Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Mon Mar 20 2023

Valid 12Z Mon Mar 20 2023 - 12Z Thu Mar 23 2023

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

A persistent Pacific jet streak will spill moisture across the
Pacific coast and Intermountain West through mid-week, leading to
widespread moderate to heavy snowfall across much of the area
terrain. The moisture shifting onshore within the Pacific jet will
be enhanced by confluent mid-level flow, resulting in IVT which
has a high probability to exceed 500 kg/m-s, especially late D1
into D2, producing PW anomalies that reach as high as +3 to +4
standard deviations above the climo mean. Within this moisture
plume, synoptic deep layer ascent will gradually increase in
response to shortwaves embedded within the flow, and persistent
LFQ diffluence as the primary jet axis pivots across the
Southwest. A more pronounced closed mid-level low off the CA coast
Tuesday morning is progged to drop onshore the central CA coast by
Wednesday morning, with spokes of vorticity around it helping to
amplify the large scale trough across the West during D3, and lead
to pronounced surface low development across Nevada. This
evolution will create more pronounced omega through the column as
a strong low pressure moves onshore CA, while secondary
development occurs along an area of greater baroclinicity across
the Great Basin on the leading edge of WAA. This widespread more
intense ascent will occur concurrently with the plume of greatest
PW departures, and Tuesday-Wednesday appear to be quite wintry
across much of the West. Snow levels in general will be around
4000 ft each day, but will likely surge to as high as 6000 ft
across the Great Basin and Four Corners Wednesday before crashing
behind a cold front late in the forecast period.

On D1, heavy snow will be most pronounced across the Four Corners
area, including the Wasatch, San Juans, Sangre de Cristos, CO
Rockies, and atop the Kaibab Plateau due to enhanced ascent in
advance of a weak shortwave moving overhead as well as increased
upslope flow. In these ranges, WPC probabilities for more than 6
inches are generally 50-80%, and locally more than 2 feet of snow
is possible in the higher terrain of the San Juans. While this
area will experience the heaviest snow D1, additional moderate
probabilities for more than 6 inches exist in the Sierra, the
Absarokas, NW WY ranges, Sawtooth/Salmon River ranges, and into
the Blue Mountains of OR. For D2 and through D3, the focus will
shift almost exclusively to the Sierra, Great Basin, and Four
Corners as the most impressive IVT comes onshore, and despite
decaying gradually with inland penetration, will be overlapped by
impressive synoptic ascent. WPC probabilities on D2 are high for
more than 6 inches across nearly all of the terrain from the San
Gabriels and San Bernadinos, through the Sierra, eastward across
NV, UT, and western CO, and along the Mogollon Rim, with similar
coverage expected on D3 with only subtle expansion northward back
into WY. 3-day snowfall across the highest terrain of the Sierra
and San Juans will likely eclipse 5 feet, producing major impacts
through snow rates and snow accumulations.


...Northern Plains through the Upper Midwest...
Days 2-3...

Dual shortwaves will eject out of the Four Corners and cross into
the Northern Plains/Upper Midwest through the middle of the week.
Each of these will likely result in surface cyclogenesis with a
low of modest intensity spreading snowfall across the region.

The first of these will eject out of WY Tuesday morning and then
race northeast towards Lake Superior by Wednesday morning while
interacting with a modest northern stream shortwave dropping out
of Saskatchewan. These will eventually phase into a more
pronounced trough across Ontario, but likely too late to really
deepen the surface low to impact precipitation across the CONUS.
However, concurrent upper level jetting streaking out of the
Pacific will arc northeast to place favorable LFQ diffluence over
the Plains and Upper Midwest by Tuesday night, working together
with the mid-level PVA/height falls to at least subtly deepen the
surface wave as it tracks out of the Rockies and along a warm
front towards Wisconsin. Downstream moisture advection will
intensify as flow surges up from the Gulf of Mexico noted by
moistening 290-295K isentropic upglide with mixing ratios of 3-4
g/kg. This will draw a plume of + 0.5 to +1 standard deviation PWs
northward, while the accompanying theta-e ridge tries to wrap at
least modestly into a TROWAL. Overall, the forcing and moisture
for this event are modest, suggesting a swath of moderate snow
versus truly heavy snow, but guidance continues to indicate at
least a chance for deformation band development NW of this wave
which could support heavier snow rates and higher snowfall
accumulations. Overall, WPC probabilities for more than 4 inches
50-70% from southeast ND northward through much of northern MN,
with locally more than 6 inches possible in the heaviest snowband.
There is also likely to be a secondary maxima along the Lake
Superior shore of the Arrowhead where enhanced moisture and
pronounced upslope flow will produce rounds of heavy snow, for
which WPC probabilities reach more than 80% for 8+ inches on D2.

As the first shortwave pulls away, a second piece of energy will
shed from a closed low over the Intermountain West, following
closely on the heels of the lead shortwave. This feature, at least
in current progs, is weaker than the least shortwave, but does
show some amplification as it races eastward and becomes sheared
into more confluent mid-level flow. The key difference with this
second impulse is that it combines with much more intense LFQ
diffluence as the core of the Pacific jet shifts northeast, which
also drives better moisture into the region as the western AR
decays through its eastward advection. The ageostrophic response
to this jet position combined with the confluent flow between a
high to the north and the developing surface low beneath the
trough will provide robust 850-600mb frontogenesis, which will
drive ample ascent into the DGZ to support a band of heavy snow.
Still quite a bit of uncertainty as to where this band will setup,
but forecast soundings suggest a favorable setup for heavy snow
despite modest SLR, and current WPC probabilities for more than 4
inches are as high as 50% across western SD, with 10-30% chances
extending towards the Coteau des Prairies.


The probability of receiving at least 0.1" accumulation of ice is
less than 10%.


Weiss