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

Valid 00Z Fri Feb 01 2019 - 00Z Mon Feb 04 2019


...Northern New York...
Day 1...

Lake effect snow will persist from Lake Ontario until the Clipper
currently over IA shifts to the south Friday morning, disrupting
the westerly flow that is causing the ongoing very heavy snow
band. Rates will drop through Friday afternoon with low Day 1 WPC
probabilities for an additional 8 inches. However given the fine
scale resolution of the LES process, this is likely underdone with
a risk for an additional foot starting after 00Z/Friday.


...Ohio Valley to Mid-Atlantic...
Day 1...

An Alberta Clipper shifts east across the Midwest tonight and the
Mid-Atlantic Friday. Particularly high snow ratios will make for
one to two inches of powder tonight across the very cold Midwest.
Moisture will be more substantial in the central OH Valley,
favoring increasing snowfall from Indiana eastward to the crest of
the central Appalachians where Day 1 WPC probabilities are high
for 2 inches.  Moderately high probabilities for 6 inches exist
along the central Appalachians crest in WV where duration is
prolonged from upslope flow after the mid-level trough axis
passage.

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 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 series of three strong waves will make for a notable
precipitation event across the west as the strong ridge breaks
down. The second wave crosses the Rockies Sunday, bringing snow to
the northern Great Plains and the third wave bringing cold air and
low snow elevations to WA on Sunday.

The initial wave is a closed low over the southern CA coast this
afternoon. This is pushing into the ridge and diminishing, but
allowing some moisture into the desert southwest. The second wave
is a closed low that approaches the northern CA coast Saturday
before opening into a wave that reaches the central Rockies on
Sunday. 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 day totals of over 4 feet are
likely. Day 2/3 WPC probabilities feature moderate to 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 Saturday night into Sunday. Strong high
pressure over the Canadian Prairies Sunday will set up a strong
front north of the developing surface low with bands of heavy snow
across MT.


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

Jackson