Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 082056
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
256 PM CST Thu Dec 8 2016

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Friday
Issued at 255 PM CST Thu Dec 8 2016

Primary issue through Friday will be the lake effect snows across
northern Wisconsin. Snow bands have increased today across the
far north in the wake of a trough of low pressure which passed
over earlier today. Some clearing occurred over eastern wisconsin
this morning in a brief subsidence region but clouds were filling
back in.

Early this afternoon, MQT Radar loop shows rather long plumes of
30 dbz to 4000 agl over Upper michigan approaching Florence and
Forest counties. Very unstable 1.5 KM lapse rates continue to
increase over northern Wisconsin today to enhance the bands. a few
of these bands of snow showers continued to drift southward
toward northeast and east central Wisconsin this afternoon.

Towards Vilas and Oneida counties where radar tends to overshoot
the bands, visible satellite loop suggests convective snow showers
picking up in intensity in some locations with IWD at a 1/2 mile
as of 11 am and manitowish waters 1/4 mile at noon briefly but
then have diminished. As usual, these narrow bands of moderate
lake effect snows are hit and miss with available observations.
Satellite appears to show a bit more persistent band from around
Gile just southwest of IWD to far western portions of Vilas and
Oneida counties. Rhinelander TV12 reported a half inch of snow in
45 minutes with a snow showers early in the afternoon.  Will let
the headlines continue as conditions for lake effect snows and
later shifts can adjust as needed.

Boundary layer winds gradually turn more westerly on Friday as
a low pressure system departs and a ridge of high pressure builds
toward the Western Great Lakes region. As a result, lake effect
snows will be on the decrease, with clouds beginning to erode
later Friday.

.LONG TERM...Friday Night Through Thursday
Issued at 255 PM CST Thu Dec 8 2016

Winter has definitely returned to northeast Wisconsin. Main issues
in the long term evolve around the system later this weekend into
Monday, then the arrival of very cold air midweek.

Uncertainty surrounding the weekend system is still fairly high.
With mainly zonal flow aloft, models are having a hard time
resolving the the strength and track of a series of systems
pushing through the southern Great Lakes Region starting Saturday
night. The general idea would be a low moving through Saturday
night into Sunday, and then another wave pushing through Sunday
night into Monday. As far as the 12 model runs go, largely
disregarded the NAM since it was a significant outlier. The 850mb
baroclinic zone is much too far north compared to a typical winter
set up and the other 12Z long range models. Used more of a
ECMWF/CMC/GFS blend through Sunday night, then focused more on the
ECWMF/CMC which had some agreement while the GFS was the outlier
with a sfc low deepening along a strong baroclinic zone. The
factors that will really drive the track and strength of the
system are where the low level baroclinic zones sets up and the
strength of the mid level shortwaves that swing through the mean
cyclonic flow first Sunday and then another Monday morning.

Based on model consensus noted above, trends with the first wave
Saturday night and Sunday have been edging further south with the
first swath of snow. Decreased PoPs and QPF totals in north
central and northern Wisconsin from previous forecasts as the main
low and better forcing swings through the southern half of
Wisconsin.

Sunday afternoon is an interesting forecast period and will need
to be watched as we get closer to the event. After 18Z Sunday,
southwest winds at 85mb begin to back southeast before becoming
east Sunday evening. 850mb temps will be cold around -10C and Lake
Michigan waters are still relatively warm, which would support
lake effect enhancement/snow showers as the winds go southeast and
east. 1000-850 lapse rates Sunday afternoon and night increase to
6-7C/km over the Lakeshore and Fox Valley, so bands of snow with
bursts of heavier snow are possible. The ECWMF and GFS hint at
this with enhanced QPF along the Lakeshore and a weak inverted
sfc trough shifting eastward after 18Z Sunday. Given the mesoscale
resolution of lake effect snow, obviously will not know the extent
of this feature until we are much closer to the actual event.

The second, more impressive system then pushes through Sunday
night and Monday. This system has more structure and show a
decent sfc low pushing northeast through Illinois into lower
Michigan. Again, better chances for snow will be through central
and eastern Wisconsin, however other details are still hazy.

The system should exit the Great Lakes region later on Monday.
There is another chance for quick hitting snow Monday night and
Tuesday as a system rides along an arctic front pushing into the
Upper Great Lakes region, however too much model disagreement is
present at this time to add any detail on this event. The bigger
impact from that Arctic front will be behind it as bitterly cold
air surges into the state by midweek. Below normal temperatures
are expected by Wednesday, and below zero low temperatures are
possible for the second half of next week, with even colder wind
chills.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1136 AM CST Thu Dec 8 2016

Mainly MVFR clouds will persist across the region
today into tonight, with periods of IFR conditions across the
north due to lake effect snows off Lake Superior. Clouds over
northeast Wisconsin thinned out this morning in the subsidence
region of a departing trough, but anticipate MVFR clouds to be on
the increase this afternoon as low level lapse rates increase.
As an upper low system departs east late tonight into Friday and
a surface ridge approaches from the west, some improvement
expected Friday from the southwest.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon CST Friday FOR WIZ005.

Winter Weather Advisory until 6 AM CST Friday FOR WIZ010.
&&

$$

SHORT TERM.....TDH
LONG TERM......Allen
AVIATION.......TDH



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