Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Sun Nov 06 2022

Valid 12Z Sun Nov 06 2022 - 12Z Wed Nov 09 2022

...Western U.S....
Days 1-3...

An extremely active pattern will persist across the western half
of the CONUS through early next week as the overlap of anomalously
cold air and significant moisture produces widespread wintry
precipitation across the area.

The primary driver of this large scale and long duration event
will be an amplifying closed 500mb low currently dropping out of
the Gulf of Alaska and along the British Columbia coast. This low
will deepen while shifting slowly almost due south along the coast
before finally pivoting inland over central CA late Tuesday and
Tuesday night. This evolution will drive slow height falls and
persistent downstream divergence for ascent, with an accompanying
Pacific jet streak surging inland at 130 kts and then arcing
poleward to enhance ventilation and ascent across the
inter-mountain west. Impressively confluent mid-level flow
overlapped with the aforementioned jet streak will drive anomalous
moisture and impressive IVT across the west, with the plume of +3
to +4 sigma according to the NAEFS ensemble tables shifting
southward with time. The overlap of this anomalous moisture and
the impressive synoptic ascent will result in widespread snow
across the West during the next three days, but there are likely
to be three areas of focus for heaviest snow.

On D1, the heaviest snowfall is likely along the OR/WA Cascades
and eastward into Washington State along the Okanogan Highlands
and northern Columbia Basin. This area will be under the influence
of strong UVVs through PVA and strong mid-level divergence, aided
by strong WAA on S/SW flow downstream of the mid-level low
dropping along the Pacific NW coast. This intense ascent with
ample moisture and falling snow levels will result in heavy
snowfall rates of 1-2"/hr within bands shifting northward, with
multiple waves likely lifting NNE through the day. SLRs will
generally be low due to the Pacific sourced airmass, but will
still result in snowfall that could be 1-2 ft in much of the
terrain where WPC probabilities for 12+ inches are 50-80%.

Snowfall will then begin to spread across the Sierra late D1,
becoming prolonged and intense through D3. Long duration mid-level
SW flow surging orthogonally into the Sierra with abundant
moisture noted by anomalous PWs will support widespread heavy snow
much of the forecast period. The initial surge on D1 will be
accompanied by lower SLRs due to a warmer column with a shallower
DGZ and stronger winds to fracture dendrites, but as temperatures
crash and the DGZ deepens, SLRs will climb allowing for more rapid
snowfall accumulation. More than 12 inches of snowfall is likely
each day across the Sierra, with abundant spillover also likely
into the Great Basin. Storm total snowfall may be 3-6 ft in the
highest terrain of the Sierra, and significant impacts to the
passes are expected.

Lastly, a SW to NE oriented band of heavy snow is becoming more
likely D2 into D3 from the eastern Great Basin through NW WY and
into southern MT. A wave of low pressure developing to the east
will wrap moisture back to the west as moist isentropic ascent
along the 290-300K surfaces intensifies. This will be collocated
with waves of theta-e lapse rates that are below 0C/km, with
intense mid-level fgen driving pronounced omega into a moist and
slowly deepening DGZ. The guidance has trended a bit slower
/farther west/ with this evolution tonight, but the setup seems to
support a quasi-stationary convective snow band with regeneration
of snowfall rates exceeding 1"/hr at times. The heaviest snow will
likely be in the higher terrain across this region, but dynamic
cooling of the column could result in snow accumulations even down
to the valley floors beneath this band. Confidence is a bit lower
than usual in placement and amounts, but should begin to focus as
more of the high-res guidance comes into the forecast timeframe.
For now, WPC probabilities feature a moderate to high risk for
more than 6 inches of snow both Monday and Tuesday /D2 and D3/
from central NV through northern UT and into far SW MT, with
locally 1-2 ft of snow likely.


The probability of significant icing greater than 0.25" is less
than 10 percent.

Weiss