759
FXUS63 KGRB 212106
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
406 PM CDT Sat Sep 21 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- There is a marginal risk of severe storms with quarter-sized
  hail and strong winds across central and north-central WI into
  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.

- Temperatures will return closer to normal from Sunday through
  the middle of next week before trending above normal again late
  next week.

&&

.DISCUSSION...
Issued at 405 PM CDT Sat Sep 21 2024

Short Term...Tonight and Sunday

Precipitation trends are the main focus from this forecast period.
This afternoon`s radar imagery showed scattered showers and isolated
elevated thunderstorms across the region. The thunderstorms were
mainly focused in central WI where the better elevated instability
and WAA prevailed. As a result, the storms struggled to stay
intact with an eastward progression and became scattered showers
as they reached east-central WI. Further west, isolated
thunderstorms continued to develop in the pool of instability and
ahead of/along an approaching cold front. The evolution of these
storms through this evening will be the main forecast challenge.

Precipitation...Model guidance was struggling to latch onto the
current situation throughout the day, which has led to a forecast
mainly based on radar/satellite trends and mesoanalysis/surface
observations. That being said, until the cold front moves closer to
the forecast area, anticipate isolated thunderstorms to continue
developing in central WI and weakening to showers as they move
eastward through late this afternoon. Given the large pool of
instability, there will still be a threat for hail with some of the
stronger storms through this afternoon. Models indicate additional
showers and storms to develop ahead of the cold front into this
evening as the instability pool slowly moves eastward into central
WI. Therefore, anticipate a threat for hail and gusty winds with any
of the stronger storms associated with the front this evening. The
The stronger storm threat will wane as the night goes on due to a
loss in instability, but non-severe thunderstorms will still be
possible through Sunday morning. The precip chances will continue in
far eastern WI through Sunday afternoon until the front completely
exits the forecast area in the evening.

Temperatures...Today was the last day of above normal temperatures
with many locations in eastern WI reaching the 80s. However, clouds
and precip prevented temperatures in central and north-central WI to
rise into the 80s. The cold front will bring much cooler
temperatures to the region with highs only climbing into the 60s.

Long Term...Sunday Night Through Saturday

Cold front that shifts across the area late this weekend will
carry any chances for showers out of northeast WI on Sunday
evening. Skies will clear from north to south the rest of the
night and with PWATs crashing down and winds becoming light,
temps could tank across northern WI. Lowered temps a bit in the
north leading to mins into the mid to upper 30s. Could be some
fog with patchy frost over far northern WI near the Upper
Michigan border. Monday will be dry as ridging aloft and at the
sfc prevails. Another cool night is expected Monday night over
the far north with fog and frost again possible.

The only blemish on a mostly dry week then arrives late Monday
night and Tuesday as upper low over the southwest CONUS this
weekend works its way east-northeast. Though it appears bulk of
showers with this feature remain over the central Great Lakes,
there could be a few showers into east-central WI on Tuesday
(20-30% chances). Rest of the week does look dry though as troughing
re-asserts itself farther east over the lower Great Lakes while
ridging with 590+ dam heights builds over the western Great
Lakes. After a brief cooldown, daytime temps will rebound back
above normal as early as Wednesday with highs in the middle 70s
through rest of the week. Does not look like we`ll see much in the
way of 80s though. Nighttime temps will also gradually warm, with
readings in the 40s to lower 50s by late this week.

Very low chances (mainly less than 20%) for rain return by next
weekend, though indications are that sprawling high pressure at
the sfc tied to the ridging aloft will continue to keep conditions
dry.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 125 PM CDT Sat Sep 21 2024

VFR conditions prevailed across the area early this afternoon, as
high clouds spread from the west from thunderstorms in
western/west-central WI. Given all guidance does not have this
area of showers and thunderstorms at this time, based the early
hours of the TAFs on satellite and radar trends. Therefore,
confidence is not high for the evolution of these showers and
storms into this afternoon. Based on trends, anticipate the
central WI TAF sites to see the best chance for thunderstorms this
afternoon, with a lower chance at the RHI TAF site. These storms
will struggle to move eastward this afternoon due to a lack in
instability. However, anticipate these storms to remain with VFR
cigs.

Another chance for thunderstorms, with MVFR cigs, will move
across the area from northwest to southeast, in association with a
cold front, mainly between 00z and 06z Sunday. A few of these
storms may be strong and produce hail and gusty winds, especially
at the central and north-central WI TAF sites. Additional showers
are possible in east-central WI Sunday morning. Decided to include
ceilings lowering after midnight as models continue to show this
solution. Winds will turn north after midnight into Sunday morning
following the cold front. Gusts up to 20 kts will be possible for
Sunday morning.

&&

.MARINE...
Issued at 405 PM 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 brief period of stronger winds close to/behind the front
as the initial surge of cold air advection arrives late tonight,
then a little lull during the day on Sunday, with stronger winds
arriving aloft later Sunday afternoon which should mix down.
Gusts to 25-30 kts are expected with waves generally building to
2-5 ft, highest waves south of Sturgeon Bay. Winds and waves will
slowly decrease Monday morning.

&&

.CLIMATE...
Issued at 405 PM CDT Sat Sep 21 2024

The record stretch of consecutive days in the 80s during
September continues. High temperatures have already reached the
lower 80s today at Green Bay, so one more day will be added to the
record. There is still a chance Wausau will see highs reach 80
later this afternoon. The streak will end after today as much
cooler air works into the region tonight and Sunday.

At Green Bay, the high today has already cracked the lower 80s,
marking the 11th consecutive day in the 80s. The old September
record of consecutive days in the 80s was 9 days set from Sep
16-24, 1891 and from Sep 1-9, 1933.

At Wausau, the high Friday was 80 degrees, marking the 10th
consecutive day in the 80s. The temperature today could still
reach 80 which would mark the 11th straight day of 80, but that
is not a sure thing. 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.....Kruk/JLA
AVIATION.......Kruk
MARINE.........Kruk
CLIMATE........JLA