Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Wed Apr 20 2022

Valid 12Z Wed Apr 20 2022 - 12Z Sat Apr 23 2022

...West Coast to the Northern Rockies...
Days 1-3...

An extended period of unsettled weather is likely across much of
the West in response to a slow moving large mid-level low moving
eastward from the Pacific. This low will initially be closed as it
pushes onshore D2, but is likely to open, at least briefly, into a
longwave trough before restrengthening across the Great Basin and
lifting into the Northern/Central Rockies by Saturday. This low
will be accompanied by Pacific jet energy to increase ascent and
drive moisture onshore, which will also be enhanced by WAA ahead
of a surface cold front which is likely to move onshore late D1
and then continue eastward into the Rockies D2. The overlap of
these features is likely to produce a large area of precipitation
extending from the Pacific Northwest as far south as central CA
and as far east as the Rockies through the three days. While snow
levels will initially surge to 4000-6000 ft on the WAA, the cold
fropa will then cool snow levels to 2000-4000 ft D2 and D3.

On D1, the heaviest snow will likely be focused in the Sierra,
Shasta/Trinity region, and the OR Cascades, where WPC
probabilities for more than 6 inches are above 80%, and more than
12 inches are likely in the higher terrain. During D2, an
impressive snow event is expected to focus into the Sierra as the
best IVT, reaching +3 to +4 sigma, sinks southward across CA. WPC
probabilities are high for more than 6 inches in the Sierra on D2,
and more than 2 feet is likely in the higher terrain. Heavy
snowfall will likely create significant impacts at the Sierra
Passes on D2, and WPC probabilities are moderate to high for more
than 12 inches at both Echo Summit and Donner Passes. By D3, the
best moisture and ascent shift eastward into the Rockies, with
high probabilities for more than 6 inches of accumulation in the
Uintas, Absarokas, NW WY ranges including the Tetons, and into the
Big Horns.


...Northern Minnesota...
Day 1...

A vertically stacked low moving across southern Canada will
occlude into a triple point low over MN, with impressive WAA
surging moisture northward from the Gulf of Mexico. This secondary
low will eject quickly eastward, but a 12-18 hour period of
moderate precipitation is likely this aftn through late tonight.
The column is marginally sufficient for wintry precip, but
impressive WAA aloft should produce enough omega to drive heavy
snowfall rates which will likely exceed 1"/hr according to the
HREF probabilities. Where this occurs, enough dynamic cooling is
likely to allow for significant snowfall accumulation, although
most areas outside of the Iron Ranges and along the north shore of
Lake Superior will eventually transition to rain and the warm
advection overwhelms the column. Before that, however, significant
accumulations are possible, which the WPC probabilities indicate
have a better than 30% chance for 4 inches. In the Iron Ranges of
the Arrowhead, locally more than 6 inches are likely.


...Northern High Plains into Northern Plains...
Day 3...

A longwave trough moving onshore the Pacific Coast will amplify
into an anomalously strong closed 5H low, progged to reach nearly
-3 sigma according to NAEFS ensemble tables as it moves into the
Northern High Plains on Saturday. At the same time a powerful
Pacific jet streak nearing 150kts will surge eastward and begin to
couple with a northern stream jet streak moving across Canada. The
overlap of the mid-level height falls and coupling jet streaks
will drive impressive ascent to produce lee cyclogenesis in CO
Friday aftn, with this low deepening as it lifts northeast into
the Northern Plains. Robust warm and moist advection downstream of
this low will drive PWs to +2 to +3 sigma, while the strengthening
WCB lifts cyclonically into a TROWAL into ND/MT late in the
forecast period. The guidance is very well clustered with both the
position and intensity of the mid level and surface low, and
confidence is increasing that significant snowfall will develop
late D3, and more extensively on D4 (beyond this forecast period).
Current WPC probabilities for more than 4 inches are 10-30% for
northeast WY into eastern MT, but much heavier snow appears nearly
certain after 12Z Saturday.

On the northern edge of the precip shield, especially early in the
event 00Z-12Z Saturday, retreating high pressure interacting with
the WAA may allow for a p-type transition to freezing rain across
parts of northern ND. At this time accretions are forecast to be
light, but WPC probabilities do indicate pockets of 10-30% for
0.1" of ice accretion D3.


Weiss