221
FXUS63 KGRB 221157
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
657 AM CDT Sun Sep 22 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Gusty north winds and building waves this afternoon into Monday
  morning will result in hazardous conditions for small craft.
  Dangerous swimming conditions on the beaches of Lake Michigan,
  mainly south of Sturgeon Bay. A Small Craft Advisory and Beach
  Hazard Statement remains in effect.

- Areas of showers and a few storms end from northwest to
  southeast today. Mainly dry conditions are expected the rest of
  the week.

- Patchy to areas of frost are expected across north central WI
  Monday morning.

- Temperatures will return closer to normal today into middle of
  next week, before trending well above normal again late next
  week where high temperatures could run as much as 10 to 15
  degrees above normal.

&&

.DISCUSSION...
Issued at 353 AM CDT Sun Sep 22 2024

Short Term...Today...Tonight...and Monday

Main forecast concerns revolve around pinning down the best
chances for showers and storms today, fog/frost potential tonight
and temps.

Showers/Storms & Clouds:

Last of the showers/storms ahead of the front are coming to an end
across eastern WI early this morning. Activity will turn to
along/behind the front the rest of the morning and early afternoon,
with a shortwave exiting the region. The heaviest activity is
forecast to develop across east central WI later this morning,
with some 0.50"+ amounts possible. Expect some mist/drizzle over
the north early this morning as well. A ribbon of MUCAPE up to
1000 J/kg will slowly exit the area this morning and early
afternoon, keeping thunder chances going, but nothing severe is
expected. All of the activity will shift south of the area by
3-5pm, with dry conditions expected tonight into Monday.

Clouds will slowly clear from northwest to southeast as the front
exits, but likely not getting to near Manitowoc until around
sunset. Could get a few daytime cu where there is enough surface
heating behind the main batch of clouds. Skies will be mostly
clear tonight with only a few high clouds over the south. Mostly
sunny skies are expected for most spots on Monday, with a few
daytime cu possible, along with some lake effect clouds over the
east where delta Ts in the low teens and some lingering low-level
moisture will remain in place.

Temperatures / Frost & Fog Potential:

The long stretch of well above normal temps comes to an end
today, as 925mb temp drop to between 10-13C behind the front.
Temps likely will be slow to climb this morning with the
clouds/precip around, but as the clouds exit, we should be able to
get into the mid 60s to around 70, with a few lower 60s in north
central WI where temps aloft are the coolest. Temps will be pretty
steady over far east central WI where clouds/precip will be
around most of the day. Not much change in the airmass for Monday,
so highs should be pretty close to today.

Leaned toward the cooler guidance for tonight, which brings lows
into the middle and upper 30s across much of north central WI.
Would not be surprised to see some lower 30s in the typical cold
spots. Will hold off a frost advisory as it looking more like
patchy to areas of frost, but if lows drop a degree or two lower,
widespread frost would be more likely. HRRR/SREF not highlighting
much of a fog threat tonight, but think with the good radiational
cooling, recent rain, mostly clear skies, light winds and a low-
level inversion, should see at least some patchy ground fog early
Monday morning.

Long Term...Monday Night Through Saturday

A fairly tranquil pattern is expected Monday night into Saturday.
Some patchy fog (entire area) and patchy frost (far north) is
possible Monday night. Meanwhile, a 500mb trough swinging across
the western Great Lakes Tuesday will bring a small chance of
showers across east-central Wisconsin. After the upper trough
passage, 500mb ridging is expected across the Great Lakes which
would result in temperatures returning to well above normal
(10-15 degrees) levels by the end of the work week. The latest
GFS guidance has some areas flirting with 80 on Thursday and
Friday.

There are some difference between the GFS/Canadian/ECMWF on how
they handle the 500mb ridging, thus confidence is very low in
the small chances of rain next Saturday. Beyond the forecast
period, the 8-14 day outlook from the Climate Prediction Center
is calling for a greater chance of above normal temperatures and
a greater chance of below normal precipitation. Looking at past
La Nina winters, which is expected this winter, there was a strong
signal for the fall to be warmer than normal at Green Bay.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 657 AM CDT Sun Sep 22 2024

MVFR/IFR and low-end VFR conditions are expected to continue this
morning, as a frontal boundary pushes south of the area. Skies
will clear and areas of showers and isolated storms will end from
northwest to southeast through the day, bringing VFR conditions.
Some patchy ground fog will be possible across north central WI
early Monday morning.

Winds will become north early this morning behind the front,
gusting to 20 kts through the day. Winds will decrease later this
afternoon and remain light tonight.

&&

.MARINE...
Issued at 353 AM CDT Sun Sep 22 2024

A period of hazardous conditions for small craft is expected this
afternoon into early Monday as gusty north winds develop behind a
cold front. A Small Craft Advisory remains in effect from this
afternoon into Monday morning. There could be a brief period of
winds to 20-25 kts later this morning as the initial surge of cold
air advection arrives behind the front. Then as winds increase
aloft, gusts to around 25-30 kts are expected this afternoon into
early Monday. Waves will build be up to 4 ft on the bay and up to
6 ft on Lake Michigan, highest south of Sturgeon Bay. Winds and
waves will slowly decrease Monday morning (north) and Monday
afternoon (south).

While winds will mainly be out of the north, there will be just
enough of a easterly component to bring 3+ waves to the shores of
Lake Michigan. This will create dangerous swimming conditions,
mainly south of Sturgeon Bay. A Beach Hazards Statement remains in
effect.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 1 PM CDT this afternoon through
Monday morning for WIZ022.

Beach Hazards Statement from 1 PM CDT this afternoon through
Monday afternoon for WIZ040-050.
&&

$$

DISCUSSION.....Bersch/Eckberg
AVIATION.......Bersch
MARINE.........Bersch