NWS Chicago Decision Support Briefing - Forecast

088
FXUS63 KLOT 251125
AFDLOT

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

.KEY MESSAGES...

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

- Near to below normal temperatures expected through next
weekend.

- Cold front could bring scattered showers and thunderstorms to
portions of the area Thursday afternoon.

&&

.DISCUSSION...
Issued at 305 AM CDT Mon Aug 25 2025

High amplitude blocking pattern aloft has become established
across western North America with anomalously strong upper
ridging across much of western Canada. Downstream mid-upper
level trough over the Great Lakes and New England will continue
to provide largely tranquil weather and slightly below to below
average temperatures (and very comfortable humidity) to the area
through much of the upcoming work week.

Only real item of note the rest of the week is a shortwave
trough progged to dig south down the west side of the long wave
trough over region on Thursday. A cold front is expected to
move across the area Thursday in association with the digging
shortwave trough. Some modest moisture pooling in advance of
this front could allow afternoon dewpoints to climb into the
mid-upper 50s, which given the abnormally cold temperatures
aloft, could allow for some weak destabilization ahead of the
front. If some weak destabilization does indeed occur, strong
frontal forcing could allow for some scattered afternoon
convection along and just ahead of the front. Given the
uncertainty in the ability for atmosphere to destabilize and
otherwise limited moisture, only made a slight upward nudge to
NBM pops into the low end chance category.

Friday into next weekend there is growing spread among the
medium range guidance and their respective ensembles regarding
how far west the upper trough and cooler temperatures will set
up. Examination of cluster analysis suggests that these
differences over the positioning of the upper trough over the
Great Lakes can be tied to how quickly the blocking ridging over
western Canada breaks down. The guidance that is slower to
break down that block, results in farther west upper trough and
colder temps locally Friday into the first part of the weekend.
The operational ECMWF is in this camp and presents a reasonable
lower bounds of temp forecast with highs only in the 60s Friday
north of I-80. Wouldn`t be surprising to see other models trend
this direction, as being too quick to weaken/breakdown a
blocking pattern is a fairly common systematic error with
medium range guidance. The only real difference to our sensible
weather is forecast temps; rain chances look low Friday through
the weekend in the full spectrum of possible solutions.

Stepping back to beach hazards in the near term. No meaningful
changes made to current headlines. Some diurnal relaxation of
the winds over the lake is expected today, which should allow
wind-waves to come down some this afternoon. Despite the lower
wind-waves, I expect dangerous rip currents to continue into at
least this evening as longer period waves/swell are likely to
continue. Looks like another (weaker) secondary cold front will
move down the lake tonight and could lead to an uptick in winds
and wind-waves later this evening and especially after
midnight. It is possible that the Lake County BHS may need to be
extended beyond its current 03z expiration time, but will let
the day get another look at things and make that decision later
today. Winds and waves should more precipitously decrease during
the daytime Tuesday with longer period waves/swell gradually
decreasing as well, so the 00z Wed end time for the BHS for
Porter County looks good.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 625 AM CDT Mon Aug 25 2025

Only forecast concern remains gusty northwest winds today.
Northwest winds will steadily increase this morning, into the
12-15kt range with gusts into the 20-25kt range expected.
These speeds and gusts will diminish with sunset this evening.
Northwest winds will increase again after sunrise Tuesday with
gusts into the 15-20kt range possible by late Tuesday morning.
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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