textproduct: Chicago

KEY MESSAGES

- Showers and storms are expected today with the highest coverage southwest of a line from Rockford to Joliet to Rensselaer, and then areawide on Monday. - The strongest storms today and tomorrow will be capable of producing localized flash flooding and wind damage.

- Heat and humidity will build Tuesday, Wednesday, and possibly into Thursday with peak heat indices in the mid to upper 90s. - There is a low (20%) chance for thunderstorms Tuesday and Wednesday followed by a threat for severe weather on Thursday.

DISCUSSION

Issued at 248 AM CDT Sun Jun 7 2026

Early morning water vapor imagery depicts short-wavelength upper-level ridging across the Midwest between aggregate troughing over the Pacific Northwest and northeastern United States. A broad upper-level low is slowly lifting northeastward from the Southern Plains toward the middle Mississippi River Valley toward the center of the ridge. Closer to the ground, an elongated stationary front is present from the northern Plains through the Lower Great Lakes. The position of the front mirrors the shape of Lake Michigan across north central Illinois and northwestern Indiana thanks to onshore flow forced by a surface high pressure system centered over Lake Superior.

Over the next 24 hours, the aforementioned surface high pressure system will meander eastward into southern Ontario leading to a east-southeasterly wind direction across the Great Lakes. The stationary front will hence remain more or less in the same place today, stretching from near Rockford to Joliet to Rensselaer. Southwest of the front, continued muggy conditions (dew points in the lower 70s) will facilitate easy-to-achieve convective temperatures around 80 degrees altogether forcing a diurnal flare in afternoon showers and thunderstorms. Poor lapse rates (near 6 K/km) tied to tropical-like moisture (PWATs near 2 inches) amidst meager shear in the convective layer (around 25kt) will promote slow-moving spatially-small cells capable of torrential downpours and wet microbursts. More specifically, the strongest storms will be capable of producing rain rates will exceed of 2 inches per hour, localized flash flooding, and highly localized wind damage. Meanwhile, it is shaping up to be a fairly pleasant and dry day northeast of the front (including across much of the Chicago metropolitan area) with relatively lower humidity values and highs in the mid 80s (upper 60s to lower 70s lakeside).

Tonight, composite outflow from afternoon convection and a synoptic turn of the winds to adopt a more southerly component should allow for the frontal boundary to lift northeastward. As a result, muggy conditions are poised to spread across the remainder of the area by daybreak Monday. The core of the upper- level low will then pass overhead throughout the day, leading to another diurnal flare of slow-moving showers and thunderstorms. Like today, poor lapse rates, high column moisture values, and meager shear will promote torrential downpours and wet microbursts with the thunderstorms tomorrow. In other words, there will be renewed threat for localized flash flooding and wind damage. Should any slow-moving storm overlap with an urban area (Rockford or the Chicago metropolitan area), flash flooding could be locally significant (flooded roadways and viaducts). Finally, cannot rule out a few funnel clouds on Monday as the center of the upper-level low moves directly overhead. Highs on Monday will be in the upper 70s to lower 80s, which with dew points in the low to mid 70s will make it feel more like Florida than the Midwest. Showers and storms may continue even after sunset until the upper-level low finally pivots away from the area by daybreak Tuesday.

Tuesday through Thursday:

In the wake of the upper-level low, ensemble guidance depicts the reinforcement of upper-level ridging in the Midwest as troughing strengthens across the Pacific Northwest. The net result will be warming temperatures locally Tuesday and Wednesday with highs poised to return to the upper 80s to locally 90 (particularly on Wednesday). When combined with the (by this point) stagnant and humid airmass, heat index values will climb into the mid to upper 90s both days. It'll be the first taste of summer-like heat of the year.

At this point, our forecast favors Tuesday and Wednesday to be dry in our local area leading to little to relief from the heat. However, periodic upper-level shortwaves shedding from the aggregate troughing across the western United States may provide just enough ascent to break through strong capping at the base of an expansive EML plume across the Plains. While deep-layer shear will not be particularly strong into the Midwest Tuesday and Wednesday, the presence of an expansive reservoir of instability (nearly dry-adiabatic mid-level lapse rates atop seasonably most low-levels) and nightly low-level jets (terminating into the mid-Mississippi River Valley) does raise concern for any Plains convection to grow upscale into eastward- moving mesoscale convective systems (MCSs). In other words, any storms that develop upstream of our area would likely continue eastward into ocean of instability. With hints of such a threat depicted in a few model solutions (REFS in particular is bullish), will plaster low-end chance (20%) PoPs nearly continuously Tuesday and Wednesday (though keep in mind that equates to an 80% chance of dry conditions).

The hot and humid pattern looks to break down on Thursday as a stout upper-level shortwave "kicks" the aggregate Pacific troughing and and associated cold front eastward toward the Midwest. While the strongest upper-level flow looks like it will be displaced behind the cold front, seasonably strong low-level flow ahead of the front (925-700 mb flow increasing from 30 to 50 kt) within a strongly unstable airmass (MUCAPE > 3000 J/kg) does raise concern for an eastward-moving squall line with a threat for destructive winds in the general region. Temperatures and heat indices on Thursday will depend on the ultimate arrival timing of the front (our current forecast has it arriving during the evening meaning daytime hours may be hot and humid once again).

Borchardt

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 555 AM CDT Sun Jun 7 2026

Main Concerns:

- Potential for TS into RFD area and the far western Chicago metro later this afternoon into this evening.

- Low chance for TS late this evening through early Monday, with a higher chance by late Monday morning-midday.

Expectations are for primarily quiet conditions at the main Chicago area terminals through tonight/early Monday. RFD area continues to have the highest chance for TS, where a PROB30 was maintained. Still may need to consider a PROB30 for DPA with later issuances, informed by satellite trends this afternoon.

For ORD, MDW, and GYY, the most likely scenario is that the stable marine layer conditions prevent TS into the vicinity through this evening. TS coverage does look high enough this afternoon-mid evening south and west of Chicago for prolonged air space impacts. While SHRA/TS may approach at times tonight/early Monday, the next best chance appears to be by late Monday morning (PROB30 mention at ORD and MDW).

Easterly winds around 10 kt today will only gradually diminish tonight/Sunday night. Direction should favor east-southeast after daybreak Monday. Outside of SHRA/TS, expect VFR conditions until possible patchy MVFR CIGs Monday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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