Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sun Mar 19 2023

Valid 12Z Sun Mar 19 2023 - 12Z Wed Mar 22 2023

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

The West becomes active once again into next week as an
atmospheric river with IVT exceeding 500 kg/m-s according to CW3E
probabilities surges onshore with minimal decay even well inland
from the coast. The primary driver of this AR onshore will be an
amplified closed low lifting onto the CA coast Tuesday with
impressive height falls and downstream confluent flow to surge
moisture onshore beneath an intensifying Pacific jet which is
progged to reach 150kts by Tuesday. However, even before this
evolution, persistent nearly zonal mid-level flow with embedded
shortwaves will also help to drive moisture onshore as reflected
by PW anomalies that are progged to be around +1 standard
deviations across CA and the Great Basin even ahead of the higher
moisture within the AR. This series of shortwaves will also help
to push waves of surface low pressure across the West through
early next week, with subsequent cold fronts tracking across the
region as well, although the strongest low will likely still be
positioned just offshore the CA coast by the end of D3. These
waves will additionally enhance regional ascent, while the cold
fronts and periodic height falls will help squelch snow levels
down to as low as 2500-3000 ft at times, within otherwise
generally 4000-6000 ft snow levels due to the broad persistent WAA.

These persistent waves of ascent and moisture will drive
widespread moderate to heavy snow across much of the terrain in
the west, although there will likely be a focus from CA through
the Great Basin and into the Four Corners owing to the presence of
the greatest IVT shifting across that region. WPC probabilities on
D1-2 for more than 6 inches of snowfall peak across the Sierra
(D1) and the Great Basin into the Four Corners D2, with maximum
totals likely eclipsing 1-2 feet in the Sierra and San Juans
during this period. More modest probabilities for 6+ inches
stretch as far north as the WA/OR Cascades on D2 as well as across
many of the ranges from the Blue Mountains of OR through the
Tetons of NW WY.

By D3, as the core of the AR shifts inland and a more substantial
surface low develops over NV, heavy precipitation with more
intense snowfall rates will redevelop across the Sierra while
impressive spillover reaches as far east as the Wasatch and San
Juans, with less intense but still impressive snow impacting the
Transverse/Peninsular ranges of CA and Mogollon Rim of AZ.
Additional snowfall across the Sierra will likely reach 2-4 feet
in the higher terrain as this seemingly never-ending parade of
snow events continues, and the PWSSI indicates a high likelihood
for major impacts across those areas once again.


...Northern Plains through the Upper Great Lakes...
Day 3...

A northern stream shortwave dropping southeast out of Alberta,
Canada will interact with a southern stream impulse lifting out of
the Four Corners across the Northern Plains on Tuesday. This will
result in a subtly amplifying trough centered over the region late
in the forecast period, with ascent being enhanced by the LFQ of
an intensifying subtropical jet streak positioned to the south.
This overlap of synoptic lift will help drive surface cyclogenesis
in the lee of the Rockies Tuesday, with this surface low progged
to lift northeast into the Dakotas by Wednesday morning but with
minimal additional deepening. Downstream of this amplifying trough
and surface low, moist advection will increase out of the Gulf of
Mexico on intensifying 290K isentropic upglide aided by residual
Pacific moisture arcing northeast within the upper jet streak.
This will result in PW anomalies reaching as high as +0.5 to +1
standard deviations according to the NAEFS ensemble tables, with
this concurrent WAA helping to enhance ascent to the north and
east of the low. The result of this overlap will be an expanding
precipitation shield, with snow expected to spread across the
Dakotas, Minnesota, Wisconsin, and the U.P. of Michigan by
Wednesday morning. Overall the forcing appears generally modest
with just slightly above normal PWs suggesting moderate snowfall
accumulations, but there is potential for a sharpening deformation
axis to produce a band of heavier snow to the NW of the low,
enhancing snowfall in that region. The placement of this band is
still uncertain due to the inherent uncertainty by D3 and some
temporal and spatial differences among the various ensemble
clusters, but current WPC probabilities indicate a moderate risk
for more than 4 inches stretching from near the SD/ND border
eastward through the Minnesota Arrowhead. Local maxima of greater
than 6 inches are possible along the Arrowhead-shore of Lake
Superior into the iron ranges where moist upslope will enhance
precipitation, and across eastern ND where the consensus indicates
the greatest risk for that more intense deformation band.

Weiss