Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 071608
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1008 AM CST Wed Dec 7 2016

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 427 AM CST Wed Dec 7 2016

A more wintry weather pattern is settling into the area. Blustery
and colder with some flurries today. Significant snow in the Lake
Superior snowbelt late tonight into Friday...then snow possible
across the entire area this weekend.

Near total splitting of the Pacific jet out near the dateline
combined with high upper heights over the northeast Pacific are
blocking the westerly flow of air into North America at high
latitudes. That is allowing cold air to build up over eastern
Alaska and Canada. To the south, a fairly strong band of
westerlies with modest amplitude is in place across the CONUS. The
mean trough position within the westerlies will progress from the
Plains into the East the next several days. After that, the flow
will briefly flatten before the trough re-forms back out to the
west during the latter part of the forecast period.

This is an active weather pattern, and much more wintry than
recent patterns. Though varying some from day to day, temperatures
are likely to be near or a little below normal for much of the
forecast period, before dropping off late. The strong baroclinic
zone near the area is likely to result in some light snow at
times. The most likely time for a widespread snowfall is during
the upcoming weekend, though some significant lake-effect will
precede that across the Lake Superior snowbelt.
&&

.SHORT TERM...Today...Tonight...and Thursday
Issued at 427 AM CST Wed Dec 7 2016

The cyclone that generated scattered snow showers across northern
Wisconsin yesterday will shift east across Ontario today. Clouds
were plentiful across most of the area early this morning. Some
clearing developed in portions of east-central Wisconsin, but
that should be short-lived. Most locations with clouds were
reporting flurries as well, with some snow showers in north-
central Wisconsin. Will keep plenty of clouds and snow flurries
across the area today. Winds will also remain quite gusty as
colder air wraps around the southwest side of the departing cyclone.

As the cyclone pulls further away tonight, a low-level trough
hanging back its wake will begin to swing south across the area.
The thermal structure of the trough looks almost like a warm
front, with slightly warmer air to the north. Snow flurries and
snow showers will likely increase as the feature swings through.
Of more importance will be the low-level flow shifting to the
north northwest. This will set up a lake-effect situation for the
Lake Superior snowbelt. Forecast soundings for IWD off both the
NAM and GFS looked very favorable for significant lake-effect in
Vilas county. Given the deep moisture, very favorable trajectory,
low shear, and high EL, am very concerned an organized snow band
could set up over western Vilas county Thursday. For those
reasons, upped snow amounts and decided to post a Winter Storm
Warning for late tonight into mid-day Friday. The heaviest snows
are likely to occur Thursday--probably in the mid-late morning.
Similar situations in the past have actually allowed significant
snows to get into western Oneida county as well. But that would
require holding the favorable trajectory for most of the day.
That`s possible, but also hard to depend on this far in advance.
Later shifts can reassess the need for expanding the warning or
possibly buffering it with an advisory.

Elsewhere, it`s possible snow showers associated with the passage
of the trough could bring a light snow accumulation to the
remainder of the forecast area. Upped PoPs from what we had in the
previous forecast, and would not be surprised if further upward
adjustment is necessary.

.LONG TERM...Thursday Night Through Tuesday
Issued at 312 AM CST Wed Dec 7 2016

Since the mean flow is expected to run nearly zonal to broadly
cyclonic across the CONUS through the extended period, this would
translate into fast-moving systems which will be difficult to time
such a progressive, low-amplitude flow. Initially, onset of colder
weather will be more of a lake effect issue for north-central WI.
However, potential for accumulating snow remains on the table
later this weekend with the first Pacific system. Main story for
next week will be the depth of the arctic air and exactly how cold
we can get.

Lake effect snow showers will continue over north-central WI
Thursday night with delta-T values in the lower 20s, favorable
trajectories and lake-induced pops around 500 J/KG. The only real
negative is hints on forecast soundings that inversion heights
will lower through the night which would limit (lessen)
accumulation potential overnight. The snowbelt region of Vilas
county could pick up an additional 1-3" of snow, which would bring
a 2-day total into the 6 to 10" range if current trends persist.
The rest of northeast WI will see cloudy skies and cool conditions
with a prevailing west-northwest wind at 5 to 15 mph. Min
temperatures to be in the lower to middle teens central, upper
teens to around 20 degrees eastern WI.

The lake effect snow showers will persist into Friday morning, but
remain somewhat limited by low-level inversion heights around 5K
ft. By Friday afternoon, a ridge of high pressure is forecast to
build into the Upper MS Valley/Midwest and start to back the winds
which would make trajectories become less favorable. The Vilas
county snowbelt could see another inch of accumulation before
tapering off later in the day. Central/east-central WI may be able
to see some mixed sunshine at times, but this will not help
temperatures as readings are only expected to be in the 15-20
degree range north, 20-25 degree range south.

While a few lingering light snow showers or flurries may still
reach Vilas county into Friday evening, the surface ridge axis is
progged to move into WI during the overnight hours and keep dry
conditions over the vast majority of the area. Even with this
ridge nearby, clouds may be hard to get rid of as WAA already
underway to our west. Keeping clouds around will not help
temperatures as colder air has already settled over the region.
Min temperatures to be in the single digits above zero central,
10-15 above eastern WI. The thickening clouds are expected to give
way to the onset of light snow, especially by Saturday afternoon
as mid-level forcing/WAA overspreads WI in advance of a mid-level
trough/surface cyclogenesis located over the central Plains. It is
conceivable that parts of central WI could pick up around an inch
of snow by 00z Sunday. Otherwise, any snow would be on the light
side with only minor accumulations expected. Max temperatures on
Saturday will remain 5 to 10 degrees below normal with readings
similar to Friday.

There continues to be some timing issues with this system as the
GFS is a good 12 hours faster than the ECMWF. This will ultimately
determine whether the better snow chances arrive Saturday night
(GFS) or on Sunday (ECMWF). Due to the fast-moving low-amplitude
flow, tend to lean toward the GFS which would bring potential
advisory-level snow accumulations to northeast WI. The snow
chances would carry over into Sunday, but in a diminishing mode
especially by Sunday afternoon. The WAA preceding the system will
help boost temperatures closer to normal on Sunday with readings
able to reach the lower 20s north-central, upper 20s to around 30
degrees east-central WI.

Only modest cold air is forecast by the models to get pulled into
WI behind the system and with trajectories maintaining more of a
westerly component, lake effect impacts for north-central WI
Sunday night/Monday appear minimal. Max temperatures on Monday
will be cooler with most locations in the teens, except lower 20s
near Lake MI. The passage of a strong cold front late Monday
night/Tuesday morning may bring a chance of light snow to
northeast WI, but the bigger story will be the first true taste of
arctic air this season with max temperatures Tuesday only in the
single digits north-central, to middle teens east-central WI.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1008 AM CST Wed Dec 7 2016

Wrap around clouds and colder air spilling into the
state will continue to produce mainly MVFR cigs today into
tonight. MVFR vsbys due to snow showers will also continue over
parts of central and north central Wisconsin today, then shift
northward to north central Wisconsin later tonight with lake
effect snow development. IFR cigs and vsbys due to snow showers
will be on the increase across far North Central Wisconsin. A few
of these snow showers may reach the RHI taf site at times.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 3 AM Thursday to noon CST Friday FOR
WIZ005.
&&

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......TDH


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