Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Mon Apr 04 2022

Valid 00Z Tue Apr 05 2022 - 00Z Fri Apr 08 2022

...Northwest to the northern/central Rockies...
Days 1-2...

Elongated and powerful 170+kt Pacific jet streak will extend
onshore the Pacific Northwest tonight before driving eastward into
the Northern Rockies Tuesday. This feature will be accompanied by
strongly confluent 500mb flow and zonal 850-700mb winds, all
acting to enhance moisture as an atmospheric river with IVT near
750kg/m/s surges onshore. A cold front well ahead of this moderate
AR will help drive snow levels down to as low as 1500 ft in the
Cascades and Northern Rockies during the period of heaviest snow,
with only slow recovery towards 2500 ft as the snow intensity
wanes and becomes more showery late D1 into D2. With high
moisture, impressive deep layer ascent, enhanced upslope flow on
orthogonal low-level winds, and relatively higher SLR in the cold
column, heavy snow is likely for much of the terrain Monday night
and Tuesday. Significant moisture lifting into the Cascades could
produce in excess of 3 feet in the higher terrain, and WPC
probabilities for more than 12 inches on D1 for the WA Cascades
and Olympics where the heaviest accumulations are likely.
Probabilities for more than 12 inches are moderate elsewhere in
the OR Cascades and further east including parts of the Northern
Rockies, Absarokas, and Wind Rivers. Additionally, with the very
low snow levels, heavy snowfall is likely as the important Cascade
Passes including Stevens and Snoqualmie.


...Northern Plains/Upper Midwest/Western Great Lakes...
Days 1-3...

A shortwave dropping out of Alberta will race southeast into the
Northern Plains before stalling in response to being captured into
a slow moving and deep closed low over the region. This low will
spin slowly over the area D2-3, potentially retrograding at times,
as reinforcing shortwaves amplify the trough to as low as -3
standard deviations with respect to 5H heights according to the
NAEFS ensemble tables. Beneath this feature, a weak surface low
will drift from MN to WI and finally MI by the end of D3. Although
this low will be vertically stacked, moist advection out ahead of
it will be impressive as low-level flow out of the Gulf of Mexico
surges PWs to +1.5 standard deviations within the warm advection,
with some of the theta-e ridging trying to wrap cyclonically into
a TROWAL D2. Although forcing and moisture begin to wane late
Wednesday into Thursday, the guidance has become more aggressive
with deformation axis development NW of the low across the
Northern Plains D3, which could enhance snowfall. Modest total
forcing should somewhat limit snowfall accumulations within this
deformation, but increasing SLRs and some upslope into higher
elevations on northerly flow could lead to moderate to heavy
accumulations.

WPC probabilities on day 1 are minimal as the pattern becomes more
favorable for heavy snow late in the forecast period. However,
both D2 and D3, WPC probabilities for 4+ inches of snow reach as
high as 70% across the Black Hills, the Coteau des Prairies, and
the Arrowhead of MN due to terrain influences and upslope flow,
with moisture off Lake Superior adding to snow potential in the
Arrowhead. Locally, more than 8 inches of snow is possible over
the 2 days in the Arrowhead and highest terrain of the Black
Hills. By D3, some heavier snowfall is likely south of Lake
Superior near the Bayfields Peninsula due to Lake Enhancement.


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

Weiss