685
FXUS63 KLOT 272329
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
629 PM CDT Wed Aug 27 2025

.KEY MESSAGES...

- Widely scattered showers and a few storms are expected
  primarily from late tonight through Thursday.

- Another period of dangerous rip currents and swimming
  conditions expected for Lake Michigan beaches late Thursday
  into Friday morning.

- Near to below average temperatures will continue through at
  least next week.

&&

.DISCUSSION...
Issued at 155 PM CDT Wed Aug 27 2025

Through next Wednesday:

Recent water vapor and radar imagery depict a well defined vort
max moving southeastward through eastern Iowa this afternoon.
This feature is progged to continue pivoting around the
southwestern flank of the broader longwave trough over the Great
Lakes and New England and across Illinois this evening. While
the low level air mass over Illinois is dry, fairly strong
isentropic ascent and shrinking condensation pressure deficits
on the 300-310K surfaces (also seen readily via a 700mb LLJ)
could be enough to allow at least isolated showers to make it as
far east as central Illinois. So, will maintain low chances
(15-20%) for a few showers this evening, primarily south of
I-80.

The lead shortwave trough is expected to move east of the area
after dark, but fast on its heels will be the next shortwave
trough dropping south into the upper Great Lakes overnight. As
the next wave approaches, a modest (25-30kt) westerly low level
jet is progged to develop over southern WI into far northern IL.
That westerly low level jet will once again strengthen
isentropic ascent on the 300-310K theta surfaces and support
scattered showers tonight. Initially, the activity should
develop near the IL/WI state line, but the main axis of showers
should shift southward with time and especially by daybreak
Thursday. Mid level lapse rates are progged to be weak, but some
guidance does suggest that low level theta-e advection could be
enough to support an axis of a couple-few hundred J/kg of
MUCAPE when coverage of showers is at peak. Warm air advection
regimes can be pretty effective at producing thunderstorms in
environments with meager instability, so will maintain a mention
of thunderstorms from midnight to mid-Thursday morning, mainly
north of I-88/I-290.

The potential for scattered showers will continue into the
afternoon hours ahead of an approaching cold front from the
north. Again, instability looks pretty meager with MUCAPE of
only 200-600 J/kg, but enough to make it hard to rule out there
being a couple of isolated thunderstorms ahead of the front.
Temperatures Thursday ahead of the front should reach the 70s,
to upper 70s across the southwest. Behind the front, temps
should fall back some on Friday to mainly the upper 60s to lower
70s. A period brisk northerly winds down the full fetch of Lake
Michigan will drive waves up again by late Thursday afternoon
or early Thursday evening. This will result in another period of
dangerous rip currents and hazardous swimming conditions late
Thursday night into at least Friday morning before winds/waves
relax (conditions should be better during the holiday weekend).
A Beach Hazards Statement has been hoisted for all southern
Lake Michigan beaches tomorrow (valid times vary modestly from
north to south based on when winds/waves arrive).

Temperatures look to slowly moderate over the weekend as mid-
upper level ridging spreads east into the Midwest in the wake of
the departing upper trough. Surface high pressure will remain
parked over the Great Lakes, so low level easterly flow should
keep the temp moderation modest in nature. Medium range guidance
suggests a continued slow warming trend heading into the first
part of next week with highs probably getting back above 80
degrees. Ensemble model guidance is providing an emerging signal
for another push of relatively cool air approaching by the
middle of next week.

- Izzi/Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 629 PM CDT Wed Aug 27 2025

Aviation forecast concerns for the 00Z TAFs:

- Period of SHRA with embedded TS potential late tonight into
  mid-Thursday morning.

- Isolated SHRA potential in the afternoon mid-late afternoon
  with the passage of a cold front.

- Wind shift to northeast 10-15 kt behind the cold front
  Thursday afternoon, with patchy MVFR cigs possible especially
  near Lake Michigan.

First of a pair of mid-level disturbances was tracking southeast
across the area late this afternoon/early this evening, though
precipitation with this initial wave is limited to some spotty
light rain moving across central IL. The second more amplified
wave was evident in satellite imagery, digging into the upper
Midwest. Warm/moist advection and associated ascent aloft is
forecast to increase into WI and northern IL later tonight
ahead of this wave, leading to the development of showers and
some embedded thunderstorms which will likely affect the
terminals into Thursday morning. Have maintained a prevailing
VFR SHRA mention in the TAFs, with a prob30 for TS for lower
confidence in actual TS coverage and duration which looks to end
from NW to SE in the 12-15Z timeframe.

While the main surface low pressure center will pass well to
the northeast of the forecast area, a trailing cold front is
expected to push across the terminals from the northeast early-
mid Thursday afternoon. The main impact from this will be a
shift to northeast winds in the 10-15 kt range, though there
will be a potential for some patchy MVFR cloud bases especially
over/near Lake Michigan. Moisture profiles don`t appear as
favorable for SHRA beyond the mid-late morning passage of the
mid-level trough axis, although a brief period of isolated to
scattered shallow SHRA can`t be ruled out along/just behind the
front during the afternoon and have maintained a VCSH mention
for these.

Ratzer

&&

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

IN...Beach Hazards Statement from Thursday afternoon through Friday
     afternoon for INZ001-INZ002.

LM...Small Craft Advisory from 4 PM Thursday to 1 PM CDT Friday for
     the IL nearshore waters.

     Small Craft Advisory from 4 PM Thursday to 10 AM CDT Friday
     for the IN nearshore waters.

&&

$$

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