134
FXUS63 KGRB 201750
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1250 PM CDT Fri Sep 20 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Record stretch of 80 degree weather for September is expected to
  continue today at Green Bay (forecast high 83F), and maybe at
  Wausau (forecast high 80F). See the climate portion for more
  details.

- Unseasonably warm temperatures will continue today and Saturday
  before temperatures return closer to normal early next week.

- There is a 40-60 percent chance of showers and thunderstorms
  Saturday night into Sunday morning. Severe thunderstorms are not
  expected. Still a lot of uncertainty in the precipitation trends
  Sunday night into the middle of next week, although there has
  been a trend toward lower rain chances at times to account for
  the timing of the systems moving across the region.

&&

.DISCUSSION...
Issued at 329 AM CDT Fri Sep 20 2024

Short Term...Today...Tonight...and Saturday

Main forecast concerns will the exiting showers this morning,
chances for additional showers/storms on Saturday and fog
potential early this morning and again late tonight into Saturday
morning. Above normal temps continue today into Saturday.

Shower/Storm Chances & Clouds:

Shortwave trough and cool front will continue to push west to east
across the area this morning. Most of the shower activity will
come to an end by 4AM, but there could be some additional
development along/ahead the front through mid-morning. Still
enough elevated instability (up to ~1000 J/kg of MUCAPE) early
this morning to keep a thunder mention. Dry conditions are
expected this afternoon into tonight. Low clouds will continue to
form ahead of the front early this morning due to the moist
south/southeast flow, but as winds turn to the west behind the
front, look for the clouds to clear out as drier air works in
later this morning, with only a few daytime cu expected this
afternoon.

A few CAMs are trying to bring an area of light rain or sprinkles
into parts of central and/or north central WI in association with
a push of WAA and LLJ poking up into western WI. Won`t bite on
this just yet, but some low-end PoPs may need to be added if this
feature does materialize. Another frontal boundary, shortwave and
jet streak will then approach the area Saturday afternoon, with
chances for showers and storms arriving across north central WI
after ~4pm. A ribbon of decent instability (500-1200 J/kg MUCAPE)
and shear (25-35 kts) is forecast to push into north central WI
in the afternoon (but uncertainty remains on just how unstable it
will get ahead of the front), bringing the chance of a strong
storm with gusty winds, brief heavy rain (PWATs climbing to ~1.3")
and small hail. But the severe weather threat will remain low
through sunset.

Fog Potential/Trends:

Areas of dense fog has developed across parts of southeast MN and
southwest WI early this morning. Expect the fog to continue to
expand a little eastward through sunrise as skies clear and with
recent rain and winds remaining light. Some spots will likely see
a low stratus deck rather than fog, as winds just off the surface
remain strong enough to limit fog formation. While it will be
drier tonight, recent rains, light winds and a low-level inversion
could lead to patchy ground fog development.

Temperatures:

The long stretch of above normal temps will continue today and
Saturday. Despite some weak CAA behind the front, 925mb temps
remain between 19-21C, supporting highs mainly between 75-85
degrees, warmest across east central WI. A touch of humidity will
remain in the air today, mainly across eastern WI, where
dewpoints in the low to mid 60s are expected much of the day.
Temps tonight will be cooler but still remain above normal, with
lows ranging from the upper 40s across north central WI to the mid
and upper 50s across much of east central WI. Temps at 925mb get
a 1-2C bump on Saturday, allowing highs to be a couple/few degrees
warmer than today in most spots.

Long Term...Saturday Night Through Thursday

500mb pattern indicated a zonal flow at the beginning of the
period. Early next week, a 500mb ridge is expected to amplify
across the western United States with downstream trough expected
over the middle portion of the country. The models diverge on
where the trough, or more likely a closed 500mb low develops by
the middle to end of next week that leads to uncertainty in the
precipitation trends beyond Tuesday. At this point, will lean
toward a dry or mainly dry solution Wednesday and Thursday based
on the expected location of the upper low along with surface high
pressure expected across the area.

After a warm afternoon on Saturday, a cold front will bring a
chance of showers and thunderstorms to the area Saturday night,
then linger into Sunday morning across eastern Wisconsin. The
models have come into agreement with the north dry out Sunday
night, with lingering chances of rain across portions of central
Wisconsin east to the lakeshore including Door County. The
Canadian model actually had a wave moving across Lake Michigan
Sunday night, which would result in higher precipitation totals
and stronger northerly winds. This model appears to be the outlier
at this time with the other models not showing a closed surface
low across Lake Michigan. Confidence is low on the rain chances
Monday night into Tuesday as the models have come in with a drier
solution.

For temperatures, the GFS mos guidance has come in cooler than
the European guidance for lows Monday morning. If the GFS guidance
is correct, will need to add frost to the forecast in the next day
or two. The old European numerical guidance was a few degrees
warmer which leads to doubt in how much frost will be around
Monday morning. Temperatures for the most part should be pretty
close to normal next week, if not a tad bit above normal at times.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1241 PM CDT Fri Sep 20 2024

Patchy MVFR ceilings lingered along the Lake Michigan counties,
where a cold front resided early this afternoon. Otherwise, SCT-
BKN cumulus clouds and VFR conditions prevailed. Daytime cumulus
should dissipate by around sunset, leaving mostly clear skies for
the overnight period. This, combined with light winds, should
allow patchy/areas of MVFR/IFR fog to form overnight, especially
across far NE WI. Any fog should mix out early Saturday morning,
with VFR conditions returning regionwide. A cold front will bring
a chance of showers and thunderstorms late Saturday afternoon into
Saturday night, but that is beyond this TAF period.

&&

.CLIMATE...
Issued at 329 AM CDT Fri Sep 20 2024

Record stretch of 80 degree weather for September is expected to
continue today at Green Bay and maybe at Wausau. If the high
reaches 80 degrees in Green Bay today, it would mark the 10th
consecutive day reaching 80 degrees in September, breaking the old
record of 9 days in a row from Sep 1-9, 1933. At Wausau, the high
on Thursday reached 84 degrees, marking the 9th consecutive day
reaching 80 degrees. The old September record was 8 days in a row
set from September 4-11, 1978 and from September 17-24, 1908. The
forecast high for Wausau today is 80 degrees, so uncertainty exist
if the record would be extended.

Except for portions of central Wisconsin, the rainfall overnight
did little to make up for the rainfall deficits over the last
month to a month and a half, especially across northern and far
northeast Wisconsin.

&&

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

&&

$$

DISCUSSION.....Bersch/Eckberg
AVIATION.......Kieckbusch
CLIMATE........Eckberg