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Area forecast discussion 
National Weather Service Green Bay WI
1030 PM CST Wednesday Dec 24 2014

Updated aviation portion for 06z taf issuance

Short term...tonight and Thursday
issued at 218 PM CST Wednesday Dec 24 2014

Fog finally lifting across Fox Valley...though visibility still below 1
Michigan at atw early this afternoon. Occasional light rain or drizzle also
persisted in the east-central as upper level trough was slow to
move through. Light precipitation will end early as upper trough exits
area...though will linger small chances near Lake Michigan as main
surface system moves across western Ohio.

Weak surface and middle-level ridge builds into the region later
tonight with winds turning westerly...but clouds persisting.
Generally cloudy with temperatures in the 30s expected for
Christmas day as surface ridge slowly drifts off to the east late
in the day. Could see a few breaks...but satellite imagery shows
a great deal of upstream cloudiness...and short-term model guidance
indicates considerable boundary layer moisture.

Confluent middle-level flow upstream over the Dakotas will result in broad
frontogenetic forcing at low to mid-levels. Models are picking up
on the signal generate a narrow band of snow along the
deformation axis from South Dakota...east across central Minnesota
into northwest Wisconsin.

Long term...Thursday night through Wednesday
issued at 218 PM CST Wednesday Dec 24 2014

A few things to focus on in the long term...but nothing of high
impact. Few waves of energy with associated precipitation through the
weekend...then turning much colder.

A deep trough will be centered over the western US at the
beginning of the long term...which will extend across into the
northeast. This positively tilted trough will remain
fairly stationary through the weekend....with waves of energy riding
up through the flow on the downstream side over the Great Lakes.
So issues in the long term through the start of next week will
revolve around timing and strength of these impulses through the
broad flow.

Models bring first piece of energy towards Wisconsin Thursday
night. All models keep probability of precipitation confined to northern Wisconsin
associated with a broad area of weak fgen. While forcing will be
increasing over the entire state overnight Thursday...dry air in
the middle- levels should keep things dry over central and east
central Wisconsin. Worried drizzle or freezing drizzle will
develop in central and eastern Wisconsin after midnight Friday as fairly
strong warm air advection develops in the midlvls and forcing aloft
increases...but left it out for now as dry air looks very deep and
saturation near surface is rather shallow. Agreed with model
consensus and ran with chance of light snow only over the north
Thursday night.

Models then begin to disagree with the timing and coverage of
precipitation starting Friday. Leaned towards the 12z GFS/European model (ecmwf) as they
were the most consistent from the previous runs and were the first
to pick up on the stronger solution for saturdays system.

Friday night a more potent low will eject out of the Southern
Plains and move towards state as a shortwave pushes through the
upper Great Lakes. Some models want to allow precipitation to overspread
the area on Friday as weak low pressure swings through the Great
Lakes. With increasing warm air advection and issues getting
precipitation to fall however precipitation type may be an issue with snow in
the north and rain in eastern Wisconsin Friday afternoon before
changing back to all snow Friday night.

A more potent shortwave will swing through the area Friday night
and Saturday morning. Models not in agreement how this system will
behave so nailing down details at this point is up in the air.
Models have slowed down this feature...and thus the arrival of
precipitation. Forcing would support precipitation overspreading the area last
Friday night...and the main precipitation effecting central and east
central Wisconsin Saturday morning. Precipitation should then diminish
Saturday afternoon as the system quickly exits the area...with
probability of precipitation lingering along the Lakeshore the longest into Saturday

After Saturday...a strengthening western ridge will allow Arctic
high pressure to build south out of Canada. So...while quiet and
dry weather can be expected starting Sunday...much colder air will
infiltrate the area. Below normal temperatures will arrive Sunday or
Monday...and by middle to late next week much of NE WI will be
struggle to get out of the single digits for highs. A marked
temperature difference will be felt by Tuesday.

Aviation...for 06z taf issuance
issued at 1027 PM CST Wednesday Dec 24 2014

Ceilings across east-c WI have improved this evening...but IFR ceilings have
moved back into the north. Temporary clearing may occur over eastern WI
with the northwesterly winds late this evening...but backing winds will
probably allow the low clouds to spread east again either late tonight or
tomorrow morning. Will keep prevailing conditions rather pessimistic
in the tafs...though allow for the temporary decr in clouds with a
tempo group. MVFR ceilings expected to linger through tomorrow.

Grb watches/warnings/advisories...


Short term.....Jkl
long term......Allen

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