textproduct: Central Illinois

KEY MESSAGES

- Conditions are favorable for localized damaging wind gusts with thunderstorms this afternoon and evening, with chances greatest north of a line from Galesburg to Mattoon-Charleston.

- Warm temperatures and high humidity will result in heat index values from 100 to 105 degrees this afternoon. Portions of central Illinois may experience similar conditions on Monday and Tuesday.

- Wildfire smoke will begin moving into central Illinois from the north early Sunday morning. Most of the area will likely be affected by Sunday afternoon.

DISCUSSION

Issued at 253 AM CDT Sat Jul 18 2026

*** SEVERE STORM POTENTIAL THIS AFTERNOON/EVENING ***

A cold front connected to a surface low moving across southwest Ontario/southeast Quebec will move into Illinois later this morning, and become a focus for thunderstorm development this afternoon. Though 0-6 km shear is forecast to be in the 15-25 kt range which may limit storm organization, several parameters will be favorable for localized downburst potential. MLCAPE values are forecast to be near 3000 J/kg in a thin stripe just ahead of the cold front. In addition, DCAPE values in the 1100-1300 J/kg range just ahead of the front will support strong rain-cooled downdrafts. The 00z HRRR indicates multiple pockets of near-severe wind gusts in the 4 pm - 8 pm time frame, with the greatest coverage of strong wind gusts north of a line from Galesburg to Mattoon-Charleston. Storms should exit the forecast area and decrease notably in coverage by around midnight tonight.

*** HIGH HEAT INDEX VALUES THIS AFTERNOON AND EARLY NEXT WEEK ***

Current forecasts support heat index values this afternoon largely in the 100 to 105 degree range. There is some spread in short-term model forecasts of temperature and moisture, with the HRRR indicating some mixing out of dewpoints into the upper 60s ahead of the front this afternoon. Westerly winds around 10 knots ahead of the front, with current upstream dewpoints modestly lower, may lend some support to the idea of tempering the daytime rise in near-surface moisture but the affect will probably be limited and not as dramatic as the HRRR indicates. Going with a blend of inputs places heat indices today in a similar range to those observed yesterday. There is a 20-30% chance of heat index values exceeding the 105 degree threshold this afternoon, mostly west of I-55. Once convection develops and produces downdrafts/outflow, the heat index values should decrease from north to south.

Following a cooler day in the post-frontal airmass on Sunday, temperatures will warm again on Monday. As winds turn back to southerly in response to the next short-wave trough, temperatures will warm most quickly west of the Illinois River. Combined with high humidity, heat index values will again be in the 100 to 105 degree range. As the next cold front moves through on Tuesday, southern portions of the area will retain the heat the longest with high heat indices persisting roughly south of I-70.

*** WILDFIRE SMOKE SUNDAY ***'

Smoke from wildfires to the north is anticipated to head back toward Illinois as the post-frontal surface high settles southward on Sunday. Smoke transport models bring the smoke into northern portions of the forecast area around sunrise Sunday morning, potentially affecting much of central and southeast Illinois by late Sunday afternoon. There are differences between the smoke forecasts from the HRRR and RRFS, with the concentration being much less in the HRRR. It is worth noting that even if the greater concentration forecast by the RRFS is realized, we are not anticipating the degree of smoke experienced by those in northeastern Illinois yesterday. Current air quality forecasts are in the moderate category for tomorrow. Should greater impacts develop, they would most likely be in an area northeast of a line from Galesburg to Effingham.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 548 AM CDT Sat Jul 18 2026

The main driver for potential sub-VFR conditions through 12z Sunday is thunderstorms along a cold front this afternoon and evening. Confidence is fairly high in their occurrence, but as a somewhat broken line of storms is anticipated will stick with PROB30 groups until confidence increases regarding location. The strongest storms will have notable downdraft potential this afternoon and evening, particularly between 21z and 02z. Smoke should filter into Illinois early Sunday morning, with the greatest potential for lowering visibility at central Illinois TAF sites after 12z.

ILX WATCHES/WARNINGS/ADVISORIES

None.