Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 210928
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
428 AM CDT Mon May 21 2018

Forecast discussion for routine morning forecast issuance

.SYNOPSIS...
Issued at 427 AM CDT Mon May 21 2018

Scattered showers today into tonight, except across the far
north and northeast, then mainly dry until the end of the week.
The week will begin with cool temperatures in central and east-
central Wisconsin and seasonable readings across the north, then
a warming trend is expected.

Little change in the large scale upper pattern is anticipated
during the period. The primary westerlies will remain across
Canada and the far northern U.S., with weaker, somewhat diffuse
branch of the flow across the remainder of the CONUS. The pattern
will support some rain today and tonight, and again toward the
end of the period. As has been the case the past several weeks,
the better rain chances/amounts will probably occur across the
southern part of the area. It`s always tough to estimate
precipitation totals at this time of year when amounts depend
primarily on convection. But the best guess is for AOB normal
amounts across the north, and near normal amounts in the south.

After being held back by clouds and east winds early in the period
(especially across central/east-central Wisconsin), readings
should warm to 10-15F degrees above normal for the middle to end
of the work week. They will likely drop back a bit for the
upcoming holiday weekend, but probably stay above normal.
&&

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 427 AM CDT Mon May 21 2018

An upper level shortwave ejecting northeast out of the southern
stream upper trough over the southwestern United States will
approach the area today, and cross the region tonight. The main
forecast challenge was aligning PoPs for the system. Models were
considerably more aggressive in generating precipitation when
compared to yesterday. That seemed reasonable given the
appearance of the system on WV satellite imagery this morning and
trends on the radar mosaic. Went with a northward advance of
precipitation a little slower than a consensus of the models,
mainly with deference to the dry air dominating the area. Despite
that, enough moisture and forcing should arrive in the area to
support likely or better PoPs across at least the southern half of
the area for this afternoon into this evening. The north is more
of a question mark as that area will remain near the upper
confluent zone between the main branches of the flow, and have the
best forcing with the upper shortwave pass to the south. In the
end, went with a sharp gradient in PoPs over northern Wisconsin,
with precipitation never quite reaching the upper Michigan border
from KLNL-KIMT. SPC grazed the southeast part of the area with a
General Thunder designation for today/tonight. A few cells with
lightning are possible, but model forecast MUCAPEs seemed low
enough to allow for keeping an explicit mention of thunder out of
the grids for now.

The showers should persist into the evening, but diminish
overnight. Getting them to end completely may not be easy, as
it`s possible a few small clusters could linger into early
Tuesday. Stuck with the dry forecast generated by the standardized
model initialization grids for Tuesday afternoon, but enough of
the models generated scattered light precipitation to support
edging PoPs up from near zero. Would not be surprised if later
shifts end up needed slight chance or chance PoPs for the
afternoon.

The reversed (from typical) north-south temperature gradient will
continue today. Expect highs to range from 10-15F degrees below
normal over the far southern part of the forecast area, to 3-6F
degrees above normal in the far north. With precipitation slow to
arrive in the north and likely not making it all the way north
through the area, allowed surface dewpoints to mix out again
across the north this afternoon. That will likely result in
another day with near critical afternoon relative humidities.
Temperatures tonight should be held up by clouds and increased
moisture. Temperatures will begin their rebound Tuesday, though
east winds will probably keep readings in east-central Wisconsin
below normal again.

.LONG TERM...Tuesday Night Through Sunday
Issued at 427 AM CDT Mon May 21 2018

An active pattern is expected by the end of the work week
into the first half of next weekend. Toughest part of the
forecast is to sort out the details and timing of precipitation
due to differences in model solutions. Above normal temperatures
are expected into the weekend.

High pressure is expected to move across the area Tuesday night
and then east of the area on Wednesday. Meanwhile, low pressure
and an associated warm is expected to trigger convection across
the plains into Minnesota Wednesday morning. The rain will try
to move towards the area, but dry air in place should slow down
the arrival of the rain. Trended toward the ECMWF solutions with
bringing chances of showers or storms to much of the region
Wednesday night. The warm front should continue to push northward
into the area Thursday, bringing continued chances of showers and
storms to much of the area. Unsettled pattern to continue
Friday and Saturday with weak disturbances moving through the
region, triggering additional shower and thunderstorm activity.
Some stronger storms are still possible Friday and Saturday,
however the 0-6 km shear is weak, in the 10 to 20 knot range.
Appears to be pulse type convection that could bring some gusty
winds, heavy rains and potentially hail.

Conditions are expected to dry out Saturday night. Upper trough
lingering across eastern Wisconsin could trigger some shower
and isolated thunderstorm activity during the late morning and
afternoon hours on Sunday. Only minor adjustment made to
temperatures to account for the warm bias spots during the day,
and the cold bias locations at night.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 427 AM CDT Mon May 21 2018

Good flying conditions are expected this morning. Then ceilings
should drop during the afternoon and evening, probably to IFR or
LIFR over the southern part of the forecast area. Less rain and
lingering dry air should result in better conditions (MVFR) in the
north. Some fog is also likely over east-central Wisconsin
overnight.
&&

.FIRE WEATHER...
Issued at 427 AM CDT Mon May 21 2018

Rainfall amounts over the next few days are not expected to
alleviate the abnormally dry conditions across the north. Dry
conditions are expected through Wednesday, resulting in elevated
fire weather conditions and worsening drought conditions. Relative
humidities are expected to be in the upper teens to middle 20s
today, and 25 to 40 percent Tuesday and Wednesday. Some relief is
expected by the end of the week as a low pressure system is
expected to bring cloud cover and the chances of showers and
thunderstorms to the region.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Eckberg
AVIATION.......Skowronski
FIRE WEATHER...Eckberg


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