Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Wed Dec 28 2022

Valid 00Z Thu Dec 29 2022 - 00Z Sun Jan 01 2023


...The West...
Days 1-3...

Persistent confluent fetch across the Pacific will transport waves
of moisture embedded within a pronounced atmospheric river (AR)
through the week and into the weekend. Both the GEFS and ECENS
probabilities indicate IVT exceeding 500 kg/m/s according to CW3E
angling into CA Thu-Sat, with periodic shortwaves lifting onto the
Pacific coast driving waves of low pressure onshore. There are
likely to be multiple impulses through the weekend, and while
guidance still features variable timing of these, each one is
likely to spread enhanced moisture onshore which will manifest at
snowfall, especially in the terrain due to snow levels that will
be elevated on pronounced WAA. The most intense impulse is likely
to approach late D3 as a a trough amplifies along the coast and
digs towards CA which will enhance both the coverage and intensity
of precipitation across CA and the Great Basin, but periods of
widespread rain and snow are likely across much of the western
CONUS through the forecast period.

Snow levels will generally be steady around 2000-3000 ft across
much of the west on D1, but then steadily climb as pronounced WAA
begins to shift onshore with the next AR, climbing above 8000 ft
in CA and 4000-5000 ft elsewhere. This indicates that heavy snow
is possible on the leading edge of this AR late D1 into D2 at many
of the important mountain passes, but will then generally become
confined to the higher terrain and only highest passes of the
Cascades/Sierra by D2, while still impacting more inland passes
across the Northern Rockies and into the Four Corners. For D1, WPC
probabilities for more than 6 inches are confined to the WA/OR
Cascades and Shasta/Trinity region of CA on the leading edge of
the incoming AR. By D2, precipitation spreads much farther inland,
so despite rising snow levels climbing to 9000 ft in CA and around
4000 ft over much of the Intermountain West, WPC probabilities for
more than 6 inches encompass much of the terrain from the Sierra
through the WA Cascades and eastward as far as the Wasatch and
Tetons. Locally more than 12 inches of snowfall is possible D2 in
the higher terrain, especially within the Sierra.

The more amplified wave lifts into CA D3 spreading the most
significant moisture onshore as the core of the AR pivots into CA.
While moisture will continue to spill across most of the West, it
will become enhanced across CA and into the Great Basin as an
impressive low pressure and its associated surface fronts shift
eastward to enhance ascent into the moistening column.
Precipitation associated with this low should be more intense,
which could dynamically cool the column to lower snow levels
across the Great Basin leading to some moderate accumulations even
in some of the lower terrain of NV. However, WPC probabilities for
more than 6 inches are again high only in the terrain, generally
from the Sierra eastward through the Wasatch, Uintas, and CO
Rockies including the Park Range. Again, isolated amounts over 12
inches on D3 are likely in the higher terrain across this region.


...Central Plains & Upper Midwest...
Days 1-2...

A broad trough across the Intermountain West will shed a vorticity
lobe across the Four Corners this evening shifting into the
Central Plains by Thursday morning. This feature will deepen at
least slightly, while a poleward arcing jet streak downstream of
the primary trough axis shifts into Canada leaving favorable RRQ
diffluence atop the central High Plains. The result of this ascent
is likely to be lee cyclogenesis across eastern CO this evening,
and this low will then shift northeast towards the Great Lakes
through Thursday night. While this low is not likely to deepen
much, the guidance has trended a bit more amplified with the
accompanying 700mb wave, which when combined with downstream WAA
should result in a swath of at least moderate precipitation from
eastern CO through the Arrowhead of MN. The heaviest snowfall is
progged to be along the Palmer Divide and high plains of CO where
the strengthening low pressure will yield enhanced fgen, some
pivoting deformation, and increased upslope flow into the higher
terrain. Impressively sloped 700-600mb fgen should drive strong
lift into the DGZ lying just above, which when combined with
theta-e lapse rates <0C/km indicate the potential for CSI,
reflected by WPC snow band probabilities suggesting 1-2"/hr
snowfall rates. The guidance has fluctuated greatly with
accumulations across this area, but overall trends have come up,
likely due to the higher rate potential which is also influencing
the pWSSI this aftn. WPC probabilities for 4+ inches are now
60-80% across the Palmer Divide and up into the I-25 corridor near
Denver, where locally 6-8" of snow is possible in the best banding.

As the low shifts rapidly to the northeast, a band of moderate
snowfall is likely to extend across Nebraska, eastern SD, and into
Minnesota, with fgen helping to cool the column and drive a p-type
transition quickly from rain to snow. Antecedent rain and
relatively lower SLR through marginal column thermals should limit
snowfall, but this could be overcome by a transient but persistent
band of fgen beneath some -SEPV to support CSI potential here as
well. The guidance has trended upward with snowfall today, and
while it still may struggle with the transition from rain to snow,
expect the dynamical cooling to lead to a more rapid transition
than prior forecasts. WPC probabilities suggest this as well, with
a stripe of 20-40% probabilities for 2+" from far NW KS through
the Arrowhead of MN. Just east of the snow axis, some light icing
is also possible, and although accretions should be a few
hundredths at most, some slippery travel is possible from eastern
SD through southern/central MN and into northern WI.

Weiss