Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 100930
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
330 AM CST Sat Dec 10 2016

Forecast discussion for routine morning forecast issuance

.SYNOPSIS...
Issued at 324 AM CST Sat Dec 10 2016

A round of snow tonight into Sunday night, then a cold snap for
the middle to latter part of next week.

Blocky pattern over the northeast Pacific and the high latitudes
of North America will remain in place through the forecast
period. To the south, a seasonably strong band of nearly zonal
westerlies extended from the Pacific Northwest to the Mid-Atlantic
States. The westerlies will take on a broadly cyclonic structure
as deep upper vortex digs through the Hudson Bay area mid-week,
then flatten back to zonal again thereafter. Consolidation of
upper ridge in the Gulf of Alaska will likely lead to southwest
upper flow by the end of the period.

Temperatures will rollercoaster between near normal and well
below normal, with the most pronounced dip occurring during the
middle to latter part of the work week when a substantial cold
snap is expected. On the other hand, the fast, low-amplitude upper
flow will continue to make discerning details of the precipitation
forecast difficult. For the fourth day in a row, the models
presented a somewhat different look to the incoming snow system.
Though it`s received less attention because it`s much farther into
the forecast, the same thing is happening with another potential
system for next weekend. Overall though, it`s likely we will
start building up a substantial snow cover across the area.
Combined with the cold snap, that`s likely to make it feel like
mid-winter.
&&

.SHORT TERM...Today...Tonight...and Sunday
Issued at 324 AM CST Sat Dec 10 2016

First the near term trends. Clouds were more plentiful than much
of the short-term guidance suggests. Radar and surface
observations indicated flurries across the area as well. Increased
clouds for the start of the forecast and carried flurries into the
morning, then stuck with the trend for getting at least some
decrease in clouds before the high and middle clouds ahead of the
next system begin to arrive.

The main short-term forecast issue is the snow potential tonight
into Sunday. The main change in the latest model runs was to have
a shortwave embedded in the northern portion of the flow arrive
in the area earlier than progged yesterday. It`s now progged to be
much stronger as well. This results in a stronger wave riding along
the surface front, and tracks that wave farther north.

Although forcing is now stronger, it`s disjointed. The strongest
isentropic lift is likely to race through (focused south of the
area) late tonight. The QG forcing with the mid-level shortwave
mainly cuts across the north on Sunday/Sunday evening, with hints
of a dry slot wrapping toward the southern part of the forecast
area. The linked jet structure from yesterday is also much more
diffuse. The overall result of these changes was to shift more of
the precipitation with the system into northern Wisconsin, at the
expense of precipitation over the south. The latest snowfall
forecast has 4 to 7 inch totals over pretty much the whole area.

Another consideration is the possibility of a lake-effect band
over Lake Michigan backing into eastern Wisconsin Sunday. 850 mb
temperatures remain quite cold, and lake temperatures are still
in the upper 40s. Both meso WRFs show this scenario as does the
Canadian regional, though they don`t agree on where the band will
make landfall.

The current snowfall forecast supports an advisory more than a
warning. However, with a good portion of the upper energy
generating the system just now arriving onshore over the Pacific
Northwest and finally within the North American RAOB network,
it`s quite possible the guidance may need a little more time to
settle on the correct outcome. If nothing else, having consistency
between two consecutive model runs would be encouraging. Plus,
there is still the potential lake-effect issue that could
dramatically affect snow totals in the east. When all taken
together, the best course of action seemed to be to keep the watch
running intact. This meshed with plans from the MKX office as
well.

.LONG TERM...Sunday Night Through Friday
Issued at 324 AM CST Sat Dec 10 2016

Once the snow tapers off later Sunday night, full attention turns
to the first arctic surge of the season as upper ridging over
eastern AK/Yukon Territory unleashes the Siberian express into the
CONUS by the middle of next week. Temperatures from Tuesday night
through Friday are expected to be 15 to 25 degrees below normal,
bringing sub-zero mins and daytime maxes only in the single digits
above zero. Snow chances appear to be limited to the passage of
the arctic front Monday night and perhaps a bit of lake effect
over Vilas county, although prevailing west winds would favor
western Upper MI.

Snow will continue to fall through at least Sunday evening as the
surface low tracks northeast into the Great Lakes and the trailing
mid-level shortwave trough shifts east into the Great Lakes as
well. Perhaps an additional inch or two of snow is possible over
eastern WI Sunday evening with accumulation numbers becoming less
the farther west you go. The snow is forecast to gradually come to
an end from west to east after midnight as the stronger lift/
forcing exits the area. Min temperatures will actually be mild
Sunday night (compared to what is coming) with readings only
dropping into the middle to upper teens central, lower to middle
20s eastern WI. Monday appears to be a quiet day, albeit with more
clouds than sun despite having a weak area of high pressure pass
by to our south. There might be a few lake effect snow showers or
flurries that would clip Vilas county, but winds will have too
much of a westerly component for any accumulation to occur
(perhaps a dusting?). Max temperatures on Monday will range from
around 20 degrees central, middle 20s east except upper 20s near
the bay and lake.

A strong cold front is forecast to sweep across WI Monday night
and is expected to bring at least a small chance of snow showers
to the region. The bigger story will be the onset of the first
arctic surge this winter with temperatures falling well below
normal. While min temperatures Monday night may end up near
normal, plan on seeing readings fall through the day on Tuesday
with temperatures by sunset flirting with zero central, mainly 5
to 10 above zero eastern WI. Wind chills by late Tuesday are
progged to be as cold as the mid teens below zero central, 5 below
to 10 below zero eastern WI.

The bitterly cold arctic air mass will settle over the Great Lakes
region Wednesday and Thursday with strong Canadian high pressure
forecast to stretch from the northern Rockies toward the Ohio
Valley. This pattern would favor a continuation of a west wind
component across WI mid-week, but will have to closely monitor any
subtle shift to the west-northwest which would allow some lake
effect snow showers into Vilas county. Otherwise, temperatures/
wind chills will be the main story with min temperatures Tuesday
night/Wednesday night ranging from 5 below to 12 below zero
central, zero to 5 below east (around zero lakeside). Max
temperatures Wednesday/Thursday to only reach the single digits
above zero. Wind chills at night could approach/surpass advisory
criteria depending on wind speeds.

Arctic high pressure is forecast to hang on in the vicinity of the
Great Lakes through Friday, although some of the models are
attempting to already bring precipitation back to the area later
on Friday. This is in association with the next weather system
that will be organizing over the central/southern Rockies. Have
crept a low pop into central WI later on Friday, but it is looking
like more snow is on the way for next weekend as this system
eventually moves up the Ohio Valley. The crux of the arctic air is
progged to lift back into Canada by Friday, but with another cold
start to the day, max temperatures on Friday will only reach 5 to
10 above zero central, 10 to 15 above eastern WI.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 324 AM CST Sat Dec 10 2016

Low clouds have proven a bit tougher to dislodge than earlier
expected. That may be due to a stronger inversion now in place
across the area. Most bases were low-end VFR, though some MVFR
ceilings were reported. Plan to have improving conditions in the
12Z TAFs, though how quickly to move the forecast in that
direction will be an issuance time decision. Flight conditions
will lower to IFR from southwest to northeast late tonight as snow
overspreads the area.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from this evening through Sunday evening FOR
WIZ037>040-048>050.

Winter Storm Watch from this evening through Sunday evening FOR
WIZ035-036-045.

Winter Storm Watch from late tonight through late Sunday night
FOR WIZ022.
&&

$$

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


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