Forecast Discussion for GRB NWS Office
FXUS63 KGRB 232320

Area Forecast Discussion
National Weather Service Green Bay WI
620 PM CDT Wed Sep 23 2020

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 302 PM CDT Wed Sep 23 2020

Mostly clear skies and warm temperatures will begin the evening
hours across the forecast area. However, an approaching cold front
and mid level trough will bring showers and thunderstorms to
north-central Wisconsin tonight. Although a portion of this area
is under a marginal risk for severe weather, the main threat is
expected to be small hail and gusty winds as storms appear to be
elevated with the best severe weather threat just to the west.

The front will slowly sink south on Thursday, as the
aforementioned mid level trough pushes southeast through the
western Great Lakes region. Despite the increase in dynamics, the
trough itself is expected to weaken as it tracks through the
region. Overall scattered showers and isolated thunderstorms can
be expected through the day, which should be fairly disorganized
in nature. The abundant cloud cover during the day on Thursday
will cause daytime temperatures to be much lower, with highs
ranging from the upper 60s across the north, with lower 70s across
the south.

.LONG TERM...Thursday Night Through Wednesday
Issued at 302 PM CDT Wed Sep 23 2020

The main concerns for the extended period will be the passage of
several rounds of rainfall and isolated thunderstorms, as well as
a shift to a much cooler pattern, as above normal temperatures
shift to well below normal temperatures next week.

Thursday night into Saturday morning...The first of a series of
upper level shortwaves will be moving out of the region Thursday
night, with showers and thunderstorms coming to an end in the
hours after midnight. By Friday morning, winds behind the cold
front will turn once more to the south and southwest, which will
bring enough WAA into the region at lower levels to push highs
back into the middle to upper 70s. The next shortwave then
approaches for Friday night into Saturday, bringing rainfall back
into the region for much of central to northern and northeastern
Wisconsin. Conditions still look fairly capped at this time, which
will help minimize thunderstorm potential in the region. That
said, instability will be increasing at the surface and ample
shear will be present in the area as well. If signs start to point
to a weaker cap aloft, severe wording may need to be included in
future updates. For now, still favor a lower thunderstorm threat
with non-severe impacts.

This weekend...Saturday is a day with considerable uncertainty, as
there is some discrepancy as to where redevelopment will occur
later in the day. If the front from Friday moves more slowly,
redevelopment may occur along it and affect mainly central to
east-central Wisconsin. If it moves faster, the next system may
arrive earlier and impact northern Wisconsin. Thus kept a low
blend of pops in the forecast. In either case, severe is not
expected at this time. Sunday, wrap around moisture will over Lake
Superior and into the region, but the overall best precipitation
will be east, where the jet and trough setup. Still, another round
of rainfall is certainly expected. Sunday will also be the last
day with near normal temperatures, as CAA will start to get going
by Monday and bring in much cooler temperatures next week.

Rest of the forecast...The stacked low will continue to intensify
for a time across the Great Lakes region to our east, bringing
with it a period of gusty winds and strong CAA aloft. Expect highs
to head towards the 50s by the middle of next week! Rainfall from
wrap around moisture will also be possible at times, for a fairly
cool and wet fall period.

.AVIATION...for 00Z TAF Issuance
Issued at 611 PM CDT Wed Sep 23 2020

Ceiling height and visibility will be the main aviation weather
concerns during the next 24 hours. A cold front will sag into the
area tonight and then linger into Thursday. Showers will develop
INVOF the front. Some thunderstorms are likely, but at this point
there is too much uncertainty with respect to timing and coverage
to warrant the explicit inclusion of TS in the TAFs. Will
reconsider carrying TS later this evening as trends may be a
little clearer on radar by then. Anticipate MVFR ceilings will
eventually develop near/north of the boundary, especially after
precipitation becomes more widespread. Most of the aviation
guidance shows this happening. Worked it into the TAFs for C/N-C
WI, but slowed it some and tempered how much CIGs will drop
because of the low-level dry air initially in place north of the



SHORT TERM.....Kurimski
LONG TERM......Uhlmann

NWS GRB Office Area Forecast Discussion