Sevastopol, Wisconsin Weather Forecast Discussion

515
FXUS63 KGRB 211104
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
604 AM CDT Sun Sep 21 2025
Updated aviation portion for 12Z TAF issuance
.KEY MESSAGES...
- On and off chances for showers and thunderstorms continue today
  through Monday. Best chance for a few strong storms is Monday
  afternoon and evening.
- Patchy areas of fog have developed overnight and will linger
  through mid-morning before burning off. Another round of fog is
  possible tonight.
&&
.DISCUSSION...
Issued at 207 AM CDT Sun Sep 21 2025
SHORT TERM (Today through Monday afternoon)
Thunderstorms: Scattered light showers continue across eastern
Wisconsin early this morning as a short-wave trough moves to the
east. A second short-wave, currently over eastern North Dakota
and western Minnesota, is expected to arrive this afternoon. Warm
air advection ahead of this wave and a weakly destabilizing
environment with MUCAPE values of 800-1000 J/kg should trigger
showers and thunderstorms this afternoon and evening. While the
risk for stronger storms is low today due to the best forcing
remaining to the west, a marginal severe weather threat exists for
late Monday afternoon, mainly south of Highway 29. A cold front
will be dragged across Wisconsin on Monday by the short-wave as it
moves across the U.P. This front, although expected to become
diffuse, may provide enough focused ascent to combine with
increased MUCAPEs of 1000-1200 J/kg and deep layer shear of 25-30
knots to support the development of marginal severe storms. The
primary hazards with any strong storms that develop are hail and
isolated microburst wind gusts.
Flooding: Localized flooding is a concern for Monday due to slow
storm motions. However, precipitable water (PWAT) values are
expected to decrease from recent levels, dropping to 1.0-1.2
inches which should reduce the heavy rain potential.
Dense Fog: Behind this mornings rain, areas of fog have developed
across parts of central and north-central Wisconsin, with a few
pockets of dense fog. This fog is expected to burn off by mid-
morning. Tonight patchy areas of are likely to develop across much
of the region.
LONG TERM (Monday Night through next weekend)
A short-wave trough and its associated surface boundary are
forecast to exit the region Monday night into early Tuesday
morning. A few lingering showers may persist Monday night, but the
area is expected to dry out by mid-Tuesday morning as a surface
high-pressure system builds in from the west and brings in drier
air. Ensemble guidance from various models supports this pattern,
showing continued height rises along the U.S./Canadian border
through the middle of the week, which should keep the forecast
area mostly dry.
The next synoptic feature of note is a surface low that is
progged to develop over the Ozarks during the middle of next week.
This low is then forecast to lift northeast, tracking toward the
eastern Great Lakes region from Thursday into Friday. While some
moisture wrapping around this system could potentially bring a few
showers to the lakeshore late in the week, the primary axis of
better moisture and instability is expected to remain shunted well
to the south of our forecast area.
Temperatures throughout this extended period are expected to
remain steady, holding at or just above normal for this time of
year. The overall pattern appears to favor a relatively tranquil
period with minimal impactful weather.
&&
.AVIATION...for 12Z TAF Issuance
Issued at 604 AM CDT Sun Sep 21 2025
Low status and fog have developed across much of the region early
this morning creating mainly IFR/low end MVFR conditions across the
region. Expect those poor flying conditions to linger through mid
morning before the status and fog begin to lift. This afternoon
there is another chance for showers and a few thunderstorms as the
next short-wave approaches the area. For now framed out the best
window for rain with PROB30 groups at each TAF site, but timing will
likely need to be adjusted. Increased thunder at RHI, AUW, and CWA as
the best instability is generally expected to reside across northern
and central WI.
Tonight another round of IFR/LIFR status is expected to develop
across much of the region along with pockets of fog. Any areas of
fog that do develop will likely dissipate early Monday morning.
Beyond the TAF period there is another chance for thunderstorms
Monday afternoon and evening as a cold front moves across the
region. A few stronger storms capable of producing hail and gusty
will be possible south of a AUW to GRB line.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION.....GK
AVIATION.......GK

NWS GRB Office Area Forecast Discussion