Gusty winds and locally heavy rains are possible with stronger cells. WPC/NBM has trended higher with between a quarter and half inch of rain on Friday over central and southeast IL but some areas will likely see over 1 inch of rain.

DISCUSSION

Issued at 300 AM CDT Tue Jul 14 2026

Patchy shallow ground fog recently develop over parts of central and nw IL. Latest CAMs like HRRR and RAP show fog formation more likely over the IL river valley into early morning. But we are starting to see 2-4 mile vsby at SPI and Paris so will expand mention of patchy fog further se. This shallow/ground fog should lift quickly by mid morning. Will likely see a repeat of patchy fog development over central IL north of I-70 overnight into early Wed morning.

A Rex Block pattern featuring a strong upper level high pressure ridge over the Upper Midwest into the southern Great Lakes and Ohio river valley, and weak troughing over the Southern Plains into the Deep South will dominate the weather over IL during this work week. Another heat wave is unfolding for IL the rest of this week especially Wednesday through Saturday. Temperatures will reach the upper 80s and lower 90s today with heat indices reaching mid 90s to near 100 this afternoon (highest over the IL river valley). Highs 90-95 Wed through Saturday with heat indices in the upper 90s and lower 100s each afternoon. A few spots could see heat indices approach 105 degrees from Wed-Sat afternoon, but currently feel we are shy of heat advisory criteria of 105-109F. Though if we get 4 or more days of heat indices of 100-105, that could translate into a heat advisory. The NWS HeatRisk climbs into the moderate category north of I-72 this afternoon and across much of central IL Wednesday, and into southeast IL too Thursday through Saturday afternoon. The IL river valley is forecast to reach major category Thursday through Saturday afternoon. People who are particularly sensitive/vulnerable to heat such as the elderly and those with health issues should therefore take precautions.

The upper level ridge starts to weaken a bit on Thursday with short waves tracking ESE over southern/se Canada with troffing into eastern Canada and New England. We may see isolated convection as early as Thu afternoon over CWA with light return south to sw flow bringing in more moisture (PWATS rising to 1.8-2.2 inches Thu afternoon, though better chances appear to be in sw IL and over MO where CAP is weaker. The best chances of showers and thunderstorms this week over CWA appears to be on Friday, especially Fri afternoon/early Fri evening with a weak disturbance moving in with increasing moisture. PWATS forecast to be 2-2.4 inches on Friday and with weak steering winds, expected locally heavy rains possible with thunderstorms Fri afternoon. Also a few stronger cells could have gusty winds/wet microbursts. Best convection chances shift east of IL on Saturday though we still have low chances of showers and thunderstorms with 20-40% Sat afternoon (highest near the IN border).

More northern stream short wave deepen the upper level trof over the northeast third of the US this weekend with upper level ridge retrograding westward into the Rockies and High Plains. A backdoor cold front coming out of the Great Lakes region late this week could push into central IL on Sunday and Sunday night and would keep a chance of showers and thunderstorms around. Temperatures trending closer to normal next Mon/Tue with highs in the mid to upper 80s and dewpoints slipping into the 60s.

The latest Climate Prediction Center (CPC) 8-14 day Outlook for July 21-27th has temperatures trending near normal over IL, while precipitation is trending near normal in western IL and 33-40% above normal in central and eastern IL.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 520 AM CDT Tue Jul 14 2026

Patchy shallow ground fog could produce MVFR vsbys through 13Z. HREF has 50-60% chance of vsbys below 3 miles at SPI early this morning, which they have been as low as 3/4-1 mile a few hours ago. More patchy ground fog could develop during overnight into early Wed morning though coverage appears too limited to mention in TAFs at this time. GFSLAMP guidance more aggressive with fog at PIA overnight while HRRR and RAP are very limited with vsby restrictions at central IL airports. Aside from the early morning fog potential, VFR conditions should prevail with few diurnally driven cumulus clouds with 3-5k ft bases between 15Z and sunset. Nearby 1023 mb high pressure over southern IA and northern MO and ridging into the IL river valley, to stay near central IL next 24 hours, keeping winds light. Winds ENE around 5 kts during the day and light and variable early this morning and tonight after sunset.

ILX WATCHES/WARNINGS/ADVISORIES

None.