textproduct: Central Illinois

KEY MESSAGES

- A high pressure system will result in low humidity and seasonable to slightly above normal temperatures through Wednesday, with highs in the low to mid 80s. - A pattern change occurs late in the week, resulting in increasing heat, humidity, and storm chances (30-60%) Friday into next week. Afternoon heat indices could climb into the low to mid 90s at times.

DISCUSSION

Issued at 117 AM CDT Tue Jun 2 2026

*** THROUGH WEDNESDAY ***

06z/Midnight sfc analysis shows an expansive 1026-mb sfc high pressure over Ontario. This feature will sink south and drive the local conditions through Wed. In the upper levels, an Omega block persists, with a meridionally-oriented ridge axis west of IL, an upper low over MT, and a positively-tilted trough over the eastern US extending into the Ohio Valley. The proximity of this trough (and the cooler temps aloft associated with it) should lead to slightly cooler temps today, although I remain concerned about the persistent cool bias the NBM has exhibited over the last week.

On Monday (yesterday), temps once again wound up warmer than forecast, while dewpoints dropped much lower than guidance, falling into the 30s across northern IL. The dry, NE flow regime will persist today, so raised temps a few degrees (low to mid 80s) and made an effort to trend dewpoints lower. These forecast edits now have dewpoints near 40F along/north of I-74 this afternoon. That will result in minimum RH values dropping into the 20-30% range, but wind gusts less than 20 mph should mitigate elevated fire risk.

The sfc high will continue to shift south on Wed, resulting in a SE component to the wind. This may result in temps and dewpoints being incrementally higher than today, but overall similar conditions can be expected. Precip is not expected today or Wed.

*** LATE WEEK PATTERN SHIFT ***

A pattern shift is still anticipated during the latter half of the week as the upper ridge becomes flattened. An upper high and the associated sfc high pressure still persist over the SE CONUS, while the northern branch of the jet stream becomes zonal and embedded waves become more progressive. The location of the sfc high in the SE US favors moisture advection into IL, gradually boosting dewpoints from the mid 50s on Thurs to near 70F by Sun/early next week.

The building instability combined with the progressive northern jet stream results in daily shower and storm chances, beginning Fri PM and continuing into early next week. The NBM probability of at least 0.25" of rain Friday through Sunday night is 50-80% area-wide. The severe storm potential will need to be monitored, but for now deep layer shear continues to be the most questionable ingredient, with guidance generally outputting values less than 30 kts. Machine learning guidance lowered the severe probs compared to the previous forecast.

Combined with above normal temps, the increasing humidity pushes the forecast heat indices into the low 90s by Sun/Mon. While below traditional heat advisory criteria (heat index around 105F), the multi-day nature of this heat, and the fact that it is the first such event of the year, could lead to impacts for sensitive populations. The presence of showers and storms does at least provide an "out", where one or more days in the extended could be cooler than forecast owing to cloud cover or precip being more extensive than currently resolved by global models. Probabilistic Heat Risk continues to focus the highest probs on Sun-next Tues, when there is a 40-70% chance of moderate heat impacts (sensitive populations) each day.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 436 AM CDT Tue Jun 2 2026

VFR conditions and east-northeasterly winds around 8-10 knots are expected through the period. Winds will weaken after sunset. Satellite imagery shows a thin layer of cirrus moving overhead currently. Aside from these clouds, mostly clear skies are expected through the 12z TAF period.

ILX WATCHES/WARNINGS/ADVISORIES

None.