Green Bay, Wisconsin 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

434
FXUS63 KGRB 142035
AFDGRB

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

.KEY MESSAGES...

- Marine Dense Fog Advisory for the nearshore areas of Lake
  Michigan through Thursday.

- Scattered showers and thunderstorms are occurring in central
  and north central WI this afternoon. Isolated storms are possible
  along the lake breeze as well.

- A broken line of strong thunderstorms will cross northern WI
  between 4 and 9 PM Thursday. Some severe storms are likely
  with the passage of a cold front. All hazards possible over
  central WI and large hail possible over eastern WI.

&&

.DISCUSSION...
Issued at 327 PM CDT Wed May 14 2025

Tonight through Thursday Morning...

Scattered showers and thunderstorms developed over central and north
central WI this afternoon, which is in an area with some weak mid
level vorticity advection. Main threat is lightning, but there is a
small chance for small hail. There are a few showers popping up
along the lake breeze from Green Bay to southern Calumet County.
There is weak instability so the showers can develop along weak
forcing, but no shear, so the updrafts cannot be sustained.

We will continue to have low level moisture flowing over the
relatively cool Lake Michigan and therefore a continued fog threat
for the overnight hours. The setup is similar to last night where
the marine fog will spill inland during the late evening hours. It
should not get quite as far inland as last night, but I would
suspect that we will have to issue Dense Fog Advisory once again for
at least the lakeshore to the Fox Valley area.

Fog and low clouds will dissipate quickly during the mid morning as
winds start to increase out of the southeast. This is in response to
low pressure approaching from the Northern Plains. Synoptic forcing
will remain weak during the morning hours, and there will be a lot
of mid level dry air in place. Sunshine will help temps quickly warm
to around 80 in central WI, but the onshore breeze will keep
lakeshore temps in the 60s. Southeast winds will be in the 20 to 30
mph range for inland areas.

Thursday Afternoon and Night

A mid level trough swinging across MT and WY will close off over the
Northern Plains early Thursday morning. The associated surface low
centered over eastern South Dakota will quickly occlude, and the
frontal boundary extending from the low (across southern MN and into
west central WI) will set up by midday Thu and wait for further
forcing. That forcing will arrive within a few hours when the next
front rounds the base of that low and pushes into southwest WI. This
is the where the initial storms (for WI) are expected to develop.


Most meso models agree on there being a line of storms extending
from central MN to the Twin Cities at 2 PM and then that line
quickly expands east-southeast as the front lifts into central WI
between 2 and 4 PM. By 5 pm, there should be a line of storms
(possibly a second line behind it) that goes from northwest WI to
Wisconsin Rapids, and potentially down through Madison and
Janesville. Storms should be fairly close together along that front
(in a line) in the northwest and central portions of WI, but much
farther apart (discrete) in southern portions.

A line of strong storms will quickly track across northern WI
between 4 PM and 8 PM, although they may be just exiting Door
County at 9 PM. As storms roll into central WI between 4 and 5 PM,
all severe hazards will be possible. The synoptic forcing with
strong vorticity advection and low level front will be plenty to get
storms to develop and punch through that EML cap. With dewpoints in
the lower 60s, CAPE around 2000 j/kg, and deep layer shear at an
impressive 40 kt (thanks to southeast winds and the surface and
southwest winds pushing in just above), the environment is conducive
for supercells. Large hail, damaging winds, and tornadoes will all
be possible as these storms move into our area. As the line speeds
eastward, it will start to encounter the stout marine layer (Fox
Valley up to Iron Mountain) and storms will start to become elevated.
There will still be a large hail threat, but the threats for
damaging winds and tornadoes will start to diminish.

Quiet weather with fairly mild temperatures in the 50s and lighter
south-southwest winds will occur for the overnight hours.

Friday through Saturday...

The closed upper low and associated weakening surface low will track
from central MN to the MN Arrowhead during the day Friday, and
straight across Upper Michigan Friday night through Saturday
morning. A piece of energy (500mb vorticity) is expected to ripple
up across the east half of WI on Friday out ahead of that upper low.
In addition, the low level jet will also be streaming across the
region and there will be some upper divergence present from the
left exit region of a strong upper jet.

The main concern with this will be the very strong winds available
to mix down to the surface during the day Friday. Gusts in the 35 to
45 mph range are looking likely for the early afternoon hours in the
Fox Valley, and this would meet Wind Advisory criteria. In addition,
a few thunderstorms are possible Friday afternoon, mainly south of
Appleton to Sheboygan. Continued gusty winds and a few showers are
possible over the rest of the area Friday evening.

We will remain in cyclonic flow Saturday, so diurnal showers are
possible across all of northern WI. Gusty northwest winds up to 30
mph will also be possible.

Sunday through Tuesday...

High pressure will cross Ontario Sunday and northerly winds will
usher colder temps into the region. Sunday night looks the coldest
with north/central areas dropping into the lower 30s, thus a frost
potential. Quiet weather will continue Monday. Our next chance for
showers and some thunderstorms will arrive Monday night as an upper
trough approaches from the Plains and we get into the warm air
advection area to give us some elevated convection.

Cronce

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 327 PM CDT Wed May 14 2025

Scattered showers and thunderstorms are occurring over north
central and central WI this afternoon and a few showers and
isolated storms are possible along the lake breeze. These will
quickly diminish around sunset. The fog and low-cloud setup is
similar to last night, so expect marine fog and stratus to begin
pushing inland by mid evening and persist across the Fox Valley
and the lakeshore areas the rest of the night.

Visibility of a half mile or less and ceilings of 200 ft or
less are likely once again for this area. There is lower
confidence in the fog spreading as far west as last night, but
the stratus has a decent chance of getting pretty far inland
(low confidence in how far west).

Fog and stratus will quickly diminish by mid Thursday morning.
Expect a line of storms to track across northern Wisconsin
between 4 and 9 PM, with all severe hazards possible in central
WI and large hail possible closer to Lake Michigan.

Cronce

&&

.MARINE...
Issued at 327 PM CDT Wed May 14 2025

Given that the environment will continue to remain favorable
(moist air over the cool open waters) for dense fog until this
pattern breaks, the Marine Dense Fog Advisory is in effect until
7PM Thursday. We may be able to expire that a little earlier on
Thursday as southeast winds kick in, so there is uncertainty in
the end time. This includes the Bay area, although we may see
less foggy conditions during the daylight hours there.

Cronce

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...None.
MARINE...Dense Fog Advisory until 7 PM CDT Thursday for LMZ521-522-
     541>543.

&&

$$