NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 211747
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1247 PM CDT Tue Oct 21 2025

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Expect on-and-off rain showers today through Wednesday. Lake
  effect showers Wednesday night into Thursday potentially mixing
  with the seasons first snowflakes over far north-central
  Wisconsin.

- Gusty northwest winds up to 30 mph are possible today, creating
  hazardous conditions for small craft on Lake Michigan and Green
  Bay.

- Below normal temperatures are expected through Thursday followed
  by a slow warming trend into the weekend.

&&

.DISCUSSION...
Issued at 246 AM CDT Tue Oct 21 2025

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

The latest RAP analysis and satellite/radar imagery show a low
pressure system located over the Boundary Waters of Minnesota. An
associated cold front is currently moving east across northeast
Wisconsin, with frontogenesis along the boundary contributing to
an arcing band of rain. Behind the front, partial clearing is
already occurring over western Wisconsin and southeast Minnesota.
However, additional shortwave activity and a weaker secondary cold
front are moving across southwest Minnesota, bringing increasing
clouds, spotty showers, and gusty winds. As the parent upper low
drops southeast across the area, the primary forecast concerns
revolve around shower trends and gusty winds.

Rain Showers: Low pressure, along with shortwave activity on the
southern flank of the upper circulation, will move across northern
Wisconsin today and meander across the northern Great Lakes
through Wednesday. After a brief break in shower activity early
this morning (post-5 am) once the initial rain band departs,
clouds and showers are expected to re-develop and increase from
west to east after 9-10 am, aided by diurnal heating. This will
lead to on-and-off shower activity through the afternoon. A more
prominent slug of deep moisture and an associated surface trough
will rotate into the region from the north this evening, resulting
in precipitation increasing in coverage. While forecast soundings
indicate sufficient moisture for showers to continue into
Wednesday afternoon, coverage over central to east-central WI
should decrease late tonight as the main focus for ascent along
the surface trough departs. However, lake effect rain showers are
expected to keep shower coverage more widespread across north-
central and far northeast WI. Temperatures look to remain too warm
for any snowflakes to mix in with the rain tonight.

Strong Winds: Northwest winds are gusting 35-45 knots over the
Dakotas early this morning, though the strongest winds are
forecast to remain west of our region through Wednesday. Despite
this, forecast soundings indicate that gusts to 30 mph will be
possible at times today. The greatest probability for these gusts
(70-90% chance) will be over central to east-central WI, where an
isolated gust to 35 mph cannot be ruled out.

Boating Hazards: The Small Craft Advisory remains on track. We
will need to consider an extension for Wednesday evening, but
current guidance is just shy of advisory criteria at that time.

Long Term...Wednesday Night Through Monday

The large synoptic-scale trough currently in place across the
Great Lakes and Northeast is forecast to lift out to the north by
Friday. The main focus through the end of the work week remains on
precipitation trends and wind impacts associated with deep
cyclonic flow. The next trough is expected to spur surface
cyclogenesis over the central/northern Plains by next Monday,
which could impact the region by next Tuesday, Oct 28.

Precipitation: A thermal trough will be present across the region
on Wednesday night, dropping 850mb temperatures to -3C to -4C.
This will create favorable ingredients for lake effect
precipitation off western Lake Superior Wednesday night into
Thursday morning, including favorable wind trajectories, lake-
to-850mb delta-Ts of 14-16C, and 850-700mb relative humidity of
80-90%. Forecast soundings and precipitation type energy data
indicate a potential for snowflakes to mix in with these lake
effect showers during this time over far north-central WI, which
could be the first snowflakes of the season for the area.

Outside of the lake effect areas, widespread clouds are expected
to persist Wednesday night and Thursday, with sprinkles or light
showers possible. Later in the period, medium-range models
indicate a compact shortwave could bring light precipitation to
the area on Friday night and Saturday. Following this, a split-
flow pattern will lead to low predictability for precipitation
until the larger system approaches late Monday into next Tuesday.

Frost/Freeze: As the upper low departs, backing winds will lead
to an influx of drier air from the west on Thursday night into
Friday. This, combined with clearing skies, will lead to frost or
freeze potential for most areas. Confidence in frost/freeze
development is lower for Door County due to persistent northwest
winds of 5 to 10 mph. Please note that frost/freeze
considerations are only valid over the Fox Valley and Lakeshore
areas where the growing season has not officially ended.

Temperatures: Will generally be 5 to 10 degrees below normal on
Thursday. A gradual warming trend is expected after that, with
temperatures returning to above-normal levels by late in the
weekend.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1247 PM CDT Tue Oct 21 2025

Mix of VFR/MVFR cigs across the region early this afternoon. Along
with the lower cigs scattered showers are developing south of an IMT
to RHI line. As a surface low continued to spin over the western
Great Lakes this afternoon and evening shower activity will continue
to increase across most of the region. Vsbys are will generally
remain VFR, however, brief periods of MVFR vsbys are possible as
showers pass directly over any terminals.

Cigs are expected to gradually descend tonight with blanket MVFR/IFR
cigs expected early Wednesday morning. IFR cigs are mostly likely at
AUW, CWA, and RHI

Southwest to west winds will also remain breezy through this evening
with periodic gusts of 20-25kts possible across much of of the
region.

Scattered showers will continue Wednesday as breezy northwest winds
off Lake Superior induce some lake effect showers. Coverage of
showers will be greatest across northern WI.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......GK