145
FOUS11 KWBC 010650
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Wed May 1 2024

Valid 12Z Wed May 01 2024 - 12Z Sat May 04 2024

...Pacific Northwest and Northern Rockies...
Days 1-2...

Active winter weather will continue across the northern tier of the
West through late week as a large trough persists over the region.
The core of this trough will be in the form of an elongated closed
low centered over Saskatchewan but extending towards the northern
Great Basin, and several impulses and associated vorticity lobes
will swing through the flow and into the Northern Plains. These
produce periods of enhanced ascent, which will wring out moisture
as snow, primarily in terrain features above 4000 ft. In general,
the forcing is pretty transient, and moisture is modest as noted by
NAEFS PW anomalies that are slightly below normal. This suggests
that any heavy snowfall will be primarily driven by where upslope
flow can enhance the broad synoptic lift already occurring.

The exception is still likely to be during D1 from the Lewis Range
in the Northern Rockies southward through the Little Belts and
maybe as far south as Yellowstone NP and the Big Horns in WY. Here,
an inverted trough traversing south out of Canada will produce some
enhanced E/NE upslope flow into this terrain, but more importantly
this will be accompanied by some stronger mesoscale ascent as a
weak theta-e ridge (remnant TROWAL) pivots south in conjunction
with a modest deformation axis. The overlap of additional moisture
and ascent within broad synoptic lift will drive heavier and more
prolonged snowfall across this region, and this is reflected by WPC
probabilities D1 and D1.5 that are 70-90% for 6+ inches reaching
as far south as the Little Belts, with lower probabilities into the
Big Horns. Event total snowfall in the Lewis Range could exceed 12
inches.

Elsewhere, WPC probabilities are highest for 6+ inches in the OR
Cascades D1.5, reaching 40-50%, with lower probabilities extending
across other ranges including portions of the Absarokas.


...Central Rockies...
Day 1...

A fast moving Pacific jet streak arcing from the Great Basin into
the Northern Plains will combine with weak height falls downstream
of a northern stream shortwave to drive modest cyclogenesis in the
lee of the CO Rockies Wednesday aftn. This low will deepen through
Wednesday night but then lift quickly into the Central Plains by
Thursday morning in response to fast progression of the driving
synoptic features. Moisture across the region during this time will
increase due to Pacific air flooding eastward combined with at
least modestly enhanced moist isentropic upglide on southerly flow
out of the Gulf of Mexico then rotating back into Colorado. The
overlap of ascent into this moistening column will wring out
precipitation across the terrain, with moderate to heavy snow
accumulating above around 9000 ft, as the guidance has warmed a bit
in the past few cycles. The general modest moisture and rapid
progression of the forcing will limit snowfall accumulations
somewhat, but WPC probabilities are 10-30% in the higher terrain of
the CO Rockies, focused across the Park Range, before colder and
drier air sinks into the region by Thursday morning cutting off
precipitation.


The probability of significant icing is less than 10 percent.

Weiss


$$