Area Forecast Discussion
Issued by NWS Green Bay, WI

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225
FXUS63 KGRB 152321
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
521 PM CST Fri Dec 15 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Saturday
Issued at 236 PM CST Fri Dec 15 2017

The main forecast challenges will be low temperatures tonight
and precipitation trends and snowfall amounts.

A weak ridge of high pressure was building into the forecast area
this afternoon, resulting in partial clearing and diminishing
snow showers or flurries. The clearing may be short-lived, as
mid-level clouds are already approaching form the southwest, in
advance of a warm front in southwest MN and eastern IA.

Temperatures tonight will be tricky due to cloud cover and
developing WAA overnight. Have used a blend of several models,
and ended up with lows in the single digits north and teens south.
If clouds thicken up later than expected over northern WI, the
typical cold spots could easily end up 5 to 10 degrees colder
than the forecast.

Models are in decent agreement with a band of light snow moving
into our southwest counties later this evening, then lifting
northeast and bringing accumulations to much of central and east
central WI (especially along and south of Hwy 29) overnight into
Saturday morning. Forcing will be provided by mid-level
frontogenetic forcing, WAA and some weak upper jet energy. With
warming temperatures in the lower to mid-levels of the
atmosphere, do not expect to see a repeat of the very fluffy snow
(with snowfall to water ratios of 30-50:1) that we had last night.
Given expected QPF of less than a tenth of an inch, and SWR
closer to 15:1, we should see amounts of 1 to 1.5 inches along
and south of Hwy 29, and a dusting to a half inch over northern
WI. Highs on Saturday should range from the upper teens and lower
20s north to the middle to upper 20s south.

.LONG TERM...Saturday Night Through Friday
Issued at 236 PM CST Fri Dec 15 2017

The upper air pattern will flatten for a time this weekend into the
middle of next week, before buckling occurs once again.  As the
pattern re-amplifies, models are suggesting that a cyclone could
impact the region by the end of next week.  Until then, low
amplitude split flow will lead to nuisant precip chances to go along
with moderating temps.  Will continue to use a blend of the
ecmwf/gfs.

Saturday night through Monday...Weak warm advection in the 850-700mb
layer will continue on Saturday night into Sunday morning.  In
addition, light onshore flow will also continue off Lake Michigan in
the low levels. The combo of these factors should result in mostly
cloudy conditions with occasional light precip. Due to dry wedges in
the mid-levels, will likely see a loss of ice crystals at times,
which could lead to freezing drizzle mixing with light
snow/flurries. Will keep this mix in the forecast, but with little
to no accumulations. Could finally see a chance without light precip
on Sunday afternoon and Sunday night, as northern Wisconsin will be
positioned between systems north and south of the region.   Then a
relatively potent shortwave will bring the next chance of light snow
to northern WI on Monday.  Mild temps with highs in the upper 20s to
mid 30s on Sunday and Monday.

Rest of the forecast...The chance of light snow will continue on
Monday night, mainly over northern WI.  Otherwise, colder temps will
filter in behind the shortwave and associated cold front on Monday
night into Tuesday.  West to northwest winds look to become strong
and gusty during this time, and probably extending into Tuesday
night as well. Then will be keeping a good eye on the potential for
a cyclone to impact the region late Wednesday into Friday.  Forecast
confidence is low this far out, with fluctuations in the
track/timing expected to occur over the next several days.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 422 PM CST Fri Dec 15 2017

Patchy MVFR cigs continue to linger over mainly north
central Wisconsin late this afternoon. A weak ridge of high
pressure drifting over the area this evening will likely reduce
these MVFR cigs a bit more while mid level clouds increase ahead
of the next weak clipper type system. Later tonight into Saturday
morning, a band of light snow and associated MVFR/IFR conditions
will overspread central and east central Wisconsin, including the
AUW/CWA/GRB/ATW/MTW TAF sites. Vsbys expected to improve to VFR
levels by Saturday noon, but cigs will be slower to improve,
likely remaining in MVFR levels for much of Saturday.
&&

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

$$

SHORT TERM.....Kieckbusch
LONG TERM......MPC
AVIATION.......TDH



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