Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Apr 26 2024

Valid 12Z Fri Apr 26 2024 - 12Z Mon Apr 29 2024


...Great Basin into the Rockies...
Days 1-2...

A mid-level shortwave will surge onto the CA coast with an
elongated vorticity lobe strung out along its southern periphery.
This entire feature will pivot eastward into the Great Basin by
Saturday morning, with amplification into a closed low expected
over the Four Corners. This will drive increasing ascent across
much of the Intermountain West, with the most intense lift moving
across the Central Rockies Saturday into Sunday. In this area,
forcing will be provided via an overlap of impressive height falls,
downstream divergence, and increasing LFQ diffluence as the
subtropical jet streak pivots around the base of the trough and
arcs poleward. This overlap of ascent will drive surface
cyclogenesis in the lee of the Rockies as well, with E/NE flow
behind the associated cold front also producing upslope flow into
the terrain of CO/WY. At the same time, mesoscale ascent will
maximize as a potent deformation axis and accompanying mid-level
fgen drive lift into the DGZ, which will fuel the potential for CSI
and even CI as reflected by cross-sections and good model
agreement of an axis of theta-e lapse rates less than 0C/km
collocated with -EPV. This indicates the likelihood for some areas
receiving snowfall rates in excess of 1"/hr, and the WPC prototype
snowband tool suggests locally 2+"/hr rates are possible.

All of this lift will work across an environment with widespread
PW anomalies above +1 sigma according to NAEFS as moisture lingers
from a recently departed low, and in response to persistent onshore
flow from the Pacific spreading moisture eastward. Although snow
levels will be moderate at around 8000 ft (using the NBM 25th
percentile as a guide), strong and dynamic ascent should permit at
least some snow into the lower elevations.

The heaviest snow is likely above 8000 ft, especially in the
vicinity of the Front Range on D2, where WPC probabilities for more
than 6 inches are 80-90% or more, and with sufficient upslope flow
wringing out the moisture, where it remains all snow, some areas
could see as much as 3 feet of accumulation when added up over D1
and D2. Other heavy snow areas in CO include the San Juans and
remaining CO Rockies where WPC probabilities D2 reach 50-70% for 6+
inches. Elsewhere across the Intermountain West, WPC probabilities
D1 for more than 6 inches are above 80% around Yellowstone NP and
across the Wind River Range, and 50-80% in the Uintas, Wasatch, and
higher terrain of eastern NV. During D2 the focus shifts into CO,
but some additional moderate snowfall accumulations are possible in
the Uintas and Wind Rivers.


...Pacific Northwest...
Day 3...

Confluent westerly flow across the Northern Pacific will gradually
back to the SW ahead of an approaching shortwave D3, and this
impulse is likely to pivot onshore the WA/OR coast late Sunday
night while amplifying. This shortwave will be accompanied by
modest upper level diffluence in the LFQ of a 90 kt jet streak,
enhancing ascent into the area. This confluent flow and overlapping
jet streak will also surge moisture eastward, with an arc of
150-250 kg/ms IVT supporting an expanding precipitation shield
beginning late Sunday aftn and expanding into Sunday night. Snow
levels across the Cascades should generally be above 4500 ft
limiting total impacts, but WPC probabilities for more than 6
inches peak above 60% in the northern WA Cascades and highest
terrain farther south towards OR.


The probability of significant icing is less than 10 percent.


Weiss