textproduct: Chicago

KEY MESSAGES

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

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

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

DISCUSSION

Issued at 139 PM CDT Tue Jul 21 2026

For those looking for a brief break from the warm and humid stretch of weather, the next days are for you. For those who want to soak up as much summer as possible, we have something for you as well this weekend into early next week.

Aloft...the weather pattern is looking a bit more like early fall in our region with a deep upper trough digging into the eastern Great Lakes. Northwesterly flow is advecting a cool and dry airmass in our direction. At the surface high pressure will continue slowly ooze southward into the upper Midwest, with deep low pressure inching eastward through eastern Canada. There is an extensive cumulus/stratocumulus field across the region and spreading into NE IL. Cloud depths should remain shallow enough to prevent widespread showers, however enhanced convergence hear the lakeshore, both on the IL and IN side should support some lake effect rain showers, not super common for July. The convergence will be maximized in NE IL this evening, and then into NW IN later tonight into Wednesday as the land breeze convergence kicks in. We are already seeing some fairly robust showers across far northern Lake Michigan. Given that lake induced equilibrium levels are just high enough with +15 C of differential between the surface and 850 temperatures, we would not be surprised for a waterspout or two to develop particularly along any focused convergence bands this evening. Winds are gusting into the mid 20 kt range (near 30 mph) along and behind the front, and waves will quickly build in the wake of the front leading to a High Swim Risk after sunset at all southern Lake Michigan beaches. Away from the lakeshore clouds should clear enough and combined with cool/dry advection later tonight, and we will see temperatures fall into the 50s in the clear areas and for points north of I-80 inland.

High pressure will shift closer to the lower Great Lakes during the day on Wednesday, and then overhead late Wednesday night and Thursday. Northerly/onshore flow will maintain the High Swim Risk on Wednesday before lighter winds arrive with the center of high pressure. Highs will also struggle to get out of the mid 70s. This will set the stage for the coolest night across the region this summer, with widespread lows in the 50s away from urban Chicago. We could even see a few 40s along the IL/WI border. With high pressure overhead or slightly shifting east on Thursday, Thursday will feature comfortable temperatures in the mid to upper 70s and some afternoon cumulus clouds.

A wave in northwest flow will pass by to our south along the eastern fringes of an expanding upper ridge/heat dome across the west and the plains states on Friday, so we will get grazed with some cloudiness, otherwise it will be dry with temperatures getting back to either side of 80 (still slightly below normal given we are at the climatological peak of average highs). 850 mb temperatures warm considerably Saturday and especially on Sunday, where upper 80s to low (to mid) 90s are easily supported under partly cloudy skies. The ridge will not be centered overhead, which will open the door to convective systems during the latter half of the weekend into early next week.

KMD

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 642 PM CDT Tue Jul 21 2026

A cold front currently about to cross the Chicago terminals will shift winds NE early this evening. Periodic MVFR ceilings, gusts over 25 knots, and isolated SHRA or sprinkles are expected for a few hours behind the front. Winds are then expected to back just west of north at around 10 knots late tonight through most of Wednesday morning. FEW/SCT high-end MVFR clouds may persist during this time.

VFR conditions are expected Wednesday afternoon and evening. A lake- enhanced wind shift will turn winds NNE around 10 knots sometime late morning or early afternoon. Winds are then expected to become VRB under 5 knots, but favor W/WNW, late Wednesday evening.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through Wednesday evening for ILZ006- ILZ103-ILZ104.

IN...Beach Hazards Statement 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.