Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 091144
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
544 AM CST Sat Dec 9 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Sunday
Issued at 319 AM CST Sat Dec 9 2017

Headline decisions and snowfall trends are the main forecast
concerns.

The sharp upper trof was moving through eastern WI early this
morning, with a rapid decrease in snowfall occurring with its
passage. Visibilities west and north of GRB had increased to 3
miles or greater. Given that most locations have only received
an inch or two of snow and additional accumulations will be less
than an inch, have opted to cancel the winter weather advisory.
There may be some patchy blowing snow along the lakeshore
counties, but snow here at the NWS office has not been as powdery
as expected, so impacts should be minimal. Will continue to
mention hazardous travel in the Hazardous Weather Outlook for
this morning.

Conditons should be briefly quite favorable for lake-effect
snow showers in Vilas county, as low-level winds become NNW and
Lake-H8 delta-t`s increase to 19-23 C. Another inch or two of
snow could fall in northwest Vilas county through midday. The
LES will taper off as a ridge of high pressure arrives in the
afternoon. Highs should be range from the mid to upper teens
northwest to the mid to upper 20s southeast.

Despite a ridge of high pressure at the surface, mid-level
WAA will result in considerable cloudiness tonight. This
should prevent a temperature freefall, though chilly readings
in the upper single digits and teens will still occur.

A cold front will sag into northern WI on Sunday. As winds
turn northwest, there will be a chance of LES in the snowbelt
of north central WI, especially Vilas county. The rest of the
region should have quiet weather, with highs in the 20s.

.LONG TERM...Sunday Night Through Friday
Issued at 319 AM CST Sat Dec 9 2017

Mean flow remains amplified through next Thursday with an western
NOAM upper ridge and east-central NOAM upper trough. Models
continue to show a series of mid-level shortwave troughs/clipper
lows diving southeast through the Great Lakes within this deep
upper trough over the next 7 days. Trying to determine the track
of each of these systems remains the biggest forecast headache.
Temperatures are still expected to warm ahead of each individual
system, then cool down behind each system.

The next clipper/associated mid-level shortwave trough is forecast
to move southeast into the Upper MS Valley Sunday night. WAA
pushing into WI ahead of the surface low will allow for light snow
to gradually overspread northeast WI after midnight once the
atmosphere saturates. Due to the late start of the snow,
accumulations would be limited to one-half to one inch with the
higher values over central WI. Min temperatures will be mainly in
the 10 to 15 degree range north, upper teens to lower 20s south.
The clipper low quickly moves across southern WI Monday morning
and is expected to reach southern sections of the Great Lakes
Monday afternoon. A good chance or likely pops still look good for
Monday morning with another one-half to one inch of snow possible.
The main concern on Monday is the potential for the loss of ice
aloft which brings freezing drizzle into play. The models have
some issues in this regard, but the majority suggest that enough
ice to be present such that any freezing precipitation would be
minimal. Colder air is forecast to begin pulling into the region
by Monday afternoon behind the clipper, but not before
temperatures reach the lower to middle 20s north-central, upper
20s to around 30 degrees over our southern tier of counties.

The main forecast issue for Monday night will be lake effect snow
showers over north-central WI as favorable trajectories and influx
of colder air (8H temperatures drop to around -20C over Lake
Superior) are in place. The one big negative is the low inversion
heights, thus accumulations should be on the light side. The rest
of northeast WI will see any lingering light snow showers end
Monday evening with mostly cloudy skies for the overnight hours.
Min temperatures to range from 5 to 10 above north/parts of
central WI, 10 to 15 degrees for the rest of central/eastern WI.
The chance of lake effect snow showers is forecast to continue
into Tuesday, mainly over Vilas County, but in a diminishing state
as a ridge of high pressure pushes toward the western Great Lakes
by Tuesday afternoon. Little if any additional accumulation is
anticipated with sky conditions holding mostly cloudy north and
partly cloudy south. Tuesday is also looking like the coldest day
of the extended forecast with max temperatures only from 10 to 15
degrees central WI, middle to upper teens for eastern WI.

This surface ridge slides across WI Tuesday night, but clouds may
already be on the increase ahead of the next clipper system diving
southeast into the Upper MS Valley by 12z Wednesday. The track of
this clipper is in dispute among the models as the CMC/ECMWF take
the surface low through the Midwest toward the mid-MS Valley on
Wednesday versus the GFS which tracks the low pressure into WI.
Due to the broad variance in potential track, have followed the
consensus solution and bring a chance of light snow to all of
northeast WI for now until confidence improves. Max temperatures
are expected to `warm` a bit with readings generally in the upper
teens to lower 20s.

The air mass behind this latest clipper is not as cold as what
occurred earlier in the week. Nevertheless, anticipate another
round of light lake effect snow showers across north-central WI
Wednesday night into Thursday. The rest of the forecast area could
see a few snow showers or flurries Wednesday night and mainly dry
conditions for Thursday. Max temperatures to continue slowly
warming with lower to middle 20s north, middle to upper 20s south.

Light lake effect snow showers to persist into Thursday night
(mainly Vilas County), then we will have to watch the timing of
the next clipper later on Friday. Models have timing issues once
again, thus pops kept fairly low for now. Max temperatures for
Friday look similar to Thursday, plus a degree or two.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 533 AM CST Sat Dec 9 2017

An upper level trof will shift east of the region this morning,
causing light snow and flurries to diminish in eastern WI. Lake
effect snow showers will continue in Vilas county during the
morning, then taper off in the afternoon as weak high pressure
arrives. Although MVFR ceilings will linger through most of the
morning, some breaks in the low clouds will occur in spots. Much
of the region should return to VFR conditions this afternoon and
tonight, though low clouds may redevelop in parts of north central
and central WI late tonight.

Gusty northerly winds will continue through this afternoon,
before rapidly diminishing towards sunset.
&&

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

$$

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



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