198
FXUS63 KGRB 291920
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
220 PM CDT Fri Aug 29 2025

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Areas of fog will likely develop overnight tonight and could
  become locally dense.

- High pressure will bring tranquil weather along with warm days
  and cool nights through the weekend.

- Showers and a few thunderstorms (40-60% chance) will be possible
  on Tuesday afternoon and Tuesday night. Temperatures will turn
  sharply colder for the middle of next week.

&&

.DISCUSSION...
Issued at 219 PM CDT Fri Aug 29 2025

Short Term...Tonight and Saturday

The latest RAP analysis and satellite/radar imagery indicate that a
high-pressure system is centered across the central Great Lakes
early this afternoon. A weak shortwave impulse has initiated a
small corridor of warm advection, resulting in an area of light
showers over Wood, southwest Portage, and Waushara counties. This
rain and associated cloud cover have limited heating over parts of
central WI where temperatures remain in the upper 50s. Meanwhile,
across far northern Wisconsin, skies are partly to mostly cloudy
due to a field of fair weather clouds. With the high-pressure
system expected to remain centered over the region for the next 24
hours, the primary forecast focus for tonight shifts to the
potential for widespread dense fog.

Within northwest flow aloft, the area of warm advection
associated with the weak shortwave impulse is expected to track
south of the forecast area into southern Wisconsin and northern
Illinois by early this evening. As this system exits, skies will
progressively clear from north to south across the region. The
high-pressure system will then become centered directly over
northeast and north-central Wisconsin, establishing favorable
conditions for radiational cooling overnight, characterized by
clear skies and light winds.

Statistical and probabilistic guidance continues to show a strong
signal for widespread ground fog development shortly after
midnight. Confidence is highest for locally dense fog over central
and north-central Wisconsin. This is attributed to a combination
of factors: the antecedent light rainfall today which moistened
the surface, and the regions generally higher elevation and sandy
soils, which are conducive to radiational cooling. A Dense Fog
Advisory may be required for portions of the area. The fog is
expected to lift relatively quickly by 8-9 AM Saturday morning as
daytime heating begins.

Temperatures across far northern Wisconsin are expected to be
warmer, generally settling into the lower to middle 40s.

On Saturday, fair weather cumulus clouds will develop with
daytime heating, contributing to partly cloudy skies. Warmer
temperatures are forecast for Saturday afternoon, with highs
reaching the middle 70s across the region.

Long Term...Saturday Night Through Friday

The overall synoptic pattern through the medium-range period is
anticipated to remain fairly stagnant, characterized by mean
upper-level troughing over the Eastern CONUS and persistent
ridging over the Western CONUS. Model guidance, including the GFS
and ECMWF, shows good agreement in maintaining this amplified
synoptic-scale pattern, anchored by the Western ridge and Eastern
trough.

High pressure will remain anchored across the region on Sunday
and Monday, leading to a continuation of warm daytime temperatures
and cool, comfortable nights. Patchy fog will be possible each
night, particularly in low-lying areas and river valleys, but it
is not expected to be as widespread or dense as Saturday mornings
potential.

By Tuesday, numerical models have been consistent in showing
stronger upper level energy dropping south through the western
flank of the eastern trough. This feature is expected to reinforce
and deepen the trough across the central and east- central CONUS.
This deepening trough will facilitate the passage of a strong
cold front across our region late Tuesday afternoon and Tuesday
night.

Forcing associated with this cold front looks rather robust,
which, when combined with forecast instability values of 300-500
J/kg of Surface-Based Convective Available Potential Energy
(SBCAPE), could lead to widespread showers and embedded
thunderstorms across the area on Tuesday afternoon and evening.
Current indications suggest instability will be insufficient for
severe weather. The Storm Prediction Center (SPC) Day 4-8
Convective Outlook continues to keep our area out of any organized
severe weather probabilities.

Behind the cold front, a very cool airmass will advect into the
region, with 850mb temperatures forecast to fall at least 10
degrees Celsius below normal for early September. Within this
significantly cooler airmass, the sensible weather is expected to
be cool and showery on Wednesday and potentially extending into
Thursday. The exact duration of the showery conditions on Thursday
will depend on the speed at which the upper trough fully
traverses the region.

Conditions will also become favorable for waterspouts over Lake
Michigan and Green Bay on Wednesday and Thursday due to the cold
air aloft over relatively warmer lake waters. Additionally,
gusty winds behind the cold front could lead to small craft
advisories at times on Wednesday and Thursday for both Lake
Michigan and Green Bay. Confidence remains too low to add to the
HWO.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1202 PM CDT Fri Aug 29 2025

An area of light rain will move across central WI, generally south
of KMFI, KISW, KY50 through about mid-afternoon before departing.
To the north of this rain, there is an area of MVFR stratus
across the central and east-central WI TAF sites. Ceilings should
gradually rise to VFR by early to middle of the afternoon in this
area as the rain moves southeast. Scattered fair weather clouds
around 3-4 kft will prevail over far northern WI until the end of
the afternoon.

Clear skies and light winds remain on track to produces areas of
ground fog, locally dense late tonight. Guidance generally point
towards fog developing in the 5-7z time frame. Trended lower with
visibilities based on the strong signal from guidance. Confidence
is higher over central and north-central WI than over eastern
portions of the state due to the rain today and the sandy
soils/higher elevation of the northern WI highlands.

The fog should burn off in the 13-14z Saturday time frame,
leaving good flying conditions for the rest of the day.

&&

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

&&

$$

DISCUSSION.....MPC/MPC
AVIATION.......MPC