Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
201 PM EST Mon Dec 20 2021

Valid 00Z Tue Dec 21 2021 - 00Z Fri Dec 24 2021

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

Persistent trough off the Pacific coast will eventually swing
eastward to come onshore D3 /Thursday/. Ahead of that trough,
shortwave ridging D1 will limit precipitation across the West.
However, a piece of energy shedding from this trough will erode
the ridge early Wednesday will begin to spread precipitation back
into the West, and initiate a prolonged period of unsettled
weather across the region. As the trough offshore amplifies and
shifts east, mid-level flow will become more confluent, which when
combined with an intensifying Pacific jet streak reaching 150 kts
will drive a modest AR onshore with the highest IVT shifting into
southern CA. Despite the southern latitude of the most robust IVT,
PWs of +1 standard deviation will overspread the West, with
anomalies reaching +2 standard deviations in CA. This will allow
heavy precipitation to occur across much of the West Wednesday and
Thursday, with snow levels generally 3000-5000 ft, but as high as
7000 ft in the core of the moisture plume. Snow levels will fall
rapidly D3 however beneath height falls and the associated cold
front, falling to around 1500 ft in WA/OR by the end of the period.

The heaviest snow D2 will likely be in two distinct regions. The
first is across the Sierra and ranges of northern CA where
shortwave energy sheds eastward to enhance ascent and drive an
increase in moisture. WPC probabilities Tuesday night and
Wednesday are above 50% for more than 8 inches. Further north into
the Olympics, WA Cascades, and the Northern Rockies, pinched flow
southeast of the approaching robust trough will drive snowfall
accumulations which have a high likelihood of exceeding 8 inches
as well, with more than 2 ft possible in the higher Cascades.

On D3, snow becomes more widespread as the trough shifts onshore,
moisture becomes more impressive, and snow levels fall from NW to
SE. The highest accumulations are likely D3 in the Sierra where
WPC probabilities are above 90% for 12 inches, and locally 2-3
feet are likely. Elsewhere, WPC probabilities for 12 inches are as
high as 60% in the northern CA ranges, the WA and OR Cascades, the
Sawtooth of ID, and into the Wind Rivers/Tetons of WY. With the
snow levels falling D3, WPC probabilities for 8 inches climb at
pass level, reaching above 50% at Santiam, Snoqualmie, and Stevens
Passes.



...Northern Tier/Great Lakes...
Days 1-2...

Shortwave embedded within fast mid-level flow will drop out of
Alberta, across the Dakotas, and then race through the Great Lakes
by Wednesday morning. This shortwave will be accompanied by modest
LFQ diffluence as a zonally oriented jet streak intensifies, to
drive a modest surface low west to east across the area. This
feature will remain progressive, and moisture is somewhat limited
noted by a modest plume of IVT within the WAA ahead of the wave.
Despite this WAA, the column remains quite cold leading to a deep
DGZ supporting higher than climo SLR, likely near or above the
75th percentile from CIPS. However, limited temporal duration of
overall moderate forcing should limit total snowfall and WPC
probabilities for 4 inches are confined to the U.P. where a high
risk exists due to the overlap of WAA snow ahead of the low
followed by LES in the cold advection in its wake. Otherwise, a
swath of 2-4" is probable from eastern ND through northern/central
MN and into WI.


...New England...
Days 2-3...

A coastal low lifting out of the Gulf of Mexico will move
northeast along the coast and approach New England Wednesday
morning before moving through the Gulf of Maine on Wednesday. At
the same time, a fast moving shortwave ejecting out of the
Northern Plains will race eastward, and the interaction of these
features may lead to significant snowfall across interior New
England, especially NH and ME. The guidance today has trended
further NW with the coastal low, likely due to a faster and
further south trailing shortwave leading to more phasing. This,
combined with a potent and coupled upper jet structure will lead
to rapid deepening of the surface low, further supporting a
further NW track. While the guidance continues to feature quite a
bit of spread in placement and hence expansion of the
precipitation shield, robust WAA into the cold air should produce
a swath of heavy snow across interior NH and ME, with a narrow
corridor of freezing rain likely along the coast and into parts of
Southern New England. The trends will need to continue to be
monitored with future runs, but current WPC probabilities indicate
a greater than 40% chance for 6 inches of snow from the Central
Highlands northeast to Canada, with the heaviest snowfall likely
reaching 1"/hr at times late D2 into D3. WPC probabilities for
freezing rain accretions of greater than 0.01" are 10-30% along
the immediate coast of ME, southeast NH, and north-central MA.


For days 1-3, the probability of ice accumulation greater than
0.25" is less than 10 percent.

Weiss