textproduct: Chicago

KEY MESSAGES

- Severe thunderstorms with the potential for damaging winds and locally heavy rainfall/localized flash flooding possible Monday night.

- Early autumn like cold front will bring much cooler temperatures, gusty winds, hazardous winds/wave on the lake, and refreshingly low humidity by midweek.

DISCUSSION

Issued at 222 CDT Sun Jul 19 2026

Satellite imagery does show smoke lingering across much of the western Great Lakes and mid-Mississippi Valley early this afternoon. Surface visibilities across our CWA have been largely in the 4-7SM range today with lower visibilities and denser near surface smoke across Wisconsin. Satellite imagery does show what appears to be an area of somewhat dense smoke upstream across central Lake Michigan and western lower Michigan which could advect into portions of our area this evening. While no where near as dense as the episode late last week, widespread smoke (with some variance in density) covers the region and will likely continue to do so locally tonight and into Monday morning. Gradually increasing southwest winds should push the brunt of the smoke northeast out over the Lake and into Michigan Monday.

High pressure in the wake of Saturday's cold front has pushed the deep, tropical moisture south into southern Illinois, with a drier and stable air mass across the western Great Lakes and upper Midwest. Water vapor imagery early this afternoon depict a vigorous shortwave trough and associated closed mid-upper level low moving east into Saskatchewan. Surface cyclogenesis associated with trough and resultant tightening southerly gradient and pressure falls is resulting in a rapid northward transport of low level moisture across the central and northern Plains.

Guidance is in good agreement with respect to synoptic features over the next 36 hours with a deepening surface low tracking east across Canada. By early Monday evening, an unseasonably strong 992mb surface low is progged to be over western Ontario with a dryline extending south-southwest from this low across western Wisconsin. This boundary is then progged to lay out more west to east across northern Iowa taking more of a cold frontal structure with greater thermal difference across the boundary. Strong mass response to this deep low will rapidly draw the very warm and moist air mass northward Monday, initially to our west, but eventually spreading east across our CWA late in the afternoon into the early evening. Seasonably cool mid level temps and moderately steep mid level lapse rates above this quickly moisture boundary layer should result in an axis of strong to extreme surface based instability in advance of the WI dryline and across northwest IL up to and even a bit north of the boundary across northern Iowa. The unseasonably strong upper trough will be accompanied by equally strong kinematic fields, resulting in a large area with strong deep layer shear, likely co-located with the extreme instability.

Synoptically, this set-up looks quite concerning for a potential outbreak of severe thunderstorms across the region. Curiously, the signal in model guidance with respect to convection tomorrow afternoon is mixed and not entirely clear. Strengthening low level jet and resulting strong low level theta-e advection looks likely to support the development of a nocturnal MCS overnight somewhere from northern IA north into central MN. HREF and RREFS suggest this activity will push east into WI Monday morning, and likely weakening/dissipating as it encounters an initially more stable air mass and as well as diurnal weakening of the low level jet. The signal in the CAMs is not overly suggestive of this MCS developing a large cold pool or leaving a significant footprint in its wake Monday.

If it were to leave a stronger cold pool, that could leave an effective boundary laid out east-west-ish to our north and that could become a focus for convective development. That scenario isn't really depicted in guidance and seems like it is on the lower end of the spectrum of probability. The more likely scenario seems to be intense convection developing along the dry line across western WI Monday afternoon, though given the strong cross-boundary shear, storms would likely be supercellular and prone to breaking off the dryline. This could lessen the threat of developing a could pool and QLCS from this activity, though should convection congeal, the southern flanks of it would have a tendency to propagate/bow southeastward into our CWA early Monday evening.

Secondary concern is for the pretty consistent signal among the various models in depicting a notable increase in convective QPF across northern IA east into northern IL Monday evening. This seems to be the result of strengthening synoptic ascent/upper level divergence in response to a 130kt+ 250mb jet translating east into the region. Most guidance suggests MUCAPE of at least 1500-2000 J/kg across our area Monday night, which given the strong shear profile would certainly support a threat for severe thunderstorms capable of damaging winds. There is still some spread in the guidance in how exactly this plays out, with some typically reliable models notably muted with convective coverage over northern IL Monday night. Certainly cannot totally discount solutions like this, so while the ceiling for potential impacts is fairly high, confidence is still somewhat on the lower end of the spectrum at this time.

The air mass over our CWA Monday night will be quite moist with PWATs nearing 2 inches. Also concerning is the potential for convection to lay out west-east across the area with fairly unidirectional flow oriented largely parallel to the synoptic boundary. So if this secondary area of convection does develop Monday evening into Monday night, then the potential would exist for some areas to pick up very heavy rainfall resulting in localized flash flooding.

Front will push out of the CWA early Tuesday morning with strong and gusty west-northwest winds in the wake of the front Tuesday. Air mass Tuesday will be somewhat less humid, but still warm with dewpoints in the 60s. A secondary cold front is expected to drop south out across WI and eventually into northern IL by early Tuesday evening. Given the lingering low level moisture and cooling mid-level temps, sufficient instability could develop to support scattered convection with this secondary front. The greatest coverage looks to be just to our north in WI Tuesday afternoon, but some threat of convection could get sneak into northern (especially northeast) IL by early Tuesday evening.

Guidance suggests that this secondary front will pack quite a punch for July with some early fall like characteristic. Strong and gusty northerly winds are expected Tuesday night into Wednesday behind this front, particularly near the lake where gusts over 30 mph appear likely. Cooler air flowing over the now quite warm lake waters will result in an unstable marine boundary layer and strong (for July) northerly winds down the full-fetch of the lake. This will likely result in unusually large and dangerous waves at southern Lake Michigan beaches Tuesday night through at least Wednesday night. Beach Hazard statements appear to be an almost certainty for that time period.

- Izzi

AVIATION /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 640 PM CDT Sun Jul 19 2026

Forecast concerns include...

Scattered thunderstorms Monday evening/Monday night. Gusty southwest winds Monday afternoon/evening. Wildfire smoke through Monday morning.

Northeast winds mainly under 10kts will slowly turn southeast later this evening and southerly overnight with some areas likely becoming light and variable. Southwest winds will increase into the 10kt range by mid/late Monday morning and then steadily increase Monday afternoon with gusts into the lower/mid 20kt range late Monday afternoon through early/mid Monday evening.

As the winds turn back southerly and southwesterly Monday morning, that should help push the wildfire smoke back northeast of the area. Until then, minor vis reductions into the 4-6sm range can be expected.

Thunderstorm trends for late Monday afternoon through early Tuesday morning remain very uncertain. Scattered thunderstorms are expected to develop across parts of WI Monday afternoon. Whether these move east into MI or evolve into a line of storms which then moves southeast into northern IL is unclear. If this were to occur, storms could be approaching the terminals by 01z-02z. Another possibility is for additional thunderstorms to develop across eastern IA and northwest IL during the evening, and move east/southeast through early Tuesday morning. Despite the low confidence, prob thunder mention is warranted for ORD/MDW for Monday evening with the new 30 hour taf period. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT Monday night for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103- ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT /1 AM EDT/ Monday night for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.