textproduct: Chicago

KEY MESSAGES

- Thunderstorm chances return Thursday, with the potential for localized heavy rainfall Thursday afternoon and evening. Strongest storms may also be capable of localized gusty winds, mainly south of I-80.

- Mainly dry, warmer weather expected this weekend into early next week.

DISCUSSION

Issued at 237 PM CDT Wed Jul 8 2026

A warm, dry afternoon is underway across the forecast area as an area of weak surface high pressure drifts slowly southeast of the cwa. A few showers and isolated thunderstorms were noted across southeast WI, where west-southwesterly return flow has brought a return of low-70s surface dew points, though drier low-levels and warmer mid-level temps across our local area should provide enough of a cap to minimize any convective threat here for the remainder of the afternoon/evening. Shower and thunderstorm chances are expected to increase across northern IL toward daybreak Thursday however, as an area of thunderstorms currently developing across IA/MN/WI eventually approaches the area. High-res CAM guidance continues to be in good agreement that this area of convection will be in a weakening/decaying mode by that time, though may push an outflow boundary into the area after 1-3 AM. Given the diurnal instability minima during the early morning, guidance is not very excited about convective development along the remnant outflow, though isolated to widely-scattered showers and perhaps an isolated thunderstorm are possible generally along/north of the I-88 corridor especially by late morning as daytime warming eventually destabilizes the boundary layer.

Scattered thunderstorm chances do increase from midday Thursday into the evening, as the synoptic cold front slowly pushes southeast into the forecast area along with some support from an approaching mid-level short wave. This wave appears weaker than some model runs had been depicting over the past few days, resulting in weaker mid-level westerly flow (20-25 kts as opposed to ~40 kts in some runs previously). Overall weaker forcing, and relatively poor forecast mid-level lapse rates, appear to be responsible for the sparser convective footprint in current CAM guidance across the cwa. There is still some support in current CAM runs for a remnant MCV (from an MCS forecast to develop along the NE/KS border later tonight) to approach the area later in the afternoon/evening which looks to yield somewhat greater shower/thunderstorm coverage during that time. Similar to earlier runs however, the exact track (while perhaps a bit south of previous forecasts) remains of somewhat lower confidence. In any case, these slow-moving storms will provide a threat of heavy rainfall, with localized amounts of 1-2" possible. Weaker mid-level flow (Corfidi propagation vectors potentially 5-10 mph to the ESE), precipitable water values approaching 2" and deep warm-cloud depths all support the likelihood of torrential downpours with some of these storms. Soil moisture, river/creek levels and storm water retention facilities continue to run quite high in portions of the cwa where repeated heavy rains occurred over the past week - especially across portions of the central and southern Chicago metro area. This raises concern for any additional localized heavy rainfall to potentially cause renewed flooding. With this in mind, have collaborated with WPC to include appropriate portions of our forecast area in their Day 2 Excessive Rainfall Outlook in a slight (level 2 of 4) risk.

The surface cold front is progged to continue to work southeast across the forecast area Thursday night, eventually slowing further and becoming nearly stationary across downstate IL/IN on Friday. This should allow showers and lingering storms to diminish from NW to SE Thursday night, while surface high pressure builds across the upper Midwest. Drier east-northeast low-level flow will bring drier and more stable conditions to much of northeast IL/northwest IN Friday, while isolated/scattered showers and thunderstorm chances linger mainly off to the south of I-80 during the diurnally favored afternoon and early evening hours. Diurnal precip chances should continue to shift south over the weekend, as the Great Lakes surface high settles a little farther southward.

Longer-range ensemble guidance continues to depict an upper- level ridge building eastward across the upper Midwest/western Great Lakes region later this weekend into early/mid next week. This looks to support a period of dry and warmer weather conditions (highs in the low-90s in many spots) in the extended period. Unlike last week however, current ensembles indicate heat within a less humid air mass. As usual on the north/northeast periphery of an upper ridge, We'll need to continue to monitor the potential for thunderstorm clusters to slide into our area, especially toward the middle of next week as a short wave is progged to rotate across Ontario and parts of the Lakes.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 107 PM CDT Wed Jul 8 2026

VFR conditions will continue through much, if not all, of the period amid persistent SW winds averaging up to around 10 knots. There are a few windows for precipitation on Thursday, highlighted by TS potential late Thursday afternoon and evening.

First, a line of TS developing from northwest Iowa to northern Wisconsin this evening is expected to gradually decay with southeast extent overnight. SHRA may survive into far northern Illinois after daybreak, so have maintained PROB30 SHRA for RFD.

Next, the residual outflow boundary from the earlier storms should wash out across the area during the morning, but foster sporadic SHRA through at least mid afternoon. Have therefore maintained VCSH during this window, though it is feasible that the Chicago terminals remain completely dry during the morning and early afternoon.

Finally, a mid-level wave crossing southern Wisconsin Thursday afternoon will provide a renewed focus for TS develop over northern Illinois between the 19 and 02Z (particularly 21-01Z at ORD/MDW). There remains uncertainty in both coverage and location (TS may remain just south of the terminals), but any TS will be slow-moving and produce torrential rainfall. Also, any initial TS originating from southern Wisconsin could push an outflow boundary with a N/NE wind shift across ORD/MDW/GYY prior to or during any TS at the terminals.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002- INZ010-INZ011.

LM...None.