Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Thu Jan 31 2019

Valid 12Z Thu Jan 31 2019 - 12Z Sun Feb 03 2019


...Great Lakes...
Day 1...

Deep closed low across eastern Canada will drift slowly northeast
today, maintaining cold advection and W/NW winds across the Great
Lakes. An extremely cold airmass advecting atop the Lakes will
produce robust instability, but as shortwave ridging expands from
the west, Lake Effect potential will decrease except off Lake Erie
and Ontario. The most significant snowfall is likely east of Lake
Ontario where strong and nearly unidirectional Sfc-700mb shear in
an environment characterized by 800 J/kg of CAPE and a moderate EL
will support a single band of intense snowfall. This will likely
focus into the Tug Hill Plateau where upslope enhancement will
further aid in impressive snowfall totals, and WPC probabilities
are high for 18 inches of snow. Off Lake Erie heavy snow is also
possible, but with 80% of the Lake now ice covered, WPC
probabilities are only high for 4 inches. Elsewhere, light snow is
possible in the eastern portion of the U.P. of Michigan downwind
of Lake Superior, as well as in far SW L.P. of Michigan from Lake
Michigan.


...Ohio Valley to Mid-Atlantic...
Days 1-2...

An Alberta Clipper rounds the ejecting deep low from the Great
Lakes Thursday night through Friday with a track from roughly WI
to NJ. Continental air over the upper Midwest will limit snow to
around 2 inches on southerly flow ahead of the mid-level impulse.
However, Gulf-moisture will be more substantial in the central OH
Valley, favoring increasing snowfall from Indiana eastward towards
the Atlantic Coast. The system is progressive and 1000-500mb RH
rises above 70% for only a short duration, so low probabilities
for four inches exist from central IN into eastern WV. High
probabilities for 6 inches exist along the central Appalachians
crest in WV/southern PA where upslope potential is greatest. The
coverage of more than 6 inches should not be widespread as the
best forcing remains below the DGZ, but high snow ratios and a
long duration of W/NW winds suggests the high terrain will receive
significant snowfall.

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 strong, and RH recovery
develops well downwind of the mountains on Friday, 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...

A closed low will push down the southern CA coast today with
precip extending inland and some higher elevation snows for the
southern Sierra Nevada and transverse ranges around the LA basin.
Moderate Day 1 WPC probabilities for two inches are over these
areas.

A more significant and widespread snow event will develop Friday
and push across nearly the entirety of the Mountain West on
Saturday. This is in response to a developing pattern changes in
the west, as a deep trough digs into California, with an
anomalously strong jet and associated moisture content.
Precipitation will spread into the Sierras and Cascades of WA/OR
on Friday, before spilling into all the ranges from the Mogollon
Rim of AZ, through the Canadian Rockies, and west to the Pacific
coastal ranges on Saturday. Significant moisture combined with
lowering snow levels and strong lift in response to increasing
700mb winds and strong jet level diffluence suggests heavy snow is
likely across all the terrain. The heaviest snow is expected in
the Sierras where 3 days totals of over 4 feet are likely. WPC
probabilities are high for 8 inches from the WA cascades all the
way into southern UT, with snow levels falling 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 by day 3.


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

Weiss