516
FXUS63 KLOT 201131
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 AM CDT Wed Aug 20 2025

.KEY MESSAGES...

- Patchy dense fog will continue through daybreak and may
  develop again late tonight.

- Choppy waves and rip currents will continue at Lake Michigan
  beaches through Thursday morning.

- Otherwise, little to no impactful weather is expected through
  the upcoming weekend and early next week. An early Fall
  temperature preview remains in store Sunday-Tuesday.

&&

.DISCUSSION...
Issued at 238 AM CDT Wed Aug 20 2025

Through Thursday:

Recent nighttime microphysics imagery depicts swaths of
southward-moving stratus across the Great Lakes within a
seasonably cool low-level airmass along the backside of a weak
low pressure system lifting into the northeastern United States.
Our local area has ended up in a local minima in stratus early
this morning, which with nearly calm winds and relatively cool
temperatures has allowed for areas of fog to develop primarily
in outlying areas. Over the next few hours, we`ll be watching
observations closely to inform any issuance of a Dense Fog
Advisory. Meanwhile, water vapor imagery depicts a subtle upper-
level trough moving southward over the western Great Lakes this
morning, which has manifested as an MCV-like feature and area
of associated showers drifting southward across southwestern
Wisconsin.

After daybreak, the upper-level trough will move southward over
the area perhaps allowing for showers to similarly spread
southward, though primarily west of I-39. Increasing north to
northeasterly flow off Lake Michigan in combination with
seasonably cool low-level temperatures will support the
development and inland spread of lake stratus. Highs
temperatures today hence look similar to yesterday and in the
upper 70s (lakeside) to around 80 far inland. A steady feed of
north to northeasterly winds will keep waves in the 2 to 5 ft
range at Lake Michigan beaches, warranting a continuation of the
inherited Beach Hazards Statement.

This evening, north to northeasterly winds will increase over
Lake Michigan in response to the building surface high across
Lake Superior, causing waves to build to 3 to 6 feet (locally
higher) along southern beaches. Tonight otherwise looks quiet
with ideal conditions for radiational cooling allowing for
overnight lows to range from the upper 50s to lower 60s. Like
early this morning, can foresee areas of fog again developing
tonight primarily in outlying areas.

Tomorrow, the center of the surface high will move over the
central Great Lakes. Relatively low humidity levels, highs
around 80, mostly sunny skies, and a northeasterly breeze will
make for a beautiful late Summer day.

Borchardt


Thursday Night through Tuesday:

Thursday night will be quiet and comfortable with lows in the
upper 50s to lower 60s outside of Chicago and mid to upper 60s in
and near the city. Friday will feature a brief warm-up back to
the lower-mid 80s away from the lake ahead of the next
approaching cold front. This front will come through at an
unfavorable time Friday night into Saturday with MUCH less
moisture to work with than during our recent stretch. Forcing
and upstream instability look respectable, however, so decaying
showers and perhaps an embedded thunderstorm may survive into
far northern Illinois in the pre-dawn hours of Saturday.

On Saturday morning, the front will be pushing through and veering
winds to northwesterly for most of the area, along with strong dry
air advection, especially at the mid-levels. Decent large scale
forcing may compensate though, to wring out some spotty
sprinkles. In the early afternoon, just ahead of the frontal
passage, modest destabilization, especially near/south of the
Kankakee River, could yield isolated showers and a non-zero
chance for a few thunderstorms. All in all, our preference for
Saturday`s cold front is the non GFS/GEFS guidance, which points
to unfavorable parameters for higher shower and thunderstorm
coverage. Temperatures will be pleasant for late August, in the
upper 70s for lower 80s, with modest humidity levels.

Deep mid-upper level troughing will set up near James Bay
(south end of Hudson Bay) early Sunday through early to mid next
work week. 850 mb temps below +10C through this time will set the
stage for highs in the upper 60s to mid 70s, dew points in the
40s and 50s, and overnight lows in the upper 40s to lower 50s
outside of Chicago! Some of the guidance members with cooler temps
aloft in recent forecast cycles would entail even cooler highs
than in the official forecast. In addition, an early season lake
effect rain (and even thunderstorm) episode may unfold primarily
to our east and northeast over western Lower Michigan and north
central Indiana. The below normal, but comfortable/pleasant,
temperature regime should persist through mid next week before
readings return back to near or above seasonal norms heading into
Labor Day Weekend. Also importantly, primarily dry conditions
should prevail through next work week, giving more time to dry out
from the recent heavy rainfall.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 631 AM CDT Wed Aug 20 2025

Only Item of Note:

- Periodic MVFR CIGs through early this afternoon.

There are no significant concerns for impacts to flying through
the period. Patchy MVFR CIGs are currently confined to over Lake
Michigan and adjacent areas downwind into extreme eastern Cook
County and northwest Indiana. With the footprint of MVFR clouds
farther east than expected this morning, confidence is a bit
lower, though forecast soundings still support TEMPO MVFR
mention. North-northeast winds will gust to near/around 20 kt
at times from mid-late this morning through the mid-late
evening, primarily at the near lake terminals, followed by
prevailing 5-10 kt speeds thereafter through Thursday morning.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Thursday morning for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Thursday morning for INZ001-
     INZ002.

LM...Small Craft Advisory until 4 AM CDT Thursday for the IL
     nearshore waters.

     Small Craft Advisory until 4 AM CDT Thursday for the IN
     nearshore waters.

&&

$$

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