Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EST Wed Dec 15 2021

Valid 12Z Wed Dec 15 2021 - 12Z Sat Dec 18 2021

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

Two areas of precipitation will produce heavy snow across the
Western United States on D1. The first is associated with a deep
trough shifting across the Great Basin and Four Corners today,
dragging a cold front eastward beneath it. Robust moist advection
from a prior AR will continue to shift eastward ahead of this
feature as pacific jet energy persists. As this feature moves
quickly eastward it will produce heavy snow in the Central
Rockies, with WPC probabilities showing a moderate to high risk
for 6 inches in the San Juans, Colorado Rockies, and Big Horns in
WY.

Immediately following this trough, a closed low dropping along the
British Columbia coast will shift into OR tonight, bringing
renewed moisture and precipitation into the region. Confluent
mid-level flow and a modest Pacific jet streak will advect
moisture onshore, through which ascent will maximize via height
falls and LFQ upper diffluence. Heavy snowfall will again spread
into the terrain of the Olympics, OR Cascades, northern CA ranges,
and Sierra Nevada with snow levels generally around 2000 ft, but
rising to around 4000 ft in the Sierra in the best WAA/moisture
plume. WPC probabilities are high for 12 inches in the OR
Cascades, Klamath/Trinity Alps, and Sierra, where an additional 2+
ft is possible on top of the 5+ feet already received across the
higher Sierra terrain. With snow levels very low across the
Pacific Northwest, some very light snow is possible at Portland,
OR, but WPC probabilities for 0.1" are only 10-20%.

By D2, the leading trough shifts well east of the area ending
precipitation across the central Rockies, but the low dropping
into OR opens into a wave and shifts into the Intermountain West.
Overall forcing weakens during this time and the moisture plume is
not as impressive as earlier, but moderate to heavy snow is likely
across much of the terrain, and WPC probabilities for at least 6
inches are moderate in the Sierra, Northern Rockies, Tetons, and
parts of the Wasatch/Uintas, and Park Range of CO.

On D3, yet another shortwave and plume of Pacific moisture will
shift towards WA state, bringing renewed heavy snow to the
Cascades and Olympics, where WPC probabilities for 6 inches are as
high as 50%.


...Northern Plains and Upper Midwest...
Days 1-2...

Dual shortwaves will move across the Northern Plains tonight
spreading precipitation across the area. The leading shortwave
will eject out of the Four Corners region as a closed low and
crosses over Lake Superior and into Ontario by Thursday morning.
At the same time, a trailing shortwave will move zonally across MT
and into ND/MN late Wednesday night. The guidance continues to
feature two primary camps on the track of the surface low. The
high-res guidance is generally NW of the global members. However,
there has been some trend towards consensus near the GFS/NAM which
were a bit more in the middle, and this was reflected with the
recent ECMWF which shifted NW, and the FV3 which dropped SE, from
its previous iterations. With the surface low expected to deep
rapidly as it lifts northeast, a track somewhat NW of the
consensus may be realistic, and this has been reflected in the
latest forecast. While precipitation will likely start as rain, an
intense deformation axis beneath a pronounced TROWAL wrapping
cyclonically around the low will drive a rapid changeover from
rain to snow, with heavy snow rates likely after transition.
Additionally, the further west solution will lead to more
interaction with the trailing shortwave, leading to an enhancement
in the areal coverage of snowfall as well. While there remains
bust potential due to the variations in the track and uncertainty
in timing of p-type transition, WPC probabilities for 4 inches
have increased across western MN and are as high as 40%. The 90th
percentile from the NBM and the WSE are as high as 8", so local
maxima of double these amounts is possible. Additionally, WPC
probabilities for 2" are up to 60% in parts of SD and NH closer to
the secondary shortwave.


...New England...
Day 1-2...

A warm front extending from a potent low pressure moving into
Ontario, Canada will lift northeast into New England, and the
accompanying WAA will spread precipitation across the area. At the
same time, surface high pressure will gradually shift off to the
east, with low-level cold air persisting as precipitation lifts
into the area. This will support a period of snow and freezing
rain this evening into Thursday morning, before changing over to
rain and ending on Thursday. While snowfall accumulations should
be light, just 1-2" outside of the highest peaks of ME and NH, WPC
probabilities for 0.1" of accretion are as high as 20% in the
White Mtns and interior ME.


Weiss