Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 PM EDT Thu Apr 07 2022

Valid 00Z Fri Apr 08 2022 - 00Z Mon Apr 11 2022

...Pacific Northwest to the Northern High Plains...
Days 1-3...

A potent shortwave diving into the Pacific Northwest will be
accompanied by a fast moving jet streak which will push a surface
cold front onshore Friday morning. Ahead of this front, WAA and
confluent mid-level flow combined with the Pacific jet streak will
surge moisture onshore as an AR, with IVT progged to reach 500
kg/m/s on D1. This will spread moisture into the Cascades Friday,
but snow levels are likely to be quite high, above 7000 ft, in the
pre-frontal airmass. However, behind the front, snow levels crash
quickly Friday night, becoming 1000-1500 ft in the Cascades, and
these lowering snow levels will expand eastward into the Rockies
and southward into the Great Basin through the weekend, and may
drop well below 1000 ft Sunday. While the best moisture and ascent
are expected Friday ahead of the trough, persistent mid-level
confluent and zonal flow will maintain moist advection and
favorable ascent into the terrain through all 3 days of the
forecast period.

For D1, WPC probabilities for more than 6 inches are less than 30%
and confined to just the northern WA Cascades due to the high snow
levels. However, by D2 heavy snow becomes more expansive as far
east as the Northern Rockies, with significant accumulations
expanding across the WA Cascades, into the OR Cascades, and in the
Olympic Range. Locally more than 12 inches of snow is likely on D2
in the WA Cascades. Precipitation expands even further on D3 with
snowfall likely as far east as the Wind Rivers and Black Hills as
precipitation continues to surge onshore in waves, but may be more
snow-showery at times, especially inland from the Cascades. WPC
probabilities for more than 6 inches are above 70% across most of
the Cascade Crest of OR and WA, with 30-50% widespread in the
terrain from the Blue Mountains of OR eastward to the Black Hills
of SD.

With snow levels falling quickly D2 and D3, heavy snow is likely
to begin to impact the Cascade Passes, and WPC probabilities are
high for more than 6 inches at many of the passes including
Stevens and Snoqualmie on D2, expanding south towards Santiam Pass
on D3. Travel could become quite difficult due to heavy snow rates
of more than 1"/hr accumulating to several inches at these passes
during the weekend. Additionally, as snow levels drop below 1000
ft, especially Sunday, even some late-season light lowland snow
accumulations are possible.


...Upper Midwest/Western Great Lakes...
Day 1...

The slow moving, anomalously deep, closed upper low will continue
to plague the Upper Midwest, beneath which a surface low will
drift from WI to MI on D1 before exiting into the Northeast D2.
Although the low will be vertically stacked and forcing will be
waning, moisture will remain above normal, especially where
trajectories off the Great Lakes inject moisture into the column.
Strong CAA/north flow behind the departing low will converge over
Lake Superior, and this additional moisture from the lake combined
with upslope flow on the south shore will lead to heavy LES on D1,
especially along the Huron and Porcupine Mountains where WPC
probabilities for more than 4 inches are above 80%. Locally, more
than 8 inches of snowfall is likely in this terrain. Additional
moderate to heavy snow is likely anywhere along the south shore of
Lake Superior from the MN/WI border eastward to the central U.P.
By late D1 into D2, forcing even from LES begins to wane and only
light additional accumulations are expected into D2.

Additionally on D1, steepening lapse rates beneath the upper low
could allow for convectively enhanced snow showers to occur
anywhere from western Iowa through the L.P. of MI. Snowfall rates
may briefly reach 1"/hr as shown by low HREF probabilities, but
accumulations within these showers is expected to be very light.


For Days 1-3, the probability of significant icing of at least
0.25" is less than 10 percent.


Weiss