Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 051717
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1117 AM CST Tue Dec 5 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 300 AM CST Tue Dec 5 2017

Several weather concerns will need to be addressed as much colder
air sweeps into the region and makes conditions feel more like
December than October. The first concern is the windy conditions
that have set up over the area with gusts approaching advisory
criteria. The second concern is rate of falling temperatures today
and possible freezing on area roads. The third concern is
potential lake effect for Vilas County today through Wednesday.

The 08z MSAS surface analysis showed the deep low pressure system
over northern sections of Lake Superior with a cold front extended
south-southwest through eastern WI. A very tight pressure gradient
existed over the Great Lakes/Midwest with most locations reporting
wind gusts of 30 to 45 mph. The radar mosaic indicated the main
band of light snow moving across northern WI.

Northeast WI residents to wake up to a completely different air
mass today as strong CAA has temperatures around 30 degrees colder
compared to Monday. The intense cyclone continues to track
northeast into Hudson Bay with the tight pressure gradient
remaining over WI between the cyclone and high pressure over the
central/southern Plains. Sustained west winds of 20 to 30 mph with
gusts of 35 to 45 mph can be expected. Do not plan on making any
changes to current headlines, allowing the wind advisory to expire
at 12z over central WI and continue the advisory over eastern WI
as downsloping helps keep wind gusts pushing advisory criteria.
The strong CAA, coupled with ongoing cyclonic flow, will bring a
chance of snow showers through the day for northern WI. The rest
of the forecast area should be generally mostly cloudy with
perhaps a few flurries. Temperatures will be steady or slowly
falling through the day with wind chills by late afternoon
dropping into the single digits below zero north-central WI,
single digits above zero elsewhere.

Winds are expected to only slowly subside tonight as the intense
cyclone stalls over Hudson Bay and gradually weakens overnight.
The cyclonic flow persists over WI, thus a good deal of clouds
should prevail, although persistent downsloping may allow for some
breaks in the clouds over eastern WI. The winds have too much of a
westerly component for much in the way of lake effect over north-
central WI, however the cyclonic flow should be enough to keep a
chance of snow showers across much of northern WI, with flurries
for the rest of the forecast area. Min temperatures to be more
typical of winter with readings around 10 above zero north-
central, middle to upper teens over eastern WI. Wind chills by
daybreak to remain in the single digits either above or below
zero.

The passage of a shortwave trough across the Great Lakes on
Wednesday will help to reinforce the cold air over northeast WI.
Winds are progged to veer to the northwest and with delta-T values
in the middle to upper teens, look for lake effect snow showers to
begin across north-central WI (mainly Vilas County). Perhaps up to
an inch of accumulation over the snowbelt of Vilas County with
primarily flurries elsewhere. Max temperatures on Wednesday to
range from around 20 degrees north-central, middle to upper 20s
eastern WI.

.LONG TERM...Wednesday Night Through Monday
Issued at 300 AM CST Tue Dec 5 2017

The long term period will feature a very amplified pattern, with a
strong ridge over the western U.S./Canada and a deep trough
across eastern U.S./Canada. This pattern will bring lake effect
snows for the snowbelts of Lake Superior and a couple shots of
snow showers for the rest of the area. Look for temps to generally
be 5-10 degrees below normal, except as the ridge briefly
flattens out across the Great Lakes this weekend, allowing temps
to moderate back to near normal. Another surge of cold air is
expected the middle of next week.

The lake effect snow machine will be in full force Wednesday
night. A more favorable NW wind, delta T`s in the 18-23C range
and sufficient lift and moisture will lead to snow showers across
much of north central WI, especially Vilas Co. Mid-level drying
above 2000-3000 ft will likely reduce the overall coverage
overnight into Thursday, but scattered snow shower activity is
expected to continue. Accumulations from Wednesday night into
early Thursday will be highest over northern WI, with an inch or
two possible. Farther south, lake effect snow showers and flurries
will be possible with little to no accumulation expected.

Lingering lake effect snow showers are possible on Thursday across
far north central WI, but overall activity should be diminishing
as winds back to the WNW and the drier air in the mid-levels.
Some sunshine is expected, especially as you work away from Lake
Superior, but model soundings are showing lingering moisture
around 3000 ft, so would not be surprised to see some clouds
stick around and/or develop with the cold air aloft.

The next chance for area-wide snow continues to be Friday, when a
fast moving piece of energy and weak frontal boundary (weak
clipper system) dive down from Canada within the upper NW flow.
While deeper moisture will accompany this system, precipitable
waters struggling to climb above 0.15" will limit snowfall totals,
with under 0.05" of QPF expected. In addition, the system will be
a fast mover, which will help limit totals. With snow ratios
around 20:1, accumulations should stay under an inch.

Lake effect snow will ramp up once again behind the weak clipper
system. A very favorable NNW wind looks to set up from late Friday
afternoon into Saturday, with impressive delta`s over 20C and
inversion heights over 8000ft, plus deep moisture. This could
lead to a brief period of heavier lake effect snow activity across
north central WI, with more isolated snow showers / flurries
farther south. Winds will back to the WNW on Saturday, so the lake
effect snow activity is expected to slowly diminish. Another
clipper-like system is set to dive across the area late Sunday
into Monday, bringing another chance of snow showers to the entire
area, along with another round of lake effect snow behind it. A
stronger system early next week now looks to track west of the
area, which will allow another surge of cold air to invade the
Great Lakes. All of these clipper systems are hard to pin down
this far out, so adjustments might be needed.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1113 AM CST Tue Dec 5 2017

Strong west winds with gusts in the 35 to 45 knot range are
expected to continue through the afternoon, particularly for
eastern WI. MVFR cigs should hold firm over northern WI, but some
clearing should work into parts of central and most of east-
central and far northeast Wisconsin this afternoon. Light snow
showers/flurries should also diminish as well. Another shortwave
will rotate through the region tonight. Should see cloud cover
surge southeast with occasional flurries and light snow returning
late tonight through the end of the taf period. Occasional IFR
vsbys possible in the heavier snow showers over northern WI.

&&

.MARINE...
Issued at 300 AM CST Tue Dec 5 2017

A Gale Warning remains in effect today for the entire Bay of Green
Bay and the nearshore waters of Lake MI. The Gale Warning
continues through tonight, with the exception of the central and
southern sections of the Bay. Sustained west winds of 20 to 30
knots with gusts of 45 knots can be expected, especially for
today.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM CST this evening for WIZ022-031-037>040-
045-048>050-073-074.
&&

$$

SHORT TERM.....Kallas
LONG TERM......Bersch
AVIATION.......MPC
MARINE.........Kallas



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