Area Forecast Discussion
Issued by NWS Green Bay, WI

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917
FXUS63 KGRB 172057
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
257 PM CST Sun Dec 17 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Monday
Issued at 256 PM CST Sun Dec 17 2017

The main forecast concerns are precipitation chances during the
late afternoon and evening, cloud trends and temperatures.

The RRQ of an upper level jet and mid-level frontogenetic forcing
will produce enough lift/saturation for several hours of light
snow over far northern WI during the late afternoon and early
evening. Would expect accumulations to remain under an inch.
A second frontogenetic band has developed over central and east
central WI, and should affect our southern couple tiers of
counties into early evening.

Satellite and surface observations showed widespread low clouds
across nearly all of the Upper Mississippi Valley and western
Great Lakes. RH timesections also show low-level moisture holding
tough through the night, so do not anticipate any clearing.
Patchy/areas of fog will also continue overnight. Lows should be
fairly mild and in the 20s.

Increasing southwest winds, warm advection and partial clearing
will make for a milder day Monday. Highs could hit 40 F where
there is less snow cover over our southwest counties, but most
places should be in the middle 30s.

.LONG TERM...Monday Night Through Sunday
Issued at 256 PM CST Sun Dec 17 2017

The potential for a winter storm over the latter half of the
work week is the primary forecast concern.

A cold frontal passage will occur Monday night, with CAA,
subsidence and 925-850 mb winds of 35 to 45 kts resulting in
strong west winds into Tuesday. Will continue the mention
of potential gale force gusts on the lake and bay in the
Hazardous Weather Outlook.

Attention turns to a potential winter storm, which looks like
it will be long-lived, impacting the region during the latter
half of the work week and possibly into the beginning of the
weekend. The initial bands of mainly light snow Wednesday
afternoon and night are expected to develop in the RRQ of an upper
jet, aided by WAA and mid-level frontogenetic forcing. The 00z
ECMWF and 12z GFS both ramp the storm up starting later Thursday
afternoon and night, as coupled upper jets develop, and low
pressure lifts northeast from the OK Panhandle. The low has
shifted farther north, and is now expected to track through east
central WI on Friday afternoon or evening. This track favors heavy
snow over the northwest half of the forecast area, with more
potential for dry slotting, and perhaps mixed precipitation or
rain in the Fox Valley and lakeshore areas. This storm has the
potential to cause significant impacts on early Christmas travel.
There will be many details to sort out with subsequent forecasts,
including the track and timing of the low, snowfall amounts and
the location of the axis of heaviest snow, and the potential for
mixed precipitation. It is still too early to pinpoint most of
these details.

Colder air will arrive in the wake of the winter storm for the
weekend, with the really cold arctic air (H8 temps of -25 to -30
C) arriving on Sunday. Some minor lake effect snow is expected in
north central WI.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1100 AM CST Sun Dec 17 2017

Moist air near the ground will keep IFR/LIFR ceilings and
MVFR/IFR vsbys in fog across most of the area through most of
the forecast period. IN addition, there may be a few hours of
light snow across northern WI during the late afternoon and
early evening. Ceilings and visibilities should slowly climb
later Monday morning, as southwest winds increase and mix out
some of the low level moisture.
&&

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

$$

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



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