Summit, Wisconsin Weather Forecast Discussion

262
FXUS63 KGRB 191057
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
557 AM CDT Fri Sep 19 2025
Updated aviation portion for 12Z TAF issuance
.KEY MESSAGES...
- Multiple rounds of showers and thunderstorms are expected this weekend,
  beginning this afternoon (Friday) and continuing through
  Sunday. The risk for widespread severe weather is low, though an
  isolated strong storm with small hail and gusty winds is
  possible each day.
- Locally heavy rainfall is possible this weekend due to slow-moving,
  training cells. The highest chances for rain will be over
  central Wisconsin on Saturday and north-central Wisconsin on
  Sunday.
- Breezy southeast winds will lead to hazardous conditions for
  small craft and on beaches south of Sturgeon Bay from late
  Friday night through Saturday morning.
- While the unsettled weather pattern will continue into Monday, drier
  conditions are expected to return from Tuesday through Thursday
  of next week.
&&
.DISCUSSION...
Issued at 253 AM CDT Fri Sep 19 2025
Short Term...Today...Tonight...and Saturday
The latest analysis reveals a pronounced split flow pattern
dominating central North America this morning. A low-pressure
system currently centered over South Dakota is advecting a deep
moisture plume into the region, evident by a precipitable water
axis extending from the south-central Plains into Wisconsin. This
axis, featuring moisture content at 200% of normal, is fostering
spotty showers within an area of weak elevated instability
(250-500 J/kg MUCAPE). Meanwhile, low cloud cover continues to
expand across the area where dewpoint depressions are 3 degrees or
less. The primary forecast concerns will be focused on shower and
thunderstorm trends as the low-pressure system and the
anomalously moist airmass gradually advance into the region.
Thunderstorms:  An upper-level closed low will track slowly
eastward, positioning itself over the northern Mississippi Valley
tonight before moving into Wisconsin on Saturday. An embedded
shortwave disturbance, currently over the Iowa-Nebraska border,
will lift northward, pushing a conveyor belt of moisture into
central Wisconsin this afternoon and expanding across the
remainder of the area tonight. Until this primary moisture surge
arrives, precipitation will be most probable along the existing
precipitable water and instability axis, which will lift from
central and east-central Wisconsin into northern Wisconsin through
the afternoon. Rainfall coverage is expected to be widely
scattered to scattered in nature.
This afternoon and early evening, most unstable cape values
climbing to 1000 J/kg could support the development of a
thunderstorm cluster or two over central Wisconsin. A strong storm
capable of producing small hail and brief gusty winds is
possible; however, weak deep- layer shear makes the threat of
organized severe weather unlikely. The likelihood of showers and
isolated thunderstorms will continue to increase and spread
northward across the entire area tonight. On Saturday, diurnal
heating will once again interact with the moist airmass, fostering
additional rounds of showers and storms. Instability is forecast
to peak around 1000 J/kg again on Saturday afternoon across
central and east- central Wisconsin, which could fuel another
strong storm or two. The severe weather risk remains low due to
continued weak deep-layer shear of approximately 20 knots.
Flooding Potential:  The primary concern associated with
thunderstorms will be the potential for locally heavy rainfall.
With precipitable water values holding at 150% of normal and meso-
beta element (MBE) velocities under 10 knots, the atmospheric
setup will be conducive to training cells, where storms repeatedly
move over the same areas. The greatest risk for heavy rainfall
will be over central Wisconsin.
Marine and Swim Hazards:  There is increasing confidence in
hazardous marine conditions developing on Lake Michigan late
tonight and continuing through Saturday morning. A Small Craft
Advisory and a Beach Hazards Statement will be issued south of
Sturgeon Bay. A consistent signal in the guidance points toward
breezy southeast winds gusting to 20 knots, which will build waves
to heights of 3 to 5 feet.
Long Term...Saturday Night Through Thursday
Thunderstorms:
The large-scale weather pattern from the end of the weekend
through early next week will be characterized by slow-moving
troughing over the northern Plains and Upper Midwest, leading to
periodic chances for showers and thunderstorms. On Saturday night,
a piece of energy rotating through the mean trough is expected to
swing across northern Wisconsin, keeping showers and
thunderstorms (40-50% chance) in the forecast. Although
instability will be gradually weakening, lingering MUCAPE values
up to 900 J/kg over east-central Wisconsin could be sufficient to
sustain a strong storm or two. The overall severe weather threat
remains low.
By Sunday, another shortwave is forecast to move through during
the afternoon and evening, re-initiating shower and thunderstorm
activity. While instability does not look as impressive as in
previous forecasts, with CAPE values in the 500-800 J/kg range, it
will still be sufficient for convective development. The severe
threat remains low. The highest precipitation chances (near 60%)
will shift to north-central Wisconsin, where convective
temperatures will be lowest. Individuals with outdoor plans this
weekend should remain aware of changing weather conditions.
A potent trough is forecast to move into the Pacific Northwest on
Saturday night, digging into the central Plains by late Monday or
Tuesday. Our region, situated within a split-flow pattern and
removed from the influence of this more organized system, is
expected to experience mostly dry weather from Tuesday through
Thursday. By next Friday, the central Plains low may begin to
exert some influence on our weather, potentially increasing rain
chances, particularly over east-central Wisconsin. Despite 850mb
temperatures remaining above normal, persistent clouds and
precipitation will likely keep surface temperatures near seasonal
normals through at least Monday.
Flooding: The potential for localized heavy rainfall will persist
through the weekend. On Saturday night, precipitable water values
of 150-160% of normal and meso-beta element velocities under 10
knots will support a continued threat of heavy rain from any
training cells into the evening. This round of rain is expected to
diminish over northeast Wisconsin by late Saturday night. On
Sunday, low meso-beta element velocities around 10 knots will once
again favor the potential for training cells. However, ensemble
guidance shows only muted probabilities for rainfall exceeding
half an inch, suggesting that any heavy rain will likely be
localized rather than widespread.
&&
.AVIATION...for 12Z TAF Issuance
Issued at 557 AM CDT Fri Sep 19 2025
The period will begin with a mix of VFR, MVFR, and IFR conditions
due to low ceilings and mist. Slow improvement is expected this
morning. Most TAF sites will generally improve to VFR conditions
(except RHI) by late morning or early afternoon. Widely scattered
showers will be possible for much of the day, but probabilities
are low that flight conditions will fall to MVFR in showers.
Southeast winds may become a little gusty (15-20 kts) at some
terminals this afternoon.
Showers and thunderstorms will become more widespread tonight as
the low-pressure system and associated moisture continue to push
into the area. Ceilings and visibilities will lower to MVFR and
IFR in and near showers and thunderstorms. The TAFs indicate the
onset of showers and thunderstorms between 03-05z.
Overall confidence is high in the general trend of improving
conditions late this morning into the afternoon followed by
significant deterioration tonight. There is medium confidence on
the exact timing of showers and thunderstorms tonight, but the
main aviation hazard will be the widespread low cigs developing
after 05z.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from late tonight through Saturday
afternoon for WIZ040-050.
&&
$$
DISCUSSION.....MPC
AVIATION.......MPC

NWS GRB Office Area Forecast Discussion