Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Tue Nov 09 2021

Valid 12Z Tue Nov 09 2021 - 12Z Fri Nov 12 2021


...Pacific Northwest and California through the Central Rockies...
Days 1-3...

Active weather with widespread mountain snow will persist today
before a waning trend begins on Wednesday. Westerly mid-level flow
angling onshore the West will become pinched south of a shortwave
moving into WA/B.C. this aftn producing subtle height falls but
impressive PVA. A weakening cold front will move eastward beneath
this feature, with pronounced warm and moist advection occurring
upstream of this front. The overlap of low-level convergence with
deep layer synoptic ascent in the moist environment will produce
widespread precipitation from central CA through WA state and as
far east as the northern High Plains. Snow levels ahead of the
cold front will be around 6000 ft, but will fall to 2000-3000 ft
in its wake, although most of the snow is likely ahead of the
front except in the Olympics and WA Cascades. For D1, the heaviest
snowfall is likely across WA where WPC probabilities for 12 inches
are as high as 70%, and locally more than 2 feet is likely.
Significant snow is likely at the Passes as well, and WPC
probabilities for 6 inches are above 80% at both Snoqualmie and
Stevens Pass. WPC probabilities for 6 inches are also moderate in
the terrain of the Sierra, Sawtooth, Northern Rockies, NW WY
ranges, and Uintas.

A short break in heavy snow is likely across the West D2 as the
primary trough shifts eastward giving way to ridging building
eastward from the Pacific. Residual snowfall is likely in parts of
the terrain, but WPC probabilities for 4 inches are above 20% only
in the WA Cascades. During D3, another shortwave will lift onshore
B.C. with spokes of energy shedding into WA state as mid-level
flow converges once again and a Pacific jet streak transports
moisture onshore. IVT forecasts reach 750 kg/m/s Thursday, and
heavy precipitation is likely in WA state once again. Snow levels
will climb within this AR, surging to 8000 ft or more late D3, and
heavy snow exceeding 4 inches is likely to be confined to just the
highest terrain of the WA Cascades.


...Northern Plains...
Days 2-3...

A Pacific shortwave moving quickly across the Central Rockies will
lift into the Central Plains Thursday while a more robust closed
mid-level low will drop southeast out of Alberta. These features
will interact into a highly amplified trough across the central
CONUS. Aloft, jet level energy will surge from the Pacific into
the Northern Plains while only slightly weakening, and will drop
southeast upstream of the amplifying trough while a secondary jet
streak strengthens downstream and arcs poleward on Thursday.
Together, these features will drive surface cyclogenesis in the
lee of the WY Rockies, with this low then deepening through
Thursday while lifting towards Lake Superior and then getting
captured to become vertically stacked in southern Canada Friday.

Intense WAA ahead of the wave will surge northward Wednesday as an
impressive moisture plume from the Gulf of Mexico lifts into the
region. This WAA should overwhelm the marginal thermal column
across the area producing primarily rain on D2. However, some of
the guidance depicts a potent band of fgen at the leading edge of
the WAA overlapping the saturated DGZ. This band is likely to be
transient, but may produce some light snow near the ND/MN/Canada
border early Wednesday. More significant snow is possible D3 as
the low wraps up over Lake Superior and an impressive TROWAL wraps
cyclonically around it to the NW. This TROWAL is likely to edge
eastward through Thursday such that the most robust forcing will
remain within the warmer portion of the storm, but enough residual
theta-e advection atop modest mid-level deformation should produce
snowfall on the back side of the system, angling slowly
southeastward into Friday. SLRs will climb during this period as
the column cools rapidly, so despite modest total forcing, several
inches of snow is possible, and WPC probabilities for 4 inches are
as high as 60% in NE ND and the MN/Canada international border.
There still exists quite a bit of uncertainty for D3 as shown by a
wide variation in the WSE plumes where the standard deviation is
as much as 4 inches, and a large range between the NBM 50th and
90th percentiles.

Some lighter accumulations are likely in the Arrowhead of MN where
upslope flow and the closer proximity of the TROWAL will exist,
and across the higher elevations of the Coteau Des Prairies of SD.

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

Weiss