textproduct: Chicago

KEY MESSAGES

- Dry and seasonably warm weather through Wednesday, then shower and thunderstorm chances return Thursday.

- A corridor of heavy rainfall may occur somewhere in the region Thursday-Friday, though confidence in local impacts remains low.

- Mainly dry, warmer weather expected this weekend.

DISCUSSION

Issued at 131 PM CDT Tue Jul 7 2026

12Z upper air analysis depicts a short-wave upper level ridge axis across northern IL, between a lingering mid-level trough over the lower Missouri/Ohio valleys to the south of more zonal westerly flow across the northern tier of the CONUS. A low- amplitude short wave within this zonal flow was evident in GOES vapor imagery across the upper Midwest, but is weakening/shearing as it encounters the aforementioned ridge. At the surface, weak high pressure was centered over our northwestern cwa early this afternoon, with plenty of sunshine allowing seasonable temperatures (low-80s warming toward the mid-80s), relatively modest humidity levels and light/variable winds. Farther to the south, into central IL, surface dew points remain in the mid-upper 60s making for somewhat more muggy conditions as well as contributing to a developing diurnal cumulus field. Despite the cu field, RAP forecast soundings depict an inversion ~800 mb which should prevent any convective updrafts from growing deep enough to support any precipitation. The surface high is forecast to drift southeast across the remainder of the forecast area tonight into Wednesday, maintaining dry weather conditions. Surface winds will turn southwesterly Wednesday, allowing slightly warmer temperatures in the mid-upper 80s.

Another slightly more vigorous short wave is progged to track east-southeast from the Northern Plains into the upper Midwest late Wednesday and Wednesday night. Height falls associated with this wave will erode the northeastward extent of the mid-level ridge across our region, with faster westerly flow to settle across the forecast area by early Thursday morning. Meanwhile, an associated surface low will track across MN/WI into northern lower MI during that time, trailing a cold front back across WI/IA/NE. Return flow ahead of this front will supply renewed warm/moist advection into the Plains and upper Mississippi Valley during the period, which in turn is expected to result in developing convection along the front to our west/northwest Wednesday night. Current 12Z CAM guidance indicates the bulk of this will occur well north/west of the cwa, though there is some potential for decaying storms to work into the IL/WI border area by sunrise Thursday morning especially if a significant outflow boundary develops.

Greater shower and thunderstorm chances appear to be Thursday afternoon into Thursday night however, as the approaching cold front slowly sags southeast across the forecast area. There continues to be a signal in various guidance for a more amplified short wave (potentially convectively-enhanced from an MCS across the Plains late Wednesday) to track east across the area in this Thursday-Thursday night period. Given increasing low-level moisture (dew points progged into the low-70s) and enhanced mid-level westerly flow (40-45 kts depending on model - and the potential to be stronger if an upstream MCV develops), the balance of instability and shear would likely support some increased risk of severe storms. In addition, forecast precipitable water values of around 2" would also support the potential for locally heavy rainfall, which we certainly don't need in the wake of recent heavy rains. SPC and WPC both have portions of our area in marginal (level 1 of 5/4) outlooks for severe weather and flood potential respectively. At this time, there's still quite a bit of spread in guidance with respect to the mesoscale features which will ultimately affect the details, so we'll have to continue to monitor things.

Based on current timing in guidance, the surface cold front is progged to gradually shift south of the forecast area through Friday-Friday night. This looks to shift thunderstorm potential off to the south, though perhaps still across the southern half of our forecast area. From the weekend into early next week, there is decent global ensemble guidance agreement in rebuilding of upper ridging across the central CONUS and upper Midwest. Initially, this supports surface high pressure across the Great Lakes region, though with a return of some 90-degree heat (though likely with some lake-cooling). Eventually however, models weaken the eastern extent of the ridge in the Monday- Tuesday time frame and indicate a trend toward a pattern of northwest flow aloft which we'll have to watch for "ring of fire" MCS potential by the end of the period.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1232 PM CDT Tue Jul 7 2026

Expect VFR conditions through the period. A lake breeze currently crossing MDW will edge westward across ORD over the next hour or two, with a chance the boundary slows or even stalls over the airfield mid-afternoon. Winds will remain VRB but may favor NW under 10 knots ahead of the lake breeze before shifting ENE/E up to 10 knots with the passage of the lake breeze. For ORD/MDW/GYY, winds will steadily veer SE around 5 knots this evening, S around midnight, and SSW overnight before settling SW around 10 knots on Wednesday. For DPA and especially RFD, the lake breeze is unlikely to cross the terminals, leading to W winds though sunset, VRB winds this evening, and SSW/SW winds overnight through Wednesday.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Air Quality Alert from midnight tonight to midnight CDT Wednesday night for INZ001-INZ002-INZ010-INZ011.

LM...None.