Area Forecast Discussion
Issued by NWS Green Bay, WI

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924
FXUS63 KGRB 261955
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
255 PM CDT Mon Jun 26 2017

Forecast discussion for routine afternoon forecast issuance

.SYNOPSIS...
Issued at 255 PM CDT Mon Jun 26 2017

Chilly tonight, then mainly seasonable temperatures with showers
and thunderstorms Wednesday and Wednesday night---and at times
during the upcoming weekend.

The main westerlies across North America are currently fairly
amplified, with a ridge over the Intermountain West and troughing
in the Great Lakes region. The flow will flatten the next couple
days as a lower amplitude upper trough reforms back over the
northern Rockies. The new trough will amplify as it progresses
east, back into the Great Lakes region by next weekend.

Canadian air ushered into the area by northwest upper flow will
result in cool temperatures tonight. After that, temperatures will
probably stay fairly close to seasonal normals for the rest of the
period, with day to day variations mainly due to precipitation and
differences in cloud cover. The forecast upper flow is pretty
energetic for mid-summer, so precipitation amounts will probably
end up above normal for the period.
&&

.SHORT TERM...Tonight and Tuesday
Issued at 255 PM CDT Mon Jun 26 2017

A cold upper system and cyclonic flow at the surface resulted in
plenty of clouds and scattered showers across the area today. The
12Z models were showing the core of the cold air associated with
this system passing across east central Wisconsin today and
exiting the state to the east before 00Z Tuesday. The cold air
mass combined with clouds and precipitation to keep temperatures
in the area from rising out of the 50s during the morning. Some
breaks in the clouds and showers, and high sun angle manged to get
readings to rise into the lower 60s at many locations south of
Highway 29 early this afternoon. Dew points were in the 40s at
most locations.

Showers, and much of the cloud-cover, should dissipate from north
to south in the late afternoon/early evening as the upper system
moves away from Wisconsin. Surface high pressure over the Plains
today should reach the Mississippi Valley by 12Z Tuesday and the
surface pressure gradient will decrease across Wisconsin as it
does, but there should still be some wind during the overnight
hours. Warm advection is also expected to kick in tonight, so that
should also help to keep temperatures from going into free-fall.
Lows are forecast to be mostly in the 40s, with a few 30s across
north central Wisconsin.

Expect increasing high clouds on Tuesday, and temperatures will
be much warmer/closer to normal due to the warm advection and
sunshine. Lower to middle 70s should prevail at most locations.

.LONG TERM...Tuesday Night Through Monday
Issued at 255 PM CDT Mon Jun 26 2017

Models have attempted to speed-up the arrival of the precipitation
with the mid-week system. Am somewhat skeptical of that given cool
stable air that will be lingering across the area from the
departing anticyclone, so stuck with a generally dry forecast for
Tuesday night, confining PoPs to the far west late. But remnants
of convection to the west should surge across the area Wednesday
morning as the LLJ becomes directed into the area. Given model
mid-level lapse rates and the stable air across the area prior to
the arrival of the convection, the potential for severe weather
with the early-day storms seems very limited. The severe potential
should instead be tied to redevelopment during the day. There are
a lot of questions about that as well though, as the 12Z ECMWF
shows the mid-level shortwave already crossing the forecast area
by 00Z. The high sun angle at this time of year can allow for a
very a quick air mass recovery after a batch of storms, but at
least some sunshine would be required and at this point that does
not appear to be a given. Plan to word the HWO for a conditional
risk of severe.

Will stick with the dry forecast for Thursday that came from the
model blend, though the relatively weak surface pattern suggests
moisture lingering from the Wednesday/Wednesday night rains could
result in scattered afternoon convection. The next substantial
chance for rain will begin Friday and extend into the upcoming
weekend as the upper trough reamplifies over the area. The
potential for severe storms is hard to ascertain this far in
advance, though the pattern seems supportive of a marginal or low-
end severe threat at most.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 103 PM CDT Mon Jun 26 2017

A cold upper system and cyclonic flow at the surface resulted in
plenty of low clouds and scattered showers. Ceilings were MVFR
across the north with a mix of MVFR and VFR farther to the south.
Most of the showers were not reducing visibility below VFR at
observation sites, but based on radar reflectivity some of the
heavier showers may bring brief MVFR visibility. There could still
be a rumble or two thunder and/or some small hail this afternoon.
Showers should gradually come to an end as the upper system
departs and daytime heating is lost. Expect ceilings to become VFR
as the system moves away from the area. MOS from the 12Z NAM &
GFS indicated no restrictions to visibility tonight, and dew point
depressions are forecast to be at least 3 or 4 degrees during the
night, so have not included fog in the TAFs. Warm advection
starts to pick up during the night as well.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....MG
LONG TERM......Skowronski
AVIATION.......MG



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