Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Thu Nov 05 2020

Valid 12Z Thu Nov 05 2020 - 12Z Sun Nov 08 2020


...Western U.S....
Days 1-3...

A wet and snowy pattern will develop across the West through the
weekend, with a significant low pressure system likely developing
by Sunday.

On Day 1 /Friday/ a residual AR will continue to push onshore
embedded within a 150kt Pacific jet streak which will gradually
shift eastward. PWs of 0.5"-1" will gradually sink southward,
while the enhanced lift within the RRQ of this jet streak will
combine with subtle height falls in advance of a mid-level
shortwave to drive ascent. A cold front dropping slowly SE through
the PacNW will gradually lower snow levels, so while precip will
begin as rain across most of the area, snow levels falling to
4000-5000ft, lower in the Olympics of WA, will cause a changeover
to snow. The heaviest snow Friday is likely in the WA Cascades,
and generally above 5000 ft, and WPC probabilities are moderate
for 6 inches, with amounts more than 12 inches likely atop Mt
Rainier.

A more robust snow event spreads across the West beginning
Saturday /D2/ and increase into Sunday /D3/. An anomalously strong
5H closed low will dig across OR and then into the Great Basin
before filling, while being absorbed into a larger gyre as a
secondary closed low drops into WA Sunday. 5H height anomalies
will reach as low as -3 standard deviations from the climo mean,
which will drive a strengthening downstream jet streak lifting
northward along the periphery of this trough on Sunday. Robust
ascent invof the 700mb wave moving from OR into the Great Basin D2
will drive heavy snowfall behind the cold front which will
continue to sink southeast through the West, causing snow levels
to collapse to as low as 3000ft in the Cascades, Sierra, and CA
coastal ranges by the end of D3. WPC probabilities indicate a high
chance for 4 inches in the OR Cascades, Sierra Nevada, and the
Shastas/Siskiyous of CA.

Heavy snow will persist across the Cascades and Sierra as well as
into the Four Corners in response to broad synoptic ascent on D3.
Guidance is indicating the heaviest accumulations may actually
occur in the San Juans due to repeated rounds of ascent through
700mb impulses and persistent height falls within confluent
moisture streams. WPC probabilities indicate a moderate chance for
6 inches in the OR Cascades, Sierra, Uintas of Utah, and other
ranges across the Great Basin. In the San Juans, WPC probabilities
are high for 12 inches, and locally more than 2 ft is possible in
the highest terrain. As height falls persist and snow levels crash
D3, light to moderate snow is likely even into the San Gabriels
and San Bernadinos of Southern California where WPC probabilities
for 4 inches are low to moderate.

During Sunday, heavier snow will begin to develop across the
Northern Rockies in response the strengthening jet streak
downstream of the closed low producing a robust divergence maxima,
which will combine with the height falls to drive rapid surface
low development in the High Plains of WY/MT. As this low develops,
intense WAA along a warm front will cause precipitation to spread
northward from CO into MT/ID, with an intensifying mid-level
TROWAL overlaying 300-305K isentropic ascent and upslope
enhancement driving a maxima of precipitation into the Northern
Rockies. Intense snow rates are likely the latter half of D3, with
heavy snow gradually spreading eastward into the High Plains, and
WPC probabilities indicate a high chance for 6 inches in the
Northern Rockies, with locally 12 inches possible. Moderate
probabilities for 6 inches exist further west into the Bitterroots
and Sawtooth ranges of ID, as well as into the ranges of NW WY.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.


Weiss