897
FXUS63 KGRB 011709
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1209 PM CDT Mon Sep 1 2025

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Patchy ground fog next couple mornings, with locally reduced
  visibility.

- High confidence of rain and some thunderstorms (60-80% chance)
  Tuesday night through Wednesday. No severe storms are expected.

- Record to near-record cold high temperatures remain likely for
  Thursday.

&&

.DISCUSSION...
Issued at 325 AM CDT Mon Sep 1 2025

After a prolonged stagnant pattern with sfc ridging dominating,
pattern change begins Tuesday. Strong cold fropa brings showers
and some storms Tuesday night through much of Wednesday. Slower
trend that showed up over last couple days has reversed some,
with majority of models and ensembles shoving main slug of showers
through forecast area just before peak heating on Wednesday
afternoon. In wake of the front, sharp cooling is expected late
this week, with coldest day still on track for Thursday. Amplified
pattern will take a while to unravel with below normal temps
persisting through the weekend.

Near term through tonight...Patchy ground fog has developed again
and is slightly more widespread than 24 hours ago as dewpoints
Sunday afternoon did not mix out as much. Similar to last few
days, any fog ends shortly after sunrise. Have dialed back on
slight chances for showers over the far north. Raw instability
data in BUFKIT points to very limited CAPES of 100-300j/kg with
dry air noted in soundings. Winds to steer any showers are even
weaker than yesterday and thus think showers will stay confined to
genesis area along lake breeze in Upper Michigan. Temps not too
far off from highs on Sunday, centered in the mid 70s, slightly
below average for the first day of meteorological fall. Tonight
will feature more ground fog, but we mix out better today lowering
dewpoints, which would also lower the crossover temps. Expect the
ground fog to stay patchy in coverage.

Showers and storms with cold fropa...Decently sharp shortwave is
present this morning over south central Canada. This wave will
push strong cold front across the western Great Lakes on Wednesday.
A bit more of a signal for showers as early Tuesday afternoon over
central and north-central WI. Given the strength of the shortwave
and warm air advection ahead of it, that idea has some merit. Have
low-end chancy pops earlier now. Bulk of showers and embedded storms
occur later Tuesday night through at least midday Wednesday as the
front crosses. No change to idea of widespread showers (60-80%
chance) with embedded thunder given dynamics of the system.
Instability still on the weaker side (MLCAPES 200-300j/kg). If the
front slows again, could achieve instability up to 1000j/kg,
which along with effective shear over 30 kts could support
stronger storm southeast and east-central. Right now, trends in
fropa timing and instability keep the severe risk very low. Heavy
rain is unlikely due to progressive nature of the front, primary
band of showers, along with Precipitable water (PWAT) values only
forecast to be in the 125% of normal range. NBM continues to
indicate very low probabilities of seeing more than 1 inch of
rainfall Tuesday night through Wednesday.

Temperatures: Behind the cold front, a very cool airmass will
spread across the Great Lakes region, with blend of models,
ensembles supporting 850mb temperatures falling as low as zero on
Thursday with some models even below zero. Thursday still appears
to be the coldest day, with EC EFI values continuing to signal
significantly below normal temperatures compared to climatology
around this date for the last 30 years. As a result, high
temperatures on Thursday may break record low maximums over a
large portion of the western Great Lakes, including north-central
and northeast Wisconsin. Temperatures are expected to remain
below normal through weekend, even after the upper trough begins
to lift northeastward. Due to colder flow aloft, scattered
showers will be in the forecast for Thursday and Friday, with
potential for enhanced activity on Friday as stronger shortwave
trough embedded in larger scale trough crosses the area. Though
the forecast is largely void of shower chances for next weekend,
agree with previous shift that predictability of precipitation
chances is low as though the trough will begin to lift out, a
more unsettled, but less organized pattern potentially sets in.

Marine: Conditions will become favorable for waterspouts over Lake
Michigan and Green Bay, and possibly for Small Craft Advisories
at times from Wednesday through Friday due to cooler air moving
over warmer lake waters. The precise timing and extent of these
hazards will need to be refined in later forecasts, but based on
sharply rising lake induced equilibrium levels, the two main
windows for enhanced waterspout potential that are emerging are
Wednesday tied to convection along the front, then also on Friday
as given it will already be sufficiently cold enough to support
waterspout potential, another stronger wave moving through will
just further enhance that potential. Did not add waterspouts
to the forecast quite yet, but we are certainly getting closer.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1209 PM CDT Mon Sep 1 2025

High pressure will remain in place across the region through the
TAF period, resulting in good flying weather. Maintained the patchy
ground fog (IFR-MVFR vsby) at GRB/RHI late tonight. FEW to SCT
fair weather clouds this afternoon will feature bases around 4-5 kft.
The clouds will dissipate after sunset this evening. Winds will
be light once again.

Clouds will slowly increase on Tuesday. The chance of showers
should not start to ramp up until mid to late afternoon over
RHI/AUW/CWA.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....JLA
AVIATION.......MPC