831
FXUS63 KGRB 021752
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1252 PM CDT Tue Sep 2 2025

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- High confidence of rain and some thunderstorms (60-80% chance)
  tonight into Wednesday. No severe storms are expected.
  Confidence increasing for rain chances Thursday evening through
  early Friday.

- Record to near-record cold high temperatures remain likely for
  Thursday. Below normal temperatures to continue through the
  weekend.

- Waterspouts are possible on the bay and Lake Michigan Wednesday.

&&

.DISCUSSION...
Issued at 447 AM CDT Tue Sep 2 2025

.Short Term...Today...Tonight...and Wednesday

Dense Fog This Morning: The light winds, shallow low-level
moisture, and clear skies has allowed for patchy ground fog to
develop overnight. The patchy fog, especially near bodies of
water, marshes, and low-lying areas, will continue across parts
of the area during the early morning. Pockets of denser fog are
expected, with visibilities under one mile. Will monitor obs and
webcams, but may need an SPS to highlight the changing visibilities
and/or if the fog can becomes a little more widespread. Will
continue with a mention in the HWO as well.

Shower and Thunderstorm Potential: Dry conditions will start the
day as high pressure drifts away from the region. Then, as a cold
front dives southeast through the state and an upper low drops
into Lake Superior, chances (20-50%) for showers arrive this
afternoon across central and north central WI. As the front
crosses the area overnight into Wednesday morning, the highest
coverage of showers (60-80%) will arrive. Coverage will taper off
as the front exits to our east by early afternoon, with more
instability showers through sunset as colder air arrives both at
the surface and aloft. Some storms are expected at times, with
MUCAPEs (long, skinny profiles) climbing up to ~500 J/kg, but
severe weather is not expected. PWATs only climb to ~1.25" and
rather weak overall forcing will limit the threat for heavy rain
(chance for more than 0.5" under 40%).

Temperatures: Another seasonably warm day is on tap for the area
ahead of the front, with highs in the 70s to near 80. As the cold
air begins to arrive overnight, look for lows to drop into the mid
40s to low 50s in north central WI, but lows look to stay in the
50s to around 60 over eastern WI as they will stay ahead of the
front. Much cooler temps will arrive in north central WI on
Wednesday with highs in the 50s, but highs mainly in the 60s are
expected over eastern WI.

Long Term...Wednesday Night Through Monday

Precipitation Chances...Following the cold frontal passage on
Wednesday, northwesterly winds and low-level moisture will combine
with energy from an upper-level low overhead and lead to lake
enhanced showers over north-central WI Wednesday evening through
Thursday morning. Anticipate rain being the main p-type, but some
forecast soundings indicate low-level temps may be cold enough for a
few snowflakes to mix in with the rain at times. However, confidence
is not high enough to include the mention of snow in the forecast at
this time. Surface high pressure will build into the area on
Thursday and bring an end to the lake enhanced showers with winds
turning more westerly. This high pressure will quickly be replaced
with a fast-moving surface low dropping southeast from the
Dakotas/MN into WI by Thursday evening, only to exit by Friday
afternoon. This low pressure system looks to bring the next chance
for widespread rainfall (not a washout), but the location of
greatest rainfall will depend on the track of the surface low, which
differs among the models at this time. Northwest upper-level flow
will remain in place through the weekend, which may lead to
additional intermittent light shower activity at times.

Temperatures...An abnormally cold airmass will follow Wednesday`s
cold frontal passage. In fact, 850mb temps may approach or dip below
0C for Thursday morning, nearing the all time climatological
minimum. Not only will this airmass be threatening record low
temperatures for September 4, but also record low maximums for both
September 4 and 5. High temperatures for Thursday (Sept 4) and
Friday (Sept 5) will range from the low 50s to low 60s. With low
temperatures for Wednesday night into Thursday morning ranging from
the middle 30s to low 40s, some locations in central and north-
central WI could also see patchy frost development. Temperatures
will remain below normal through the weekend and into early next
week.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1237 PM CDT Tue Sep 2 2025

Precipitation trends and low ceilings will be the main aviation
concerns for this set of TAFs.

SCT-BKN cumulus clouds had developed early this afternoon. A band
of showers associated with an MCV were gradually weakening as they
approached the forecast area.

Precipitation trends will be tricky, as three separate forcing
mechanisms; the MCV, a strong cold front and a short-wave trough,
move through the region ((at different times) during the TAF
period. The initial weakening band of showers could perk up a bit
as instability slowly builds this afternoon, and this could result
in a few thunderstorms in NC/C WI late this afternoon and early
evening. As this initial activity shifts east and weakens, the
frontal precipitation will arrive later this evening and spread
east through the overnight hours. Additional scattered showers
could develop late tonight into Wednesday morning as a short-wave
moves through. Have attempted to provide some timing of these
features, but this may need to be adjusted a bit.

Ceilings will quickly lower as the cold front moves through
overnight, with most locations lowering to MVFR/IFR. In addition,
there may be some minor vsby restrictions due to showers and fog.
A slow improvement in flight conditions is expected during the
mid to late morning hours on Wednesday, but most places will
probably not reach VFR before 18z/Wed.

Steady but light southwest winds will persist this afternoon and
evening, then become gusty from the west to northwest in the wake
of the frontal passage late tonight into Wednesday morning.

&&

.MARINE...
Issued at 447 AM CDT Tue Sep 2 2025

Southwest winds will increase today, gusting to around 20 kts this
afternoon and evening. A few gusts to 25 kts are possible. Waves
will build to 2-4 ft, with locally higher waves near/north of
Death`s Door.

As a cold front moves across the bay and lake on Wednesday, there
will be chances for showers and storms, especially late tonight
into Wednesday. A few waterspouts will be possible with any shower
or storm, particularly on Wednesday, as an intense upper low
drops over Lake Superior and colder air aloft arrives (resulting
in delta Ts to the favored 10-18C range). Combination of the most
favorable delta Ts, convective cloud depths over 6000ft and 850mb
winds under 40kts look to be Wednesday morning. After
collaboration with surrounding offices, decided to add a chance
of waterspouts to the nearshore forecast. As the upper low slowly
drifts north of the region on Thursday, the waterspout potential
may continue, but delta Ts may actually be a little too high.

Periods of gusty westerly winds are possible from Wednesday
through Saturday, which may result in hazardous conditions to
small craft at times. Small Craft Advisories may be needed.

&&

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

&&

$$

DISCUSSION.....Bersch/Kruk
AVIATION.......Kieckbusch
MARINE.........Bersch/Kruk