203
FXUS63 KGRB 272001
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
301 PM CDT Wed Aug 27 2025

.KEY MESSAGES...

- Scattered showers and isolated storms (40-80% chance) tonight, but
  the risk for severe weather is minimal.

- There is a 50 to 90% chance for low temperatures less than 37 degrees
  across north-central Wisconsin Thursday night which could
  result in patchy frost.

- Mainly dry with slowly warming temperatures towards seasonal
  averages through the Labor Day weekend.

&&

.DISCUSSION...
Issued at 301 PM CDT Wed Aug 27 2025

Thunderstorm Chances Tonight:

GOES Water Vapor/RAP 500 mb analysis showed northwest flow aloft
across the Upper Midwest this afternoon. A cold front was dropping
southward from Ontario in association with a lobe of mid-level
vorticity rotating through the parent trough. Despite meager
instability, some upstream lightning strikes have been observed
ahead of the front across southern Canada and across Lake Superior.

The front will swing southward tonight bringing scattered showers
and storms across the area. Within the northwest flow regime,
boundary layer moisture return ahead of the front will be modest,
limiting instability, with HREF most unstable CAPE forecast less
than 750 J/kg tonight and RAP soundings showing skinny CAPE
profiles. As a result, severe storms are not expected, but small
hail could not be ruled out with any storms given the cold mid-level
temps. With the limited moisture and progressive nature of the
system, rain amounts look to be under 1/2 inch in most locations. A
few showers may linger across eastern Wisconsin into Thursday
morning.


Frost Potential Thursday Night:

Some post-frontal stratus or fog will likely persist into Thursday
in the cold advective regime before a drier airmass works southward
through the day. By evening, surface high pressure will build across
the Upper Great Lakes. The ECMWF EFI shows values below -0.8 across
north-central into northeast Wisconsin for minimum temperatures
Thursday night, highlighting strong ensemble agreement for anomalous
cold. The NBM continues to show probabilities for minimum temps less
than 37 degrees of 50-90% across north-central Wisconsin with small
probabilities (~10%) for near freezing temperatures along the
Michigan border. A frost advisory may be needed for parts of north-
central Wisconsin early Friday.


Quiet Holiday Weekend:

Weather impacts through the holiday weekend look minimal as surface
high pressure slowly drifts across the Great Lakes. A somewhat
muddled/blocky mid-level flow pattern is expected with split flow
featuring northern stream ridging progressing slowly east across
southern Canada and a weak mid-level trough meandering across the
northern plains into parts of the Midwest. With the influence of
high pressure into early next week, chances for rain look to remain
mostly west with slow moderation in temps as the mid-level ridging
builds across the area.


Looking Ahead: Next Rain Chances

Timing details with blocking type patterns often exhibit low
predictability, so confidence erodes early next week. Global
ensembles support a return to Great Lakes troughing by the
middle of next week although cluster analysis among global
ensemble members shows substantial differences in the depth of
the trough and to some extent the timing of the trough.
According, the interquartile spread (25th-75th percentile) for
high temps increases considerably by next Wednesday/Thursday
following a period of near seasonal temps early in the week. NBM
rain chances for midweek remain low to medium (30- 40%) owing
to the model spread/lower predictability.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1234 PM CDT Wed Aug 27 2025

VFR conditions will prevail into this evening with west to
southwest winds occasionally gusting up to 20 kts. A cold front
will drop south overnight with scattered showers and isolated
storms (20-40% chance for thunder). Added a PROB30 group for
thunder at KRHI, KAUW, KCWA for now. Some patchy fog will be
possible overnight (30-50% chance), especially across central
Wisconsin. A period of MVFR/IFR post-frontal stratus late
tonight into Thursday morning will give way to clearing skies
through the day. Winds will shift northerly behind the front
with some gusts up to 20 kts on Thursday.

&&

.MARINE...
Issued at 301 PM CDT Wed Aug 27 2025

Small Craft Advisory has been issued for the nearshore waters of
Lake Michigan from Sturgeon Bay south to Sheboygan on Thursday
afternoon/evening as waves increase with northerly flow behind
a cold front. Conditions will improve Thursday night as high
pressure builds over the area.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...None.
MARINE...Small Craft Advisory from noon to 10 PM CDT Thursday for
     LMZ542-543.

&&

$$

DISCUSSION...JM
AVIATION...JM
MARINE...JM