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Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI
348 am CST Sat Dec 20 2014

Today and tonight...forecast confidence medium.

A shortwave trough over SW Iowa will move across northern Illinois today. Very
light snow has developed over south central WI ahead of this feature
and expect the snow to move east and continue. The snow will
diminish from west to east during the afternoon with a dusting
expected.

Southerly flow will then develop tonight as a broad north-S shortwave
trough moves into the Great Plains. A focused area of 925-850 mb
warm advection and frontogenesis will move from SW to NE across southern
WI late tonight into sun am. Forecast soundings and thermal/moisture
profiles suggest freezing drizzle with highest confidence over south
central WI. Thus expecting a light coating of ice to form on
surfaces late tonight. A Freezing Rain Advisory may be needed.

Sunday...forecast confidence...medium

Warm advection aloft along with a shortwave will produce light
precipitation on Sunday. The main forecast issue remains precipitation
type. Overall temperature profile looks like snow in the morning...with
rain/snow in the afternoon...but soundings show a loss of ice
crystals at times...leading to possible drizzle. Along with snow
chances...mentioned freezing drizzle Sunday morning while surface
temperatures remain below freezing.

Monday through Friday...forecast confidence...medium

A nearly stacked low pressure system will slowly move through the
area Monday and Tuesday. Consensus of model solutions suggest
that best chance for precipitation will be Monday afternoon and evening as
a strong shortwave lifts through. Looks like mainly rain for this
first event.

Models then strengthen and dig the upper trough farther south
Tuesday night into Wednesday...as a second surface low pressure
system rapidly deepens and lifts northward into the Great Lakes.
Models are still trying to figure this one out. Solutions range
from the farther European model (ecmwf) which brings little to no snow to southern
Wisconsin...to the GFS which suggests several inches of snow for
the forecast area...particulary the eastern half. The Canadian is
more in line with the GFS. Obviously a lot of time for this
complex situation to change...but definitely something to keep an
eye on.

Dry weather is then forecast for Christmas between low pressure
systems. Another low pressure system is forecast for Friday...with
pretty big differences between the GFS and European model (ecmwf). The GFS is far
more progressive...sliding a shortwave through quickly and
bringing a little light snow. The European model (ecmwf) continues to show a deeper
upper level system and consequently a strong surface low. The
current European model (ecmwf) track would mainly snow northwest...with a rain/snow
mix southeast. Not much confidence in either solution at this
point.

After milder temperatures for the first half of the week...should see
highs back around normal by Christmas.

&&

Aviation/12z tafs/...a shortwave trough passing across northern Illinois today
will result in very light snow...continued ceilings of 1.0-3.0 kft...and
visibilities of 3-5 miles at times. The snow will end from west to east
across southern WI during the middle to late afternoon. Southerly flow will then
develop late tonight and a push of relatively mild and moist air aloft
will result in areas of freezing drizzle. There is a fairly high
confidence of a light coating of ice accumulation on surfaces over
south central WI with a little more uncertainty over southeast WI at this
time. Ceilings will lower for late tonight and sun am from 600 feet to
2000 feet with visibilities of 2-5sm.

&&

Marine...a Small Craft Advisory may be needed for Sunday afternoon
into Monday evening. Brisk southerly winds will develop as low pressure
approaches from the northern plains. The southerly fetch will likely generate
high waves from Port Washington northward.

&&

Mkx watches/warnings/advisories...
WI...none.
Lm...none.
&&

$$

Today/tonight and aviation/marine...gehring
Sunday through Friday...ddv

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