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 /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1222 AM CDT Tue Jul 21 2026

Key messages for the 06Z TAF period:

* Still seeing a chance for thunderstorms after 06Z into the predawn hours.

* Gusty northerly winds Tuesday evening into Wednesday.

The potential is still alive and well for a period of scattered thunderstorms overnight into the predawn hours. A newly- developed cluster of convective showers is already moving across NW IL and closing in on RFD VCY. RFD may see showers and maybe a storm nearby prior to 06Z, with chances building all around after 06Z. Storms could linger into the predawn hours of Tuesday, possibly even stretching into daybreak at MDW and especially GYY. Some storms could be on the stronger side with high winds and heavy rain leading to reduced vsbys. Given current expectations, felt hanging onto PROB30s for storms was appropriate. Look for tactical AMDs as confidence builds.

W winds will build in the wake of this wave, going NW during the morning and gusting to 20 to 25 kt for the late morning and afternoon. Winds will subside some but remain breezy for the evening and night as direction turns northerly. Winds may toggle on either side of northerly from this evening through the better part of Wednesday. NNW is favored through the early evening before flopping to NNE for the night, then likely back to NNW by dawn on Wednesday.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 6 AM CDT early this morning for the IL and IN nearshore waters.