Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 132342

Area Forecast Discussion
National Weather Service Green Bay WI
542 PM CST Sat Jan 13 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Sunday
Issued at 209 PM CST Sat Jan 13 2018

The main forecast concerns center around the possible need for a
wind chill advisory tonight and how fast to bring the snow east
Sunday afternoon.

The 19z MSAS surface analysis showed a broad area of Arctic high
pressure centered over northern MN. This high pressure dominated
the central CONUS with very cold air on its east side, more clouds
and not quite as cold on its west side. Visible satellite imagery
showed clear skies across WI.

The Arctic high pressure is forecast to move from the Upper
Midwest toward the Ohio Valley by daybreak. The combination of a
very cold air mass, mostly clear skies and light winds will allow
temperatures to plummet tonight, especially over our typically
cold spots of north-central WI. There will be a gradual increase
in high clouds later tonight ahead of our next weather system, but
these clouds will have little impact on temperatures. For now,
have min temperatures ranging from zero to 5 below near Lake MI,
20 below to 25 below zero over parts of north-central WI. Due to
anticipated wind speeds being 5 mph or less over the forecast
area, prefer to not issue a wind chill advisory at this time.

Mid/high clouds will continue to overspread northeast WI Sunday
morning as a clipper system drops southeast into northern MN. As
winds slowly start to increase in the morning, wind chills could
briefly surpass advisory thresholds, however due to the small time
frame, will hold off on any headlines and issue a SPS. By Sunday
afternoon, the clipper reaches southeast MN with a better surge of
WAA into WI as southerly winds take over. There is also a strong
shortwave showed by the models reaching the MN/WI border by 00z
Monday. Expect the leading edge of light snow to reach north-
central/central WI by mid to late afternoon, so will have
increasing pops for this part of the forecast area. Eastern WI to
only see clouds continue to thicken, but remain dry until the
early evening hours. Max temperatures for Sunday to range from 5
to 10 above north-central, 15 to 20 degrees east-central WI.

.LONG TERM...Sunday Night Through Saturday
Issued at 209 PM CST Sat Jan 13 2018

Primary forecast issue is the snow potential Sunday night into
Monday night. Combination of system snow with lake effect snow
potential have prompted the start of winter headlines for far
northeast Wisconsin.

North to south elongated surface low is progged to slowly drop
into the region Sunday night to spread snow over the area from
west to east. While an accumulation of an inch or two of snow
accumulation is likely across the state Sunday night, south winds
and height 850 temps of -14 C late Sunday night will likely lead
to a south to north LES band of snow over Lake Michigan. This
band may begin to clip far northern Door County late Sunday night.
But then the potential of this band moving westward over Door
County increases as the boundary layer component turns more
easterly as the surface and 850 lows track over central Wisconsin
toward Monday morning. Anticipate system snow will linger into
Monday and perhaps Monday evening as the 850 low is still progged
to be drifting over central Lake Michigan Monday evening. Track
of the 850 mb low may allow the the remains of the convergent band
to orientate more into far northeast Wisconsin as winds over Door
County are even more easterly and areas to the south more
westerly. Combination of LES potential and system snow producing
forecast snow amounts of more than 10 inches over at least far
Door County. Will issue a winter storm watch since a 3rd period.
Its likely winter weather headlines will eventually be needed for
more of the area, but amounts were more borderline away from Lake
Michigan. Later shifts can take another look at that.

Cold air advection will produce lake effect snows through tuesday
night, followed with milder air returning later in the week and
into the first half of next weekend. A system tracking north of
the area late next week may produce a chance of mostly light snow
across the north.

.AVIATION...for 00Z TAF Issuance
Issued at 541 PM CST Sat Jan 13 2018

VFR conditions will continue through Sunday morning as arctic high
pressure exits the area. High clouds will gradually move into
central WI tonight, followed by mid/high clouds through much of
Sunday. Chances for light snow across central WI will then
increase in the middle to late afternoon on Sunday, as clipper
system moves into the area. MVFR conditions will be possible as
visibilities decrease and cloud decks lower. Depending on the
speed this system overspreads the region, GRB and MTW may also see
light snow by the end of the TAF period. &&

Winter Storm Watch from late Sunday night through Monday
afternoon for WIZ022.


SHORT TERM.....Kallas
AVIATION.......Uhlmann is the U.S. government's official web portal to all federal, state and local government web resources and services.