Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Fri Dec 10 2021

Valid 12Z Fri Dec 10 2021 - 12Z Mon Dec 13 2021

...Pacific Northwest to the Northern Rockies and the Sierra
Nevada...
Days 1-3...

Zonal mid-level flow D1 will become progressively more confluent
and back to the SW as an impressive trough digs out of the Gulf of
Alaska and amplifies along the West Coast of the CONUS. This will
be accompanied by a series of cold front draped from surface lows
moving into British Columbia, and am amplifying Pacific jet streak
which may exceed 150kts as it stretches into the middle of the
country and amplifies downstream of the sharpening trough
offshore. This will drive increasing IVT onshore, with a slow
southward progression, maximizing at over 500 kg/m/s into central
CA on D3. For WA/OR, snow levels will vary between 2000-4000 ft
during the period of strongest IVT and heaviest precipitation, and
then fall below 1000 ft D3 as precipitation lightens. WPC
probabilities are high for 8 inches in the WA Cascades are high on
D1, with high probabilities extending into the OR Cascades,
Northern Rockies, and Sawtooth/Bitterroots on D2 before waning on
D3. Snow totals may eclipse 5 feet over 3 days in the highest
terrain, and significant snowfall of 2-3 ft is likely at the
important passes of Stevens, Snoqualmie, and Lookout.

Further south into CA, the heavy snow will likely begin D2 in the
Shasta/Trinity ranges and then extend into the Sierra and become
intense as the AR focuses orthogonally into that area D3. Snow
levels will likely be higher here than points north as the AR is
accompanied by WAA, and snow levels are progged to climb above
6000 ft during the peak AR, and 4000-5000 ft otherwise. WPC
probabilities are high for 8 inches D2 in the northern CA ranges,
and high for 12 inches in the Shasta/Trinity ranges and extending
into the northern and central Sierra on D3. Total snowfall could
exceed 3 feet near Mt. Shasta, and WPC probabilities for 72-hr
snowfall exceeding 12 inches is high at Black Butte Pass, Snowmans
Pass, and Donner Pass.


...New England...
Day 2...

A significant low pressure system moving through the Great Lakes
will extend a warm front eastward into New England beginning late
Friday night. Warm advection associated with this front will
spread precipitation from SW to NE across the region, overrunning
shallow cold air associated with Canadian high pressure. While the
WAA is likely to be robust and eventually overwhelm the low-level
cold air, there is model consensus that a period of freezing rain
will occur in the terrain and elevated valleys of NH/VT/ME
Saturday morning into the aftn. Robust ascent during this time
could produce heavy precipitation, which usually does not
efficiently accrete. However, a steep shallow inversion is progged
to keep surface temps in the mid to upper 20s, leading to a high
probability for at least moderate freezing rain, with the best
chance for significant accretion occurring across interior ME. WPC
probabilities for 0.1" of ice are as high as 40% in the White Mtns
of NH/ME, and isolated totals of 0.2" are possible.


...Central Rockies to the Upper Midwest...
Days 1-2...

...Major Winter Storm will continue into Saturday...

A shortwave digging out of Alberta will continue to amplify into a
longwave trough across the Central Rockies today and strengthen
towards a negative tilt across the Great Lakes on Saturday. As
this occurs, nearly ideally coupled upper level jet streaks will
will sharpen as the zonally oriented jet streak shifts into
eastern Canada, while the amplifying poleward arcing jet streak
lifts into the Central Plains. The overlap of the RRQ of the
leading jet streak and the LFQ of the trailing will lead to robust
upper ventilation, which when combined with PVA and height falls
will cause a lee surface cyclone to deepen rapidly while it shifts
from eastern CO into the Upper Midwest and then through the Great
Lakes region. This rapidly deepening cyclone will be accompanied
by robust moist advection from both the tropical Pacific Ocean and
the Gulf of Mexico, and NAEFS IVT is progged at the 99th
percentile through the Upper Midwest by this evening. The
combination of robust synoptic ascent in a region of impressive
moist advection will produce heavy precipitation in a swath from
the CO Rockies eastward across NE/SD/MN and then northeast into WI
and the U.P. of Michigan by Saturday morning.

For D1, snowfall should wane across the Rockies as the forcing
shifts eastward, and WPC probabilities for 4 inches are moderate
across the San Juans ans Sangre De Cristos. However, heavy snow
will rapidly overspread the region from eastern SD and MN this
aftn, reaching the U.P. late tonight. Within this swath of
precipitation, there is likely to be two bands of heaviest
snowfall. The first is associated with the RRQ of the departing
upper jet streak through which modest fgen will develop leading to
a translating band of heavy snow which may reach 1"/hr from
northeast NE through southern MN and into WI. However, this band
is progged by many models to become overwhelmed by a more
impressive fgen band on the edge of the intense WAA, collocated
with the LFQ of the trailing and poleward arcing jet streak and
within the blossoming TROWAL. This band is likely to be quite
intense as theta-e lapse rates fall to <0C and fgen becomes
positioned beneath unstable or neutrally stable air noted by SEPV
near 0. This indicates the likelihood for CSI, with a potential
for upright convection and thunder snow, and snowfall rates may
exceed 2"/hr at times as noted by both HREF probabilities and the
WPC snowband prototype tool. While light to moderate snow will
exist surrounding these bands, the heaviest snowfall is expected
where these bands occur, and WPC probabilities on D1 are high for
8 inches from far SW MN through central WI, including the
Twin-Cities of MN. Locally in excess of 15" is possible in a few
locations. This low will rapidly lift northeast bringing an end to
the snow by the aftn of D2, but WPC probabilities indicate a
moderate risk for an addition 4 inches or more from eastern WI
into the U.P. of MI where a combination of synoptic snow within
the pivoting deformation axis and some enhancement through LES is
likely.

There is also likely to be a narrow corridor of light freezing
rain as the WAA shifts northward through parts of IA, WI, and the
L.P. of MI this evening. WPC probabilities for 0.01" are above 40%
in these areas, but a changeover to rain should minimize the
overall impact.



~~~Key Messages for December 9-11 Winter Storm from the Central
Rockies to the Great Lakes~~~

...Significant winter storm will continue...
A strong low pressure system will track from Colorado into the
Central Plains today, then into the Upper Midwest and Great Lakes
tonight into Saturday.

...Heavy snow will expand into the Upper Midwest and Great Lakes
through Saturday morning...
Additional snowfall of several inches will lead to storm total
snow of 1-2 feet across the highest terrain of the Central
Rockies. Further east, a swath of 8-14 inches of snow with locally
higher amounts is likely from eastern SD through the Upper Midwest
and into the U.P. of Michigan.

...Widespread impacts to travel and infrastructure expected...
All forms of travel will become dangerous as heavy snow and strong
winds create slippery roads and near whiteout conditions at times.
Additionally, the snow is expected to be heavy and wet in some
areas which could produce scattered power outages.


Pereira