textproduct: Chicago

KEY MESSAGES

- There remains a threat for severe weather this evening, but some areas may get missed by thunderstorms entirely.

- A series of cold fronts will lead to cooler temperatures by midweek.

MESOSCALE

Issued at 236 PM CDT Mon Jul 20 2026

MCS continues to push southward across western IL with extensive cloud cover across our CWA and even some light rain over about the southwest half of our CWA. Skies are quickly clearing in the wake of this MCS across southwest WI and eastern IA, with that clearing line likely to reach our northwest CWA by around 20z. Nearly full sunshine behind the MCS should allow for some heating and recovery of the boundary layer, but currently satellite imagery would certainly suggest strong MLCINH is in place given the complete absence of any cloudiness.

The band of convection has developed over southern MN in an area of transverse wave clouds in the wake of the morning MCS, suggesting that those storms are elevated. Along the western flank of that remnant cold pool boundary layer destabilization is quite evident in visible satellite imagery with some cumulus developing in an axis of strong convergence/confluence. Conceptually, it would make sense that the portion of the line of convection along the MN/IA border nearer to that confluence axis would be more likely to thrive/explode than the portion of the line farther east heading into the remnant cold pool.

Recent trends in the Warn of Forecast System (WOFS) guidance has been for more intense convection toward early evening to be progressively farther west. This could be an indication that the influence of the stable cold pool being better resolved by the model.

Not far behind this initial band of currently elevated convection, a cumulus field is developing along the actual cold front in MN into WI. This area seems prone to intense convective initiation later this afternoon as large scale forcing arrives. The big question is how far southwest that convection develops. Ultimately the convection should follow the front into northern IL this evening, but can't completely rule out a scenario where we get gapped between the storms over IA and the storms over WI.

Assuming storms get into northern IL and northwest IN, which seems like about a 60% chance at this point, then severe weather would certainly be a concern given the strong shear. Even with storms likely rooted about the boundary layer as they move across our CWA, significant elevated instability and strong effective shear would support a severe threat.

-Izzi

DISCUSSION

Issued at 236 PM CDT Mon Jul 20 2026

Tuesday...

RRFS and HRRR both have a signal for additional smoke concentrations back behind the leading cold front tonight, but there is a muted signal for any visibility reductions. Outside of this, a strengthening upper jet and colder temperatures aloft will promote cumulus development during the afternoon. A secondary and more noteworthy front will sweep southward across the Great Lakes during the afternoon hours possibly accompanied by low-topped showers and sprinkles, but not before temperatures recover back to the low to mid 80s. Northwesterly winds will increase and gust in the 20 to 25 mph range during the afternoon. The surface high, quite strong for mid July, will strengthen further Tuesday night and dip south across the upper Midwest. A sharp upper trough will pass over the Great Lakes and push a notably cooler and drier airmass into the lower Great Lakes region overnight into Wednesday. There is a limited QPF footprint on several guidance sources, and cloud depths may get just tall enough for some sprinkles or low topped showers later in the day into the evening, highest confidence into NW Indiana. This will set the stage for some seasonably comfortable overnight lows (in the 50s) for many areas outside the urban core.

KMD

Wednesday through Sunday...

Wednesday onward, deep cyclonic upper-level flow will only gradually shift eastward as a surface high pressure system meanders through the Upper Mississippi River Valley. The net result should be a period of pleasant conditions with no precipitation, highs in the upper 70s to around 80, and overnight lows in the upper 50s to around 60. Choppy waves will make swimming hazardous on Lake Michigan beaches Wednesday and perhaps into early Thursday.

Ensemble guidance depicts the redevelopment of an expansive upper- level ridge across the central United States toward next weekend which would encourage temperatures locally to climb back into the upper 80s to perhaps lower 90s. There is one ripple in the monsoonal moisture that should be able to progress toward the Ohio Valley on Friday, but most guidance sends any precipitation with this wave well south of the local area. There is still a signal for a convective event sometime in the July 27-29 timeframe in the general region with northwest flow in place on the northeast fringes of the strong upper ridge. This would pose the next chance of meaningful rainfall should this system move toward our latitude.

Borchardt/KMD

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1241 PM CDT Mon Jul 20 2026

Key messages for the 18Z TAF period (update at 236 PM CDT):

* Occasional MVFR vsbys this morning due to wildfire smoke

* A period of thunderstorms this evening and overnight

The complex of thunderstorms west of the terminals will continue to trek southeastward. A steady overcast sky with a few sprinkles will occur from KDPA westward. Winds will become a bit gusty from the southwest behind this complex. These winds will help mix out some of the wildfire smoke and haze for a period later this afternoon and evening.

Confidence has increased in a period of thunderstorms for many areas this evening, though we only have medium confidence in direct impacts to the Chicago terminals, as there is a play where storms pass by to the north and also the southwest. The highest confidence period of TS on station in the 3z-7z window, though trends will need to be monitored. Gusty winds will accompany the leading edge of storms, and not likely during the entire period.

A cold front will shift into NE IL late tonight, and will bring some additional smoke and haze, though current visibility progs suggest no worse than MVFR. Winds will also shift to W and then NW into Wednesday.

KMD

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT tonight 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/ tonight for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.