Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EST Fri Feb 01 2019

Valid 12Z Fri Feb 01 2019 - 12Z Mon Feb 04 2019


...Central Appalachians into the Mid-Atlantic...
Day 1...

Weak Alberta Clipper and mid-level impulse will move from the OH
VLY to off the Mid-Atlantic coast today. Moisture is somewhat
limited, but enhanced forcing due to upslope on NW flow into the
Appalachian Crest will produce a high risk for 4 inches of snow,
with 6 inches or more possible in a few locations. Additionally,
some light freezing rain/freezing drizzle is possible along and
west of the Crest as low-level saturation of the column supports
precipitation in an environment characterized by a mostly dry DGZ
but sub-freezing temperatures and very cold ground. Freezing rain
accretion should be only a few hundredths of an inch.

Downsloping flow east of the crest of the Central Appalachians
will reduce precip to the Chesapeake Bay/Delaware Water Gap.
However, the mid-level trough axis is fairly strong, and RH
recovery develops well downwind of the mountains, allowing bands
to redevelop over the Delmarva and southern NJ. There may be some
subtle enhancement in the snowfall in this region in response to
better PVA/height falls associated with the shortwave, but RH is
modest so WPC probabilities remain low for 2 inches.


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

Two distinct lows moving into the west coast will bring widespread
precipitation across nearly all the west through the forecast
period.

The first is an impressive low which will meander off the
California coast before lifting onshore Sunday morning. This will
be accompanied by a pronounced atmospheric river event noted by
high probabilities for IVT exceeding 500 kg/m/s and PWATs reaching
+4 standard deviations above the mean. Snow levels will initially
be relatively high, up to 4000 feet all the way to the US/Canada
border, before slowly falling through day 2. Ample forcing due to
height falls, jet diffluence, and increasing 700mb winds will
produce heavy snow in the Sierras and Washington Cascades on day
1, before spreading across the rest of the Mountain West on day 2.
WPC probabilities are high for 8 inches of accumulation above 4000
feet from the Washington Cascades, southeast as far as the Kaibab
Plateau of Arizona., The highest snow amounts will certainly be in
the Sierras, where days 1-2 accumulations will likely exceed 3
feet.

The second impulse will drop southward along the Washington coast
Sunday into Monday, while moisture transport remains robust on
strong S/SW 700mb flow. Snow levels will drop steadily as an
arctic front drops southward and 500mb heights crash. Widespread
heavy snow is again expected across much of the West. The heaviest
snow will again be in the Sierras where an additional 1-2 feet is
possible, with WPC probabilities remaining high for 8 inches from
the Northern Rockies southward into the San Juans of Colorado.
Snow levels will fall to as low as 1-2 kft from WA to WY, and as
low as 5 kft into AZ/NM. This will permit even light snow
accumulations down to the valley floors across much of the western
states north of Arizona Saturday night into Sunday. Snow levels
will be low enough by day 3 that even the cities of Seattle and
Portland have increasing chances for some light snow accumulations
Sunday night.


...Northern Plains...
Day 3...

A trough ejecting out of the Four Corners region will work in
tandem with a jet streak arcing across the Dakotas to spawn
surface cyclogenesis Sunday night into Monday. As this low tracks
northeastward and deepens, it will interact with substantial
moisture transported eastward from the Pacific to produce a swath
of snowfall from eastern MT through northern MN. Strong
frontogenesis along the arctic front dropping southward will
enhance snowfall rates across MT, but the overall pattern suggests
a transient feature with much of the snow driven by synoptic jet
level diffluence and overrunning lift, as the better mesoscale
forcing develops more into Monday. A potential dry slot may also
temper accumulations, but WPC probabilities are currently high for
4 inches, with low probabilities for 8 inches existing across
eastern MT into ND.


The probability for significant icing is less than 10 percent
nationwide days 1-3.


Weiss