textproduct: Chicago

KEY MESSAGES

- There remains a 30% chance for a severe storm to develop before daybreak with damaging hail as the main threat.

- A series of cold fronts will move through the area today leading to a period of cooler temperatures tonight through Wednesday night.

- Large waves will make swimming dangerous at all southern shore Lake Michigan beaches this evening through early Thursday morning.

- A trend toward warmer and more seasonable temperatures is expected toward the end of the week.

DISCUSSION

Issued at 151 AM CDT Tue Jul 21 2026

Per recent GOES-19 water vapor imagery and a hand surface analysis, a 994 mb surface low pressure system is moving eastward through central Ontario early this morning. A cold front extends southward away from the center of the low pressure system through central Wisconsin and across northern Iowa. A ribbon of lower 70 degree dew points extends from the middle Mississippi River Valley northeastward toward the Door County Peninsula ahead of the front. Regional RAOBS and recent AMDAR soundings out of ORD/MDW depict an inversion between about 850 and 750mb above which lapse rates exceed 7 K/km. The resulting thermodynamic profile is supporting a broad region of MUCAPE >2000 J/kg ahead of the front early this morning. Meanwhile, an upper-level short-wavelength trough is racing into the Great Lakes region anchored by a seasonably strong upper-level jet streak (105kt at 300mb, per the 00Z MPX RAOB). Pressure falls on the order of 6mb/6hr in eastern Ontario ahead of the low pressure system are supporting a 925-850mb low-level jet across the Lower Great Lakes, as confirmed by 45 to 50kt of flow at 2km sampled by the KLOT VWP.

Over the past few hours, have been watching a roughly 50 mile wide area of percolating radar returns first develop in eastern Iowa and then cross into northwestern Illinois. The radar returns appear to be associated with isentropic ascent (via the LLJ) interacting with the base of the capping inversion and are prototypical of nocturnal attempts for thunderstorms to initiate within a steep mid-level lapse rate environment. Thus far, attempts for sustained convection have been subdued likely owing to synoptic-scale subsidence along the right exit region of the aforementioned upper-level jet streak centered in Wisconsin and on the northeastern flank of an expansive MCS moving into northern Missouri. However, do have concern that continued moistening at the base of the cap will eventually allow one or two storms to develop at some point early this morning, particularly in the 3 to 6 AM window over the Chicago metropolitan area as the upper-level shortwave makes its closest approach. Provided a storm does develop, strong effective shear (60kt in the convective layer oriented due easterly) would support splitting supercells with a threat for damaging hail. Will maintain a 30% chance of storms through the morning for this threat keeping in mind it equates to a 70% chance a storm doesn't develop.

The threat for storms will end by daybreak as the low-level jet peels eastward and subsidence increases in the wake of the upper-level short-wavelength trough. Winds will gradually turn northwesterly through the morning as the frontal boundary works through the area, though not before temperatures have an opportunity to bounce into the low to mid 80s. Later this afternoon, a secondary front will race southward down Lake Michigan leading to a northerly wind shift and more noticeable drop in temperatures. The southward advection of a seasonably cool airmass (850mb temperatures tumbling toward +10C) atop the relatively warm waters of Lake Michigan (near +23C) will set the stage for a rare July lake effect rain shower set-up along and behind the front along the northeastern IL and northwestern IN shoreline. In fact, would not be surprised for a waterspout or two to develop particularly along any focused convergence bands overnight. Waves will quickly build in the wake of the front leading to a High Swim Risk after sunset at all southern Lake Michigan beaches.

On Wednesday, an upper-level wave embedded in resident broad upper-level cyclonic flow will ripple over the Lower Great Lakes. Another bout of low-level cold air advection will keep temperatures seasonably cold with highs around 70 degrees. Can easily envision stratocumulus clouds filling the skies with perhaps a few sprinkles by late morning owing to steep low-level lapse rates outside of continued scattered lake effect rain showers. Continued northerly winds down the fetch of Lake Michigan will maintain dangerous swimming conditions at all Lake Michigan beaches through the entire day.

Wednesday night, the cyclonic shear axis of the aggregate upper-level troughing will shift eastward toward Lower Michigan, setting the stage for clouds to clear. Overnight lows will be cool if not locally chilly with a few upper 40s readings not out of question near the Wisconsin state line.

Our brief taste of fall during the middle of summer will be short-lived as large-scale upper-level ridging begins to rebuild across the central United States toward the end of the week. Accordingly, temperatures will incrementally increase Thursday onward with a return of highs in the upper 80s to lower 90s by this weekend. We continue to see a signal for a convective event in the general region sometime in the July 27-29 timeframe.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 615 AM CDT Tue Jul 21 2026

Key messages for the 12Z TAF period:

* Breezy northerly winds Tuesday evening into Wednesday.

* A low chance for light showers this evening and tonight, highest at GYY.

The predawn showers and storms have moved away for the day. Westerly winds will open up to gusts to near 20 kt by 12Z and continue building closer to 25 kt out of the SW by late morning. Winds will remain breezy through the evening and night as winds go northerly, favoring east of north after mid-evening and back to west of north by predawn Wednesday.

This evening and overnight, some lake-enhanced showers are expected to develop and push north to south over and around the lake. The ORD and MDW TAFs were kept dry in this cycle for low confidence as well as little to no potential impacts aside, perhaps, from a high MVFR cig. GYY has the greatest chance at seeing a few showers, where a PROB30 was introduced in this cycle.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 4 PM CDT this afternoon through Wednesday evening for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from 6 PM CDT this evening through Thursday morning for INZ001-INZ002.

LM...Small Craft Advisory until 10 PM CDT Wednesday for the IL nearshore waters.

Small Craft Advisory until 4 AM CDT Thursday for the IN nearshore waters.