textproduct: Chicago

KEY MESSAGES

- Intermittent showers are possible at times through Thursday afternoon.

- Active pattern looks to bring stormy weather to the region this weekend into early next week.

DISCUSSION

Issued at 205 PM CDT Wed Aug 5 2026

A shortwave trough is traversing across southern IA with a cold front extending southwest IA into southeastern WI. Near the intersection of the shortwave and front is a cluster of showers and embedded thunderstorms that will continue to spread into northern IL and eventually northwest IN through the evening. While lightning coverage within the cluster has been rather limited, recent amdar soundings out of MDW do show some decent instability present which may support an isolated lightning strike or two through midnight. However, the main concern locally will just be scattered showers that at times could contain locally heavy downpours. Given the more scattered nature of the showers the threat for widespread flash flooding is low. Though, the combination of the slow front progression and the steering flow parallel to the frontal zone may result in some training of showers which could allow a localized flash flooding threat to materialize if more robust showers traverse over urban and/or poor drainage areas.

As the front slowly meanders across the area tonight, showers should taper from north to south. However, with the frontal boundary forecast to stall across the southern half of the area (namely near or just south of Pontiac to Valparaiso line) some showers will likely linger through the night and into the day on Thursday. That said, where rain does subside the lingering low- level moisture and light winds will create favorable conditions for fog development. With skies expected to remain overcast tonight fog coverage and intensity is uncertain, but with the strong signal in guidance have opted to introduce 20-30% fog coverage to the forecast for tonight mainly outside of the urban core of Chicago.

Aside from a 20-30% chance for some lingering showers and isolated storms Thursday afternoon south of the Kankakee River Valley, mostly dry conditions are expected for Thursday. Skies will unfortunately remain mostly cloudy through much of the day which will only allow temperatures to warm into the upper 70s to near 80. Clouds are expected to clear out from north to south Thursday night which may set up another favorable period of patchy fog. With fog coverage once again being a point of lower confidence have opted to forego a formal mention in the forecast at this time.

By Friday the bulk of the showers should be well south of our area leaving us with another dry and seasonably warm day with highs in the mid-80s. However, another shortwave trough is forecast to dive across the area Friday afternoon/evening which may result in a few isolated showers or storms. Forecast soundings Friday afternoon and evening do show some decent drying in the mid-levels which should keep a cap in place and therefore hinder shower/storm coverage. For now have opted to keep the 20-40% POPs offered from the NBM but suspect we will be able to lower these should these trends continue. Heading into the weekend, a strengthening upper ridge in the southwest CONUS will begin to amplify the northwest flow over the Great Lakes. As this occurs guidance hints that several shortwaves should be able to pivot through the pattern and result in periods of showers and thunderstorms locally. Currently the two favored timeframes for stormy weather are Sunday and again on Monday when the forecasted shortwave looks to be interacting with a frontal zone. Though, it should be noted that things could shift as we get closer so confidence on exact timing and coverage of thunderstorms is low at this time. As we get towards the middle of next week, the storm threat should gradually shift south of the area but again a lot of this will depend on how storms play out earlier in the week. Bottom line is that the early part of next week looks active so make sure to keep tabs on the forecast. Regardless, temperatures do look to remain seasonable (highs in the 80s and lows in the 60s) through at least early next week.

Yack

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 656 PM CDT Wed Aug 5 2026

Key aviation forecast messages:

- Downward trend in SHRA coverage this evening. Stalled surface front may lead to VCSH at times through Thursday afternoon.

- MVFR to near IFR CIGs late tonight/early Thursday AM with a period of reduced VSBYs in BR possible.

- Low confidence in wind directions on Thursday, dependent upon position of stalled surface front. Either SSW or NE.

The back edge of an area of lighter showers is moving across the Chicago terminals early this evening. Have introduced TEMPOs at MDW and GYY to account for this as it moves through over the next 1-2 hours. Looking ahead to the rest of the evening, the potential for additional showers has trended down, likely due to the local environment being worked over from earlier thunderstorm activity this afternoon. The surface front is expected to eventually stall somewhere within the area which may lead to additional isolated to widely scattered shower development late tonight through the day on Thursday. Confidence in whether this will occur directly over the terminals remains low. For now account for this with VCSH mention early Thursday AM through the afternoon.

Given an expected lower coverage of rain across northern Illinois the remainder of the evening, confidence in IFR (or lower) VSBYs in BR/FG developing late tonight have decreased as well. In fact, the drier hi-res guidance suggests there may be little in the way of VSBY reductions. However, given this is a newer trend, have opted to simply nudge the inherited VSBYs up by 1-2SM and will continue to monitor things through the evening and early overnight hours. MVFR to near IFR CIGs are still anticipated late tonight through much of the day on Thursday, lowest early Thursday morning.

Wind directions on Thursday will be highly dependent upon the position of the stalled surface boundary. If it sags farther south than expected winds at the terminals may end up northeasterly. Some hi-res guidance does have the surface boundary retreat northward slightly by mid morning and have accordingly maintained the inherited light SSW direction with this update. If the trend toward a more southerly position of the boundary continues wind directions may need to be switched to a northeast direction with later updates.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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