Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 100927
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
327 AM CST Sun Dec 10 2017

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Monday
Issued at 326 AM CST Sun Dec 10 2017

Precipitation trends and snowfall amounts are the main forecast
concerns.

An area of light snow and flurries associated with WAA was moving
out of far eastern WI early this morning. A cold front was
situated over northern upper Michigan and southern Lake Superior.

The cold front is expected to sag south across the forecast area
today. Low-level winds will turn NNW in the Lake Superior snowbelt
this morning after the front moves through, setting the stage for
another round of minor LES over Vilas county. Lake-H8 delta-t`s
will increase to 17-20 C, so have opted to raise pops to likely in
NW Vilas county and mention accumulations up to an inch. Overcast
skies should give way to partial clearing this afternoon and
early evening as a surface ridge moves in and subsidence/drying
occurs. High temperatures should be in the lower to middle 20s
northwest to the upper 20s elsewhere.

Another clipper low pressure system will impact the forecast
area late tonight (after midnight) through Monday morning,
with most locations receiving 1 to 2 inches of snow. Slightly
higher amounts (2-3 inches) are expected in far north central
WI. The snow will taper off Monday afternoon, except in the
Lake Superior snowbelt, where developing NNW winds and increasing
over-water instability will generate more LES over Vilas county.
Lows tonight will be in the upper single digits and teens, with
slowly rising temperatures overnight. Highs on Monday should
reach the middle 20s north to the upper 20s and lower 30s south.

.LONG TERM...Monday Night Through Saturday
Issued at 326 AM CST Sun Dec 10 2017

The static pattern of a western NOAM upper ridge/eastern NOAM
upper trough to persist through Thursday, before the arrival of an
upper trough flattens the ridge for Friday/Saturday. Until this
flattening occurs, the northwest flow into WI will continue with
at least two more clippers through mid-week and perhaps another
clipper next weekend. Timing/track of each clipper remains the
main forecast challenge, which would affect potential snow
amounts. Temperatures to be at or below normal through Thursday,
then trend up as the flow turns more west-northwest into WI.

Gusty northwest winds will be left over the western Great Lakes
region Monday night as the initial clipper low pressure tracks
into the eastern Great Lakes. 8H temperatures over western Lake
Superior to approach -20C and with favorable trajectories in
place, anticipate lake effect snow showers to persist through the
night over north-central WI. The only negative remains the low-
level inversion heights, thus accumulations should only be at an
inch or less (mainly over the snowbelt area of Vilas County).
Subsidence behind the departed system should allow skies to become
mostly clear to partly cloudy over the rest of the forecast area
Monday night. Min temperatures will be in the 5 to 10 above zero
range north-central, mainly 10 to 15 degree range elsewhere.

These lake effect snow showers are expected to continue into
Tuesday over north-central WI, but tend to become more widely
scattered in the afternoon as drier air arrives ahead of a surface
ridge that is forecast to reach the Upper MS Valley by 00z
Wednesday. Perhaps another one-half inch of accumulation for parts
of Vilas County, otherwise partly cloudy skies should suffice for
the rest of northeast WI. The air mass overhead remains quite
cold, thus max temperatures will struggle to reach 10 to 15 above
north, 15 to 20 degrees south (or about 10 to 15 degrees below
normal).

This surface ridge slides into the western Great Lakes Tuesday
night and with winds diminishing to accompany the snow on the
ground, temperatures should easily fall (especially if skies can
stay mostly clear). Right now, Tuesday night would be the coldest
night of the extended forecast with min temperatures around zero
north-central, single digits above zero elsewhere except around 10
above near Lake MI. The next clipper of interest is forecast by
most models to move from near Lake Winnipeg toward the Mid-MS
Valley on Wednesday. Assuming this track holds, this would keep
any snow to our west. Therefore, have kept Wednesday dry across
the entire forecast area with a slight bump-up in temperatures.
Look for readings in the middle teens north-central, upper teens
to around 20 degrees elsewhere.

Yet another clipper slides toward northern sections of the Great
Lakes Wednesday night into Thursday morning. Moisture looks rather
limited with this system, however enough is present to warrant at
least high chance pops in the forecast. Another shot of cold air
overspreads WI on Thursday behind this clipper and with northwest
winds expected, more lake effect snow showers are forecast across
north-central WI. Too early yet for accumulation numbers, but at
least a minor accumulation would be possible. Max temperatures on
Thursday to range from the lower to middle 20s north, middle to
upper 20s south.

The latest round of lake effect will continue through Thursday
night, before a ridge of high pressure builds into WI on Friday
and trajectories become unfavorable. Skies may briefly become
partly cloudy over the rest of the forecast area Thursday night
into Friday morning, however clouds are expected to be on the
increase by Friday afternoon ahead of our next clipper. The onset
of weak WAA into WI ahead of this clipper may be enough to allow
for a slight chance of light snow to develop. Temperatures on
Friday should be closer to normal as the western NOAM upper ridge
breaks down. Look for readings in the middle 20s north, upper 20s
to around 30 degrees south.

This next clipper is forecast to be stronger than its predecessor,
but models track the surface low differently (the GFS through
northern sections of the Great Lakes/the ECMWF through central
WI). Either solution would be a chance of light snow to northeast
WI starting Friday night and continuing into at least Saturday
morning. A more northern track could also allow for some rain to
mix with the snow on Saturday over central/east-central WI.
Temperatures will moderate a bit ahead of the clipper with
readings around 30 degrees north-central, lower to middle 30s
elsewhere on Saturday.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1036 PM CST Sat Dec 9 2017

Cold cyclonic flow will remain across the area during the period.
Clouds will be most plentiful and have the lowest bases in north-
central Wisconsin. An area of snow showers and flurries will drop
southeast across the area overnight, likely followed by MVFR
ceilings moving into the north and west portion of the area.
&&

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

$$

SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......Skowronski



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