Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 AM EST Wed Dec 11 2019

Valid 12Z Wed Dec 11 2019 - 12Z Sat Dec 14 2019

...Northern Plains to Great Lakes...
Days 2-3...

A shortwave will dive southeast in the northern stream out of
Alberta, Canada, racing across Montana and into the Northern
Plains Thursday morning. A weak wave of low pressure will develop
beneath this feature along a stationary boundary. As the mid-level
wave and surface low move eastward through Thursday, mid-level
moisture confluence and a stripe of enhanced 850-700mb fgen will
follow to the north, producing a swath of moderate snowfall. The
heaviest snow Day 2 is likely across northern WI and into the U.P.
of MI where WPC probabilities are high for 4 inches. Elsewhere, a
stripe of light snow is likely from eastern MT through the
northern L.P. of MI.

A second wave will follow immediately on the heels of this first
low, accompanied by the LFQ of a Pacific jet streak racing
eastward. Moist advection from the Pacific combined with
PVA/height falls and modest mid-level fgen will produce an
enlarging area of snowfall from the Dakotas into Minnesota on D3.
At this time the overall forcing looks modest and the wave moves
quickly, so WPC probabilities for 4 inches are less than 30
percent, highest across the Arrowhead of MN.

...West...
Days 1-3...

Long duration nearly zonal flow from the Northern Pacific will
transport ample moisture into the Western CONUS beginning today.
PWAT anomalies will climb to 1-2 standard deviations above the
climo mean D1-D2 before gradually shifting southward. However,
repeated perturbations embedded within the fast flow combined with
nearly continuous, despite weakening, LFQ jet diffluence will
maintain periods of moderate to heavy snowfall through the
forecast period. WPC probabilities for 12 inches are high on D2 in
the Washington and Oregon Cascades, as well as the Sawtooth, Grand
Tetons, northern Wasatch, and northern Rockies, shifting into the
Colorado Rockies on D3 while falling further west. 3-day snowfall
totals may exceed 3 feet in the Washington Cascades and portions
of the Tetons.

...Southern Appalachians...
Days 2-3...

A shortwave rotating through the base of an amplifying trough
digging through the MS Vly will spawn surface cyclogenesis on
Friday, lifting into the Mid-Atlantic Saturday. Mid-level
divergence ahead of the shortwave combined with the approach of
the LFQ of the Pacific jet streak, and increasing isentropic
ascent atop the surface wedge of high pressure will produce
widespread precipitation beginning late Thursday night. Initially,
the surface temps in both the valleys and terrain will be below
freezing despite rapid WAA driving a warm nose above 0C. This will
produce an environment favorable for freezing rain. However, the
WAA will quickly change all the precipitation over to rain on
Friday, and WPC probabilities are less than 20% for 0.25", focused
in the terrain of western NC and SW VA. Despite the low
probabilities for significant freezing rain, there is a good
chance for accretions of around 0.1" before the changeover to rain.


Weiss