NWS Chicago Decision Support Briefing - Forecast

521
FXUS63 KLOT 250526
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1226 AM CDT Mon Aug 25 2025

.KEY MESSAGES...

- Dangerous swimming conditions for Lake Michigan beaches in
northwest Indiana through Tuesday.

- Fall-like temperatures through midweek.

&&

.UPDATE...
Issued at 827 PM CDT Sun Aug 24 2025

No significant changes to the going forecast this evening.

Isolated sprinkles and showers persist mainly across southern
Lake Michigan and into northwest Indiana as of 815 pm, in
association with a mid-level short wave trough transiting the
area and low-level instability provided by relatively warm Lake
Michigan waters and the colder air aloft. Any lingering
sprinkles from the northern suburbs south to about downtown
Chicago should be ending shortly as diurnal instability
diminishes post-sundown over land, while a few showers linger
mainly along the Indiana shore and northeastern Porter county
into late evening with lake-induced instability maintained there.
Isolated lightning and waterspout potential for northeast
Porter county also appears to be diminishing quickly as we
approach late evening.

Otherwise, a cool night is in store for the area with lows in
the upper 40s expected away from the core of the Chicago metro
area (where mid-50s lows are anticipated in the heart of the
city). Going forecast has all of this handled nicely, therefore
no significant changes are warranted.

Ratzer

&&

.DISCUSSION...
Issued at 245 PM CDT Sun Aug 24 2025

A pair of well-defined mid-level waves across northern
Wisconsin continue to track southeast this afternoon. While the
initial wave has generated a higher coverage of showers and some
storms from Green Bay for Traverse City, the second wave has
been more subdued thus far. However, scattered showers have
recently developed across central Wisconsin southeastward to
around Milwaukee. The second wave will brush northeast Illinois
and northeast Indiana late this afternoon while interacting with
an already robust diurnal cumulus field over the CWA. With
convective cloud depths marginally sufficient for some rain
drops reaching the surface, isolated showers remain possible
through sunset. Closer to the wave, less mid-level warming and
higher low-level moisture will support deeper convective cloud
depths and a potential for waterspouts with any shower over
portions of the Indiana nearshore. Also cannot rule out a
lightning strike with any deeper shower as far southwest as
northeast Porter County through early evening.

Dangerous swimming conditions for the northwest Indiana beaches
will persist through Monday and possibly through Tuesday. A NNW
wind shift and increase in speeds combined with continued CAA
behind the approaching wave will result in an renewed increase
in wave heights this evening and tonight. It is possible that
the higher waves could affect the Cook County shore and warrant
a Beach Hazard Statement, but held off given expected wind and
wave directions remaining west of north and the highest waves
occurring overnight.

The area will remain under the southwest periphery of a
longwave trough drifting from eastern Ontario to across Quebec
early to midweek. Small mid-level perturbations within the
broader NNW flow aloft will yield diurnally enhanced cumulus
Monday/Tuesday afternoon, but a drier low-level profile and
lower inversion precludes the need to include a mention of
precip. Meanwhile, thermo profiles over Lake Michigan will be
marginally favorable for lake effect clouds and perhaps isolated
lake effect rain showers over northeast Porter County through
Monday.

The unseasonably cool airmass advecting over the area today
will settle over the western Great Lakes early this week, with
850 hPa temps as low as 5C on Monday and Tuesday yielding
afternoon max temps around 70 and nighttime lows in the mid 40s
inland to the mid 50s in Chicago. Temps will slightly moderate
midweek before another cold front with a chance of showers and
perhaps storms crosses some or all of the forecast area late
Thursday into early Friday.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1226 AM CDT Mon Aug 25 2025

Only forecast concern is gusty northwest winds later today.
Northwest winds will continue through the period, generally
under 10kts through sunrise and then increase into the 12-15kt
range this morning with gusts into the lower 20kt range. Speeds
and gusts will diminish quickly with sunset this evening with
speeds well under 10kts later this evening and overnight. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement through this evening for INZ001.

Beach Hazards Statement through Tuesday evening for INZ002.

LM...Small Craft Advisory until 10 AM CDT this morning for Calumet
Harbor IL to Burns Harbor IN.

Small Craft Advisory until 4 PM CDT this afternoon for Burns
Harbor to Michigan City IN.

&&

$$

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