Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EST Wed Dec 09 2020

Valid 12Z Wed Dec 09 2020 - 12Z Sat Dec 12 2020

...Pacific Northwest to the Northern Rockies...
Days 1-3...

Two rounds of snowfall are likely from the Pacific Northwest into
the Northern Rockies this forecast period.

The first is today as a shortwave currently moving across WA state
lifts eastward into the Northern Rockies while deamplifying.
Modest upper level divergence in the RRQ of a Pacific jet streak
arcing towards the Great Lakes will aid in providing ascent and
transporting moisture into the region. The combination of these
features will produce a round of moderate to heavy snow,
especially in the favored upslope regions of the Northern Rockies.
WPC probabilities on D1 are high for 4 inches, with local amounts
above 10 inches possible.

A subsequent shortwave follows quickly behind this first one owing
to short wavelength ridge/trough pattern across the NW. This
second shortwave appears to be more intense and is strung out more
NW to SE as it rotates into the amplifying longwave trough across
the West. Although ascent is forecast to be modest, it could be of
a more prolonged duration as the vorticity lobe shears out due to
fast flow to the south. This suggests an extended period of
moderate snow across the WA and OR Cascades. WPC probabilities are
moderate for 4 inches on D2.5 and D3, with total accumulations
potentially reaching 8 inches on the OR side.


...Western to Northern New York...
Day 1...

A shortwave diving out of Canada as a clipper type impulse will
race across New York driving a cold front across New England.
Modest ascent associated with this feature due to height falls and
PVA will spread precipitation southeastward, with flow behind the
feature shifting to the W/NW. While the impulse itself is modest,
there is potential for several inches of snow where lake
enhancement and upslope can occur, most notably in the Tug Hill
Plateau and into the Green Mountains of VT where orthogonal flow
will efficiently upslope to enhance snowfall. WPC probabilities
are high for 4 inches in these areas, waning into D2 as the best
ascent drops southeast and dry advection commences into Thursday.


...Southwest/Central High Plains...
Day 2-3...

A cutoff low will gradually move from the CA coast towards the
Four Corners region while filling into Thursday afternoon. This
feature will then advect further northeast while weakening, while
at the same time a stronger shortwave races southeast to replace
the first in the base of the longwave trough. The interaction of
these features will spread precipitation across the Southwest as
mid-level divergence produces ascent Thursday into Friday, with
weak surface cyclogenesis possible in the lee of the Rockies. As
the second shortwave dives southeast Friday, the amplifying
longwave trough will drive intensifying downstream jet streaks
which may couple to enhance upper diffluence. This suggests a
surface low will develop and strengthen as it moves northeast
towards the western Great Lakes by Saturday morning. Guidance is
still very different in its evolution of this feature, but a blend
of the EC/ECENS/GEFS was weighted heavily due to agreement.

The heaviest snow during this time period is likely in the
mountains of the Southwest, including the San Juans and Sangre De
Cristos where prolonged mid-level divergence and upslope
enhancement will drive heavy snowfall. WPC probabilities for 8
inches are high in these ranges, and local totals above 1 foot are
likely. Elsewhere within this broad divergent region across the
Four Corners and into the High Plains lighter amounts are likely,
with less than 4 inches progged into the Front Range and I-25
corridor of Colorado.

NW of the main surface low, an area of wintry precipitation is
also likely where WAA spreads precipitation northward into colder
air. There is likely to be a stripe of moderate to heavy snowfall
across the Central Plains during D3, and while most of the snow
may be of moderate intensity, there are some signals that an fgen
band will develop NW of the low to enhance snowfall across NE/KS.
WPC probabilities in this area are 20-30% for 4 inches, with less
than 4 inches forecast elsewhere.


The probability for significant icing (0.25 inches or more) is
less than 10 percent all 3 days. 


Weiss