182
FXUS63 KLOT 212342
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 PM CDT Tue Jul 21 2026

.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.

&&

$$

Visit us at weather.gov/chicago