Larsen, Wisconsin Weather Forecast Discussion

656
FXUS63 KGRB 161749
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
1249 PM CDT Tue Sep 16 2025
Updated aviation portion for 18Z TAF issuance
.KEY MESSAGES...
- Patchy dense fog early this morning, and again late tonight into
  Wednesday morning, may impact travel across far northeast and
  east-central WI.
- Isolated strong storms could produce gusty winds over far
  northern Wisconsin on Wednesday afternoon and evening.
  Additional scattered showers and storms are possible at times
  Thursday through Monday. No severe storms are expected.
- Temperatures about 10 to 15 degrees above normal through
  Wednesday, before returning to near normal from Friday into the
  weekend.
&&
.DISCUSSION...
Issued at 248 AM CDT Tue Sep 16 2025
Short Term...Today...Tonight...and Wednesday
The latest RAP analysis and satellite/radar imagery show low
pressure over the Lake Winnipeg region and shortwave energy over
the northern Rockies early this morning. These systems are
upstream of a sharp ridge centered over the southern Plains to the
central Great Lakes. Ahead of the upstream troughing, a narrow
moisture axis extends from the Gulf Coast to western Wisconsin.
Elevated instability upwards of 1000-1500 j/kg exists within this
moisture axis, but a lack of moisture convergence has kept the
moisture plume relatively free of precipitation activity. Fog has
remained relatively patchy this morning, mainly confined to the
lakeshore near Manitowoc and bayshore near Oconto. Patchy ground
fog could yet develop elsewhere, but widespread fog looks unlikely
as many locations over central and north-central WI have dewpoint
depressions of 3-5 degrees. As large-scale features slowly shift
east over the next 36 to 48 hours, the focus of this forecast
largely centers around thunderstorm potential as a cold front
drops into the area late tonight into Wednesday.
The persistent and sharp mid-level ridging over the Great Lakes
will gradually weaken through Wednesday as shortwave energy
gradually shifts east. The moisture axis will thin out over
Wisconsin today. While elevated instability will exist (1500-2000
j/kg), the atmosphere will remain capped, and forecast soundings
show little in the way of convective clouds during peak heating
this afternoon. As a result, precipitation chances have been
removed for today. High pressure will remain across the central
Great Lakes tonight.
Thunderstorms: A cold front will move into northwest Wisconsin
and the western Upper Peninsula late tonight. Elevated instability
(1500-2000 j/kg) will remain present above 7.5-10 kft. A few CAMs
product show shower activity within modest moisture convergence,
but many others remain dry. Therefore, a slight chance of showers
has been kept across far north-central WI after 4 AM Wednesday.
The front will sag into far northern WI on Wednesday afternoon
during peak heating. Surface-based instability of 1500-2000 j/kg
and DCAPE of 1000 j/kg could lead to brief strong storms capable
of downburst activity. Organized severe weather is not expected
given deep layer shear of around 10 kts. The highest chances for
thunderstorms (up to 40%) will occur north of Hwy 8 on Wednesday
afternoon.
Dense Fog: Dewpoints will be creeping up over the next 24 hours.
Minimum temperatures tonight are forecast to fall below the cross-
over temperature which could lead to patchy dense ground fog.
Probabilities of visibilities under 1 mile are under 20%, so
widespread dense fog seems unlikely.
Long Term...Wednesday Night Through Monday
A blocking upper air pattern develops over North America by
Thursday with an anomalous ridge through central Canada and an
upper low stalled to the south over the northern Plains into this
weekend. The pattern should begin a slow shift eastward late week
or early next week as the Canadian upper ridge weakens. As the
trough from the northern Plains moves into the Great Lakes,
thunderstorm chances will be the focus of this forecast.
Thunderstorms: Shortwave energy moving across central Canada
combined with high pressure over northern Manitoba and Ontario
will push a cold front into northern WI on Wednesday night, where
it will stall into Friday morning. This front will provide the
focus for thunderstorms over north-central and far northern WI at
times during that period. Most unstable instability will be decent
at 1000-1500 j/kg over northern WI on Wednesday evening. Winds
aloft are very light at or around 10 kts. Weak deep layer shear
favors pulse storms that could briefly become strong and produce
gusty winds in downbursts (DCAPE up to 1000 j/kg). The most
favorable time for stronger storms would be on Wednesday evening.
Convergence doesn`t look quite as strong on Thursday afternoon
when instability only rises to about 1000 j/kg over north-central
WI. While there could be some storms over northwest to north-
central WI where convergence is strongest, the threat of severe
storms doesn`t appear as high. Then, shortwave energy will
traverse the region at times this weekend into early next week.
Convective parameters look relatively unimpressive this weekend.
However, stronger dynamics are projected to move across the area
on Monday, which will make for a higher risk of stronger storms.
Confidence is low with details this far in the forecast.
Temperatures: Near-normal to above-normal temperatures are
forecast over the entire period.
&&
.AVIATION...for 18Z TAF Issuance
Issued at 1248 PM CDT Tue Sep 16 2025
Good aviation conditions are expected across the region for the
afternoon with clear skies and light winds across the area. Patchy
fog then develops again overnight tonight, with ground fog likely
across portions of central and north-central WIsconsin again
tonight, with the denser fog likely across north-central
Wisconsin. Probability of fog was too low for ATW and GRB to
include in this TAF tonight, but may see this added back in if
confidence increases by the next TAF issuance. Some fog will also
be possible off of Lake Michigan overnight, which may bring some
lower ceilings and visibility to the shoreline including MTW.
Once the fog burns off again tomorrow, another period of quiet
flying conditions is expected into early Wednesday afternoon.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION.....MPC
AVIATION.......Uhlmann

NWS GRB Office Area Forecast Discussion