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 /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 100 AM CDT Thu Jul 9 2026

Forecast concerns for the terminals...

- 20-30% chance for isolated showers early Thursday morning, mainly near RFD.

- Scattered showers and thunderstorms expected Thursday afternoon and evening. Any of which could result in localized heavy downpours and reduced visibility.

- MVFR ceilings Thursday night into Friday morning with potential for patchy fog after midnight.

A busy weather and complicated TAF period ahead as several disturbances will be pivoting through northern IL and northwest IN. Prior to the arrival of the disturbances, increasing VFR clouds are expected tonight with light southwest winds that will become more westerly by Thursday morning. As we head into Thursday morning, the line of decaying line of showers and embedded thunderstorms (currently in IA and southwest WI) will be moving through northwest IL. With radar continuing to show these showers eroding suspect that mostly dry conditions will prevail through Thursday morning, but with an embedded convectively enhanced low being noted within the showers cannot fully rule out some isolated showers developing around daybreak and lingering through the morning (20-30% chance). Due to the lower confidence on coverage of any morning activity have opted to maintain VCSH at all TAFs with a PROB30 at RFD where confidence is slightly higher that something could materialize.

Regardless of what develops Thursday morning, scattered showers and thunderstorms are expected to develop along the outflow boundary from tonight's storms that should be draped over northern IL. While exact timing as to when the afternoon activity will kick off remains uncertain, latest forecast guidance trends seem to be leaning towards a 21-22z initiation with thunderstorms likely to persist through at least 01-02z as the boundary slowly drifts south through the terminals. Though, there is a chance that some showers could linger through the evening especially if the boundary is slower than forecast. For now have opted to maintain the broad VCSH mentions with targeted PROB30s for the better thunder windows at each location. Any showers/storms this afternoon will be capable of producing locally heavy downpours which would result in IFR visibilities and possibly even some ponding issues particularly near the saturated areas from last week's rains.

Showers and storms should taper after 03z as the boundary drifts south and the associated disturbance pivots east of the area. In the wake of the boundary a northeast wind shift will occur with speeds initially around 8-10 kts but those will be waning to around 5 kts after midnight. Ceilings will also lower to MVFR behind the boundary where they look to remain for the rest of the forecast period. While there is a signal for some IFR ceilings as well, confidence on their coverage is low so have opted to handle with a SCT012 mention for now. Furthermore, there is also a decent signal for patchy fog to develop near Lake Michigan and ooze inland overnight some of which could be locally dense. At this time the 30-hour TAFs will have a 6SM BR mention to tease the fog, but more refined TEMPOs may be needed for lower visibilities as confidence grows.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.