535
FXUS63 KGRB 211151
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
651 AM CDT Sat Sep 21 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Unseasonably warm temperatures will continue today, then will
  return closer to normal Sunday into middle of next week before
  trending above normal again late next week.

- There is a Marginal risk of severe storms with strong winds and
  hail across central and north-central WI late this afternoon and
  early this evening.

- A period of gusty north winds and building waves Sunday into
  Monday morning will likely bring a period of hazardous
  conditions to small craft on Green Bay and Lake Michigan.

&&

.DISCUSSION...
Issued at 352 AM CDT Sat Sep 21 2024

Short Term...Today...Tonight...and Sunday

Main forecast concerns will be assessing chances for showers/storms
this afternoon into Sunday, the potential for strong or possibly
severe storms and fog early today. The long stretch of well above
normal temps continues today then cooler air arrives on Sunday.

Severe Weather Potential & Shower/Storm Chances:

Areas of elevated showers and storms have developed across MN and
western/northern WI during the early morning hours, associated
with steep mid-level lapse rates of 7.5-8.0 C/km, FGEN/WAA and
the nose of a low-level jet. This area should continue to
develop/percolate through the morning hours, with some of the
activity making it to central and north central WI. Then as the
nocturnal effects wane after sunrise, most of the showers/storms
should come to an end. This will set the stage for the next round
of showers and storms ahead of a cold front and shortwave. Still
some uncertainty on timing and degree of instability, along with
whether a weak capping inversion will limit activity, but looking
like isolated to scattered showers/storms should arrive after 4pm
central and north central WI. The broken line of storms will
track southeast across the area during the late afternoon and
evening hours. MUCAPE of 500-1500 J/kg, SBCAPE up to ~600 J/kg,
deep layer shear of 30-40 knots, and 0- 3 km helicities of 150-250
support potential for a few strong to marginally severe storms
with gusty winds and hail. Inverted-V signatures on soundings
will promote gusty winds of 30-45 mph making it down to the
surface, with some higher gusts, along with some hail, possible in
the stronger storms. SPC has kept the Marginal risk west of a
Rhinelander to Wautoma line, where the best instability and shear
will reside late this afternoon and early evening (4-10pm).
Decreasing instability and a strengthening low-level inversion
will end the severe threat in the late evening, but the
showers/storms will continue to spread southeast across the area,
but uncertainty remains on just how widespread the activity will
be. On Sunday, showers and a few storms are expected, especially
in the morning as the front slowly completes its trek across the
area. The last of the showers look to end by around sunset over
far east central WI. Most spots will see under a half inch of
rain, but localized higher amounts will be possible as locally
heavy rain will be possible with the storms as PWATs climb to
~1.5" ahead of the front. The fairly quick moving nature of the
storms will limit any flooding threat.

Fog Potential/Trends:

Recent rain, low-level moisture, light winds, clear skies and a
low-level inversion has allowed from some patchy ground to form
early this morning, mainly across northern and central WI. The
patchy fog will continue through a little after sunrise, then
will burn off/mix out.

Temperatures:

The stretch of well above normal temps will last one more day, as
925mb temps remain between 20-23C today. Clouds will hinder
temps at times, especially this afternoon over the west/north,
with possible showers/storms knocking down temps over north
central WI. Won`t make much change to the highs, with most spots
in the upper 70s to mid 80s. If we get a little more sun over the
east, some upper 80s will be possible. Lows tonight will range
from the upper 40s across north central WI to the upper 50s and
low 60s in east central WI. A return to near normal temps arrive
on Sunday as 925mb temps fall to between 10-14C and more
clouds/precip, with highs in the 60s to around 70.

Long Term...Sunday Night Through Friday

A fairly tranquil period is expected. 500mb trough moving across
the western Great Lakes region could bring a small chance of
showers to east-central Wisconsin Monday night and Tuesday. Once
the trough exits the area, building 500mb ridge late next week
will result in dry conditions with above normal temperatures
returning.

For Sunday night, some patchy fog is possible north and west of
the Fox Valley. Patchy frost is also possible across north-
central and far northeast Wisconsin, especially near the Upper
Michigan border. Patchy frost may also be possible Monday night
across north-central Wisconsin. Latest numerical guidance is
suggesting that high temperatures by the end of next week could
run as much as 10 to 15 degrees above normal. Also, the Climate
Prediction Center 8-14 Day Outlook is calling for greater chances
of above normal temperatures continuing.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 650 AM CDT Sat Sep 21 2024

Some patchy ground fog over northern WI will quickly burn off
shortly after sunrise. Otherwise, look for VFR conditions today
with some mid-clouds spreading across the region. Could see some
light showers or sprinkles at RHI this morning. Another batch of
showers or a storm could reach central and north central WI later
this morning or early afternoon, but think conditions will stay
VFR. A better chance for showers and storms, along with lower
ceilings/visibilities, arrives later this afternoon into tonight
as a cold front slowly works across the region. The strongest
storms, most likely across central and north central WI, could
produce gusty winds and hail. Will keep MVFR conditions overnight,
but some models are hinting at lower ceilings after midnight.

Light and variable winds will become south and gust to ~15 kts
late this morning and afternoon. Winds will shift to the north
behind the front late tonight into Sunday morning.

&&

.MARINE...
Issued at 352 AM CDT Sat Sep 21 2024

A period of hazardous conditions for small craft looks to develop
on Sunday as gusty north winds develop behind a cold front. Could
have a burst of stronger winds close to/behind the front as the
initial surge of cold air advection arrives, then a little lull
during the day, with stronger winds arriving aloft later in the
afternoon which should mix down. Gusts to 25-30 kt are expected
with waves generally building to 2-5 ft, highest toward the open
waters of Lake Michigan and south of Chambers Island on the bay.
Winds and waves will slowly decrease Monday morning.

&&

.CLIMATE...
Issued at 352 AM CDT Sat Sep 21 2024

The record stretch of consecutive days in the 80s during September
continues at Green Bay and Wausau. High temperatures are expected
to climb into the 80s today, so one more day will be added to the
record. The streak will end today as much cooler air works into
the region tonight and Sunday.

At Green Bay, the high on Friday was 85 degrees, marking the 10th
consecutive day in the 80s. The old September record of consecutive
days in the 80s was 9 days set from September 16-24, 1891 and
from September 1-9, 1933.

At Wausau, the high Friday was 80 degrees, marking the 10th
consecutive day in the 80s.The old September record of consecutive
days in the 80s was 8 days set from September 4-11, 1978 and from
September 17-24, 1908.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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