NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 140551
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
Issued by National Weather Service Milwaukee/Sullivan WI
1251 AM CDT Wed May 14 2025

.KEY MESSAGES...

- Areas of dense fog along the lakeshore this evening may
  affect eastern counties overnight. Marine Dense Fog Advisory
  for nearshore areas south of Sturgeon Bay.

- There is a chance (30-40%) Wednesday afternoon for central
  WI.

- Showers and storms are most likely (50-80% chance) Thursday
  afternoon and evening. Severe storms are possible with the
  passage of a cold front with all hazards possible.

&&

.DISCUSSION...
Issued at 856 PM CDT Tue May 13 2025

Dense fog of 1/4 mile visibility or less is slowly spreading
inland from Lake Michigan in Kewaunee and Manitowoc Counties. A
Dense Fog Advisory is in effect for these counties through 10 AM
CDT Wednesday. This fog bank may continue inland into the Fox
Valley tonight, but confidence is lower for widespread dense fog
in this region due to advective winds weakening and warmer
temperatures persisting in the valley itself. As mid-level
clouds clear late tonight into early Wednesday morning,
temperatures will fall to near dewpoint temperatures and areas
of fog, locally dense (1/4 to 1/2 mile visibility), will
develop across much of eastern Wisconsin, with patchy fog at 1
to 3 mile visibility across central Wisconsin. If temperatures
cool quickly enough tonight, Dense Fog Advisories may also be
needed in the Fox Valley and Door County late tonight into
Wednesday morning.

PREVIOUS DISCUSSION

A closed upper low spinning over the Upper Great Lakes region
will continue to bring moist, southeast flow into eastern WI
through Wednesday night. Dewpoints have increased into the lower
60s east to the mid 50s central and clouds are widespread.
There are a few light showers out there west of the Fox Valley.
Marine fog is looming over the open waters of Lake Michigan, and
the light easterly breeze tonight and abundant low level
moisture is expected to help usher that fog inland overnight.
The lakeshore cities and towns may see fog as early as 9pm, but
it should take a little longer for it to overspread the rest of
the Fox Valley, probably after midnight. For areas west of the
Fox Valley, fog should be more patchy and less dense. A Dense
Fog Advisory may be needed tonight for eastern areas.

Another round of pop-up showers and storms is expected farther
inland from the lake stabilizing flow on Wednesday afternoon as
sfc based CAPE increases to 500 to 750 J/kg from central to
north- central WI. Again with limited effective shear, no
severe storms are expected. With more clouds to start the day
and limited mixing, highs on Wednesday will come down across the
board with readings in the 70s all areas, though staying in the
lower 60s near the Lake Michigan shore.

MRC


Thursday through Tuesday:

The main focus for Thursday remains on the potential for severe
thunderstorms across most of Wisconsin. Questions remain about storm
coverage and mode, especially across the southern portion of the
state.

A strongly capped elevated mixed layer will be building into
Wisconsin starting Thursday morning, as a warm front surges north
across the region. This EML will feature steep mid level lapse rates
on the order of 7.5-8 deg C/KM, atop an inversion centered between
850 and 750 MB. A few showers or perhaps a thunderstorm or two may
be ongoing along the leading edge of the EML early Thursday, mainly
across the northern portions of the state, but for the most part the
strong cap should limit shower/thunderstorm activity through at
least early afternoon. At the surface, south to southeast flow will
support warming temperatures away from Lake Michigan, with mid to
upper 80s possible over south central Wisconsin, and upper 70s to
low 80s over the central and northern portions of the state. The
southeast flow will result in a gradient with temperatures
stretching from Racine and Milwaukee northwestward toward Green Bay,
with progressively cooler readings east of this line (i.e., toward
the lake), with locations right along the lakeshore likely remaining
in the 60s.

Surface moisture will also increase through the day, with dewpoints
expected to rise into the upper 50s to mid 60s, highest with
southward extent. This is expected to result in a large region of
2000+ J/KG of MLCAPE across much of the state by early afternoon.

A shortwave associated with the upper low centered over South Dakota
will surge eastward toward the region during the afternoon hours,
along with a cold front at the surface, with the front currently
expected to reach the Mississippi River between 1 and 4 PM. At the
same time, increasingly fast flow at 500 and 300 MB will be
overspreading the area from the west, resulting in 30-50 kt of deep
layer shear across the region.

Mid level dry air remains the greatest wrinkle in the forecast for
Thursday. While most model guidance suggests quite a bit of dry air,
especially in the 700-500 MB layer, there are differences among the
various models. Ultimately this will boil down to if and where the
combination of large scale lift and a warm and moist low level
airmass is sufficient to overcome the dry air.

With all of that said, if/where convection is indeed able to
overcome the dry mid levels, convective initiation is expected to
occur during the early to mid afternoon hours along and just ahead
of the front. Initial mode would likely be discrete to semi-
discrete, with more than sufficient shear profiles for supercells.
Tornadoes and large to very large hail will be possible with any
sustained supercells. With time, convection should become more
linear in nature, likely from north to south, since frontal and
upper level forcing will be stronger across the northern half of the
state. The risk for straight line winds will increase with any
linear segments, along with a continued hail and a QLCS tornado
risk.

Convection is expected to move east-northeastward, and if current
trends hold would be expected to cross into the cooler lake-induced
airmass and eventually over Lake Michigan by late evening (10 PM-
Midnight).

Quiet weather is expected Thursday night and early Friday morning as
a subtle surface ridge passes through the area. By later Friday,
additional convection will be possible as the upper low begins to
pivot southeastward into Minnesota, and associated surface energy
pushes across the state. While dewpoints will only be in the low
50s, continued steep lapse rates and fast flow aloft will support
1000+ J/KG of MLCAPE and around 30 knots of deep layer shear Friday
afternoon. Hail should be the greatest risk with this activity,
though a potentially well mixed boundary layer (i.e., relatively
high cloud bases) may also be supportive of downbursts.

The upper low is then expected to finally move through the region on
Saturday, with continued chances for showers, before rain chances
end Saturday night.

High pressure is expected to build south for Sunday and potentially
into Monday. By the early portion of next workweek, additional
shortwave energy will bring renewed chances for showers and
thunderstorms to the western Great Lakes.

Boxell

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1225 AM CDT Wed May 14 2025

Light east to southeast winds tonight with dense fog being the
primary impact early this morning. VSBYS less than a mile and as
low as quarter-mile have pushed as far west as Brown and Calumet
counties but looks likely to push a bit further west through the
early morning hours. There does seem to be a limit on how far
north the limited VSBYs have been able to reach but Door County
and even parts of far eastern Oconto and Marinette counties
closer to the Bay may see VSBYs under a mile. This is also a low
cloud deck with VLIFR CIGS as well. KATW/GRB will likely start
to see impacts from this over the next hour or so. This will likely
remain in place through daybreak with the sun gradually allowing
it to dissipate in the morning. Otherwise into the day expect
some potential for a few storms toward central and north central
WI with northeastern and eastern WI likely remain more so on
the dry side. All eyes will then turn to Thursday will potential
for strong to severe storms in the afternoon for central WI
with continuing potential into the evening north northeast WI.
Breezy southeast winds also expected Thursday, particularly
toward central WI.


&&

.MARINE...
Issued at 1019 AM CDT Tue May 13 2025

Expanded the Marine Dense Fog Advisory northward one zone to
include areas south of Sturgeon Bay. The marine fog is expected
to stick around through at least Wednesday morning due to light,
moist easterly flow over the cold open waters. An additional
northward expansion to include the Door Peninsula will be
possible, but more uncertainty remains.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 10 AM CDT this morning for WIZ038>040-
     048>050.
MARINE...Dense Fog Advisory until 1 PM CDT this afternoon for LMZ542-
     543.

&&

$$

DISCUSSION...
AVIATION...
MARINE...