textproduct: Central Illinois

KEY MESSAGES

- Hot and humid weather will prevail for the next several days, with peak afternoon heat index readings peaking 100 to 110 degrees through at least Friday.

- Rain chances will gradually return by the end of the week into the 4th of July holiday weekend. The latest NBM indicates a 40-70% chance for thunderstorms on July 4th.

DISCUSSION

Issued at 343 AM CDT Wed Jul 1 2026

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

07z/2am upper air analysis shows a prominent 595dm high centered over the Ohio River Valley while a 555dm low spins across Manitoba. 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 temperature forecast, suggesting readings in the middle to upper 90s in a few spots both today and Thursday. Based on consistency and a continued very moist boundary layer that will temper heating, have undercut the NBM and gone with highs in the lower to middle 90s both days. NAM/RAP forecast soundings show mixing up to about 5000ft this afternoon, which is 1000ft deeper than yesterday. This has the potential to bring slightly lower dewpoints down to the surface later in the day as evidenced by the RAP/HRRR showing readings mixing into the upper 60s. Think this may be a bit too extreme, especially considering the very moist ground conditions and actively growing crops. Have therefore used an NBM/CONSALL blend to nudge dewpoints down into the lower to middle 70s this afternoon as opposed to the upper 70s to near 80 degree readings from yesterday. Despite the lower dewpoints, corresponding peak heat index readings will still top out around 105 degrees.

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

A series of short-waves ejecting northeastward from the southern Rockies to the Great Lakes will flatten the prevailing upper ridge...resulting in a west-to-east zonal flow pattern across the Midwest Friday through Sunday. The first wave will interact with a nocturnal low-level jet streak to trigger an MCS across South Dakota/Nebraska Thursday evening...with the storm cluster then pushing mainly eastward into Wisconsin/northern Illinois by Friday morning. There is still some uncertainty regarding the exact track of the convection, but the synoptic pattern strongly suggests it will remain north of the KILX CWA. Have included 30-50 PoPs as far south as the I-72 corridor late Thursday night into Friday morning, but think the bulk of the precip will stay north. The big question for central Illinois will be where the residual outflow boundary lays out, as this will be the focus for convective re-development Friday afternoon and evening. The 00z RRFS shows the boundary firing along/north of I-72, and this seems reasonable based on the expected morning storm trajectory. Any afternoon storms that develop will have access to rich boundary layer moisture with surface dewpoints in the middle to upper 70s and corresponding strong instability with SBCAPEs in excess of 3000J/kg. The missing factor for organized severe weather will be wind shear, as the NAM/GFS show 0-6km bulk shear of just 20-25kt. The latest Day 3 SPC Convective Outlook highlights all of central Illinois with a Marginal Risk (level 1 of 5) for severe weather on Friday. Given the lack of shear and a strong forcing mechanism, the low risk is appropriate. Think the scattered cells that form Friday afternoon and evening will be capable of sporadic strong wind gusts as well as locally torrential downpours.

A second short-wave will trigger another MCS across the same area (South Dakota/Nebraska) Friday evening: however, this system may take a more E/SE track across north-central Illinois late Friday night into Saturday. As a result, the NBM has increased PoPs into the 40-70% range for July 4th. While wind shear will remain meager and the overall severe weather risk will likely stay low, am concerned that the storms will be efficient rain-producers and will produce pockets of very heavy rainfall. WPC now shows a Marginal Risk (5-15% chance) for excessive rainfall for both Friday and Saturday accordingly.

While the holiday weekend will not be a complete wash-out, central Illinois residents should keep a close eye on the forecast and be aware that some outdoor activities may be impacted. After that, the upper ridge will re-develop further west across the Desert Southwest. This will lead to weak downstream troughing over the Midwest/Great Lakes and a slight cooling trend. Rain chances will decrease substantially and high temperatures will drop back into the 80s by next Monday through Wednesday.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1250 PM CDT Wed Jul 1 2026

VFR conditions are forecast to continue across the central IL airports through 18Z/1 pm Thu. Aside from a few high/thin clouds and FEW-SCT diurnal cumulus clouds with 3-5k ft bases today and Thu, skies will remain mostly clear. SW winds will gust to around 20 kts this afternoon before decreasing to less than 10 kts by sunset and veering more south tonight. SW winds near 10 kts will prevail after 15Z Thu.

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.