059
FXUS63 KLOT 232320
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
620 PM CDT Sat Aug 23 2025

.KEY MESSAGES...

- Slight chance (20%) for a few thunderstorms this afternoon
  east of I-55.

- Fall-like temperatures early/mid week.

&&

.DISCUSSION...
Issued at 234 PM CDT Sat Aug 23 2025

A cold front bisecting the forecast area at this time will
continue to track eastward this afternoon, exiting the southeast
CWA by sunset. Ahead of the front, low-level inhibition has
mostly eroded and a cumulus field continues to bubble. However,
thicker mid to upper-level cloud cover associated with a mid-
level wave that crossed the area this morning has slowed heating
southeast of a Pontiac to Valparaiso line. Though the front
will provide a low- level forcing mechanism, the lack of deep-
layer forcing (subsidence behind the wave) will greatly limit
support for precip. Additionally, substantial mid-level drying
within a layer of modest shear has already shown signs of
entrainment (wispy tops of cumulus per local assessment and area
webcams) that will further limit the development of deep
convection. Overall, expectations are for isolated showers to
develop east of I-55 over the next 3-4 hours, but with lower
thunderstorms chances (roughly 10% or less) than previously
expected.

A notably cooler airmass will advect over the area tonight
through Monday, with 850 hPa temps as low as 8C on Sunday and 5C
on Monday. Persistent and stronger NW/NNW winds through Monday
will result in dangerous swimming conditions for portions of the
Indiana shore (primarily Porter/La Porte counties).

A well-defined trough currently over central Saskatchewan is
progged to quickly track southeast across the western Great
Lakes on Sunday. The wave will brush the northeast half of the
CWA during the afternoon and enhance an already developed
diurnal cumulus field. While a stout 10kft inversion will limit
vertical growth, convective cloud depths upwards of 5-6kft
should be able to yield isolated sprinkles or showers. Have
therefore included slight chance (<20%) of showers for much of
northeast Illinois and northwest Indiana mid to late afternoon.

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 Sunday
night and Monday.

Otherwise, expect a dry and cool stretch as we head into the
last week of August. High temps on Monday may struggle to rise
above 70 for some locations while typically colder outlying
areas Monday night will likely experience lows in the mid 40s.
Temps should moderate back to near seasonable levels late in the
week as a weak and mostly dry cold front stalls near or over
the area.

Kluber

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 620 PM CDT Sat Aug 23 2025

Aviation forecast concerns for the 00Z TAFs:

- Breezy northwest winds gusting 20-25 kts diminishing quickly
  this evening, then developing again on Sunday.

- Slight chance (<25%) of scattered VFR SHRA midday/afternoon
  Sunday.

Surface cold front has pushed well east/southeast of the
terminals as of early this evening, with breezy northwest
surface winds gusting 20-25 kt bringing cooler/drier air into
the forecast area. Surface winds will ease with sunset, losing
gusts while SCT/BKN VFR cloud bases 4500-5500 ft also decrease
in coverage.

Similar conditions are expected again Sunday, with northwest
winds becoming breezy and gusting 20-25 kts again by midday.
Strong mid-level disturbance digging into the upper Midwest will
likely support a more extensive VFR stratocu deck by mid-late
morning, along with the potential for some scattered showers to
develop as cooler air moves in aloft. Lightning potential looks
quite low, and current guidance depicts point probabilities for
showers at less than 25%. For now have maintained dry TAFs,
though may end up eventually needing a prob30 or VCSH mention
during the midday/afternoon hours on Sunday if coverage looks to
be of greater coverage.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement through Monday evening for INZ002.

LM...Small Craft Advisory until 4 PM CDT Monday for Burns Harbor to
     Michigan City IN.

&&

$$

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