textproduct: Central Illinois

KEY MESSAGES

- Hot and humid weather will prevail for the next several days...with peak afternoon heat index readings exceeding 105 degrees through at least Thursday.

- Rain chances will gradually return by the end of the week into the holiday weekend. The latest NBM indicates a 30-50% chance for thunderstorms on July 4th.

DISCUSSION

Issued at 235 AM CDT Tue Jun 30 2026

**** Hot and Humid Weather Continues ****

07z/2am upper air analysis shows a prominent 596dm high centered over the Ohio River Valley while a 558dm low spins across southern Saskatchewan. As the high remains essentially in place and the low lifts northeastward, the primary convective zone will focus from Nebraska to Michigan through Thursday. Meanwhile further south, central Illinois will remain high and dry with typical summertime heat and humidity. The NBM continues to be about 2-4 degrees too warm with its high temperatures forecast, suggesting readings potentially climbing into the upper 90s in a few spots both today and Wednesday. While there will be considerably less high cloud cover around and upper heights will remain high, copious boundary layer moisture will mitigate temps. As a result, am expecting highs mostly in the lower 90s today...then the middle 90s for both Wednesday and Thursday. The heat coupled with dewpoints well into the 70s will lead to heat index readings peaking at or above 105 degrees through at least Thursday. The heat will ease a bit by the end of the week due to decreasing upper heights and increasing cloud cover/PoPs.

**** Thunderstorms for the Holiday Weekend ****

A series of short-wave troughs ejecting from the southern Rockies into the Great Lakes will flatten the prevailing upper ridge axis currently over the Midwest, resulting in the return of flat west-to-east zonal flow by the end of the week. Once this happens, additional waves traversing the Plains will lead to increasing rain chances during the Friday through Sunday timeframe. The 00z Jun 30 model suite continues to suggest the first wave tracking through the region on Friday, albeit slightly further north than previously thought. With the main baroclinic zone still draped to the north, think the primary focus for showers/thunder on Friday will be along/north of I-80. The showers will be much less numerous further south into central Illinois, but have included 40-60 PoPs as far south as the I-74 corridor. Once the initial wave passes, it will nudge the frontal boundary into north-central Illinois on Saturday, setting the stage for scattered showers/thunder for July 4th. The latest NBM shows a 30-50% chance for thunderstorms during the afternoon and evening. Stay tuned to later forecasts for more details. After that, another wave and its associated cold front will pass through the region on Sunday...bringing more showers. As noted by the past few model cycles, all synoptic models feature the Ohio River Valley upper ridge flattening by the end of the week...then shifting/re-developing further west across the Desert Southwest by early next week. As ridging builds in the Rockies, weak troughing will form downstream over the Midwest...pushing the cold front through central Illinois Sunday night into Monday morning. The end result will be decreasing precip chances/temps by Monday as highs drop back into the middle to upper 80s.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1144 AM CDT Tue Jun 30 2026

VFR conditions will persist through this TAF cycle as an expansive heat ridge suppresses cloud cover and precipitation chances. The only failure mode could be patchy shallow fog around the 12z period, which may briefly lower visibility into the MVFR range.

Surface winds will remain from the south around 10 kts.

ILX WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until midnight CDT Thursday night for ILZ027>031-036>038-040>057-061>063-066>068-071>073.