410
FXUS63 KGRB 311930
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
230 PM CDT Sun Aug 31 2025

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- No hazardous weather expected through Tuesday, with patchy
  ground fog tonight the only minor impactful weather.

- High confidence of rain and some thunderstorms (70-90% chance)
  from late Tuesday night through Wednesday.

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

&&

.DISCUSSION...
Issued at 229 PM CDT Sun Aug 31 2025

Short Term...Tonight and Monday

The latest RAP analysis and satellite/radar imagery show a very
slow moving ridge both aloft and at the surface, centered from the
Great Lakes to the northern Mississippi Valley early this
afternoon. Weak warm and moist advection continues over northwest
Wisconsin and the western Upper Peninsula, where there is an
indication of a modest low-level trough. Clouds have been
plentiful over northwest Wisconsin so far today, and while some
convective allowing models generate light showers this afternoon,
the dry air and a persistent cap are expected to prevail, leading
to the removal of any slight chance of showers from the forecast.
With little significant change to the synoptic pattern expected
over the next 24 hours, the primary forecast concerns remain the
potential for dense fog tonight and isolated showers on Monday.

The relatively stagnant synoptic pattern will persist through
Monday. The weak trough currently noted will largely lose
definition tonight as the dominant upper-level ridging and surface
high pressure remain entrenched across the region. Diurnally
based fair weather clouds are anticipated to dissipate across most
of the forecast area by early this evening. The notable exception
will be over far north-central Wisconsin, where lingering
remnants of a weak warm advection pattern and low level
convergence could allow clouds to persist longer into the night.

Dense Fog: Patchy ground fog is highly likely to develop again
late tonight due to light winds and mostly clear skies. Very
patchy dense fog is possible, with highest probabilities over
north-central WI where dewpoint depressions are lowest.

Thunderstorms: The low levels are expected to be a bit drier on
Monday across most locations, with the exception of far north-
central Wisconsin. Models indicate the development of weak
instability, on the order of 200-500 J/kg, over Vilas County on
Monday afternoon. Consequently, a slight chance (up to 20%) of
afternoon showers has been maintained for this specific area.
Instability values are insufficient for any severe weather
concerns.

Temperatures: High temperatures on Monday are expected to be
similar to todays maximums, remaining seasonal for this time of
year.

Long Term...Monday Night Through Sunday

The broader upper-level pattern is forecast to undergo a
significant change beginning Tuesday, with a reinforced upper
trough developing across the central and eastern U.S. Latest model
guidance continues to show good agreement with this amplified
synoptic pattern.

Thunderstorms: The arrival of a cold front has trended slower,
now expected to reach late Tuesday night and Wednesday. This front
will be the primary forcing mechanism for widespread showers and
isolated thunderstorms. Strong height falls, combined with
instability values of 200-400 J/kg, will support a high chance
(70-90%) of precipitation. While thunderstorms are possible,
instability looks insufficient for severe weather development
across the forecast area.

Heavy Rain: Despite widespread precipitation chances, the risk of
heavy rainfall and subsequent flooding appears very low.
Precipitable water (PWAT) values are forecast to be in the
120-130% of normal range, but the weak instability and relatively
progressive nature of the front will likely prevent excessive
rainfall. The National Blend of Models (NBM) reinforces this risk
assessment, indicating only a 5-15% chance of rainfall exceeding 1
inch.

Temperatures: Behind the cold front, a very cool airmass will
advect into the region, with 850mb temperatures falling at least
10 degrees Celsius below normal by Thursday. Thursday appears to
be the coldest day, with NAEFS 850mb temperatures forecast to be
below the 0.5 percentile from 00Z Thursday to 00Z Friday,
potentially approaching the climatological minimum for the date.
As a result, high temperatures on Thursday may approach or even
fall short of record low maximums for many locations across north-
central and northeast Wisconsin. Temperatures are expected to
remain below normal through next weekend, even after the upper
trough begins to lift northeastward.

Boating Hazards: 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 forecast cycles.

The predictability of specific precipitation chances is low from
Friday through Sunday as the trough lifts out and a more unsettled
but less organized pattern potentially sets in.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1156 AM CDT Sun Aug 31 2025

High pressure will remain in place across the region through the
TAF period resulting in good flying weather in general. Spotty
afternoon showers will be possible north and northwest of KRHI.
The rest of the area should see few to scattered fair weather
clouds with bases around 4-5 kft.

Patchy ground fog will be possible again late tonight that could
reduce visibilities to IFR or LIFR.

Then the weather will repeat again on Monday.

&&

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

&&

$$

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