Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Sat Nov 05 2022

Valid 12Z Sat Nov 05 2022 - 12Z Tue Nov 08 2022

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

An impressive atmospheric river (AR) moving onshore this morning
characterized by IVT of nearly 1000 kg/m/s according to the CW3E
diagrams, will shift slowly southward while weakening to 250-500
kg/m/s into early next week. Within this plume, PWs are progged to
exceed 3 sigma above the climatological mean according to the
NAEFS ensemble tables, with anomalous PWs spreading across much of
the Pacific coast and inter-mountain west. This AR will sink
southward in response to a rapidly amplifying longwave trough
aligned along the Pacific coast, with this sharpening of the
trough leading to significant height falls, and pronounced
downstream divergence collocated with mid-level WAA. Additionally,
lobes of vorticity will repeatedly shed cyclonically around this
larger scale gyre, further aiding in deep layer ascent through the
period. Snow levels within the most robust IVT and downstream of
the primary trough axis will rise to 7000-8000 ft thanks to
pronounced WAA, but will crash quickly in its wake as height falls
and the accompanying surface front drop southeastward, becoming as
low as 2000 ft later today in the PacNW, and then those same snow
levels spreading across much of the west into early next week.

For D1, the heaviest snowfall is likely in the Cascades, Olympics,
and spreading as far east as the Uintas and ranges in NW WY where
WPC probabilities for 8+ inches are above 70%, and locally 1-2
feet is possible in the higher terrain. This is despite SLRs that
will likely fall below the November median, on average, within the
Pacific sourced airmass.

On D2, the AR shifts towards the Sierra, and a prolonged heavy
snowfall event will likely occur there, but at the same time the
upper low sharpens leaving more intense divergence/height
falls/PVA into the Pacific Northwest with impressive snow
spreading into the Okanogan Highlands. WPC probabilities for more
than 8 inches of snow peak along the Cascades and into parts of
inland WA north of the Columbia Basin, and with snowfall rates
likely exceeding 1-2"/hr, total accumulations approaching 2 feet
are probable, and it is possible a top-10 2-day snow event will
occur in the vicinity of Omak, WA. Additional heavy snow of more
than 1 ft is likely in the Cascades and down into the
Shasta/Trinity ranges. Another interesting event may occur early
on D2 near Portland, OR as an intense band of fgen shifts
southeastward atop the front at the same time snow levels continue
to crash to near 2000 ft. While accumulating snow is not currently
forecast for the lowlands around Portland, heavy snow rates may
dynamically cool the column to produce snowflakes down into the
lower elevations, and the recent WSE plumes suggest some light
snowfall is possible even down into Portland itself. After
coordination with WFO PQR no deterministic accumulations were
produced, but it is worth monitoring due to increasing model
consensus in a band of heavy precipitation that may produce snow
well below the background snow levels.

During D3, the heaviest snowfall continues in the Sierra with
lowering snow levels, while also extending across the ranges of
the Great Basin and into the Absarokas and NW WY ranges northeast
of the Snake River Valley. WPC probabilities are 50-70% for more
than 8 inches in the vicinity of the Absarokas, and more than 90%
in the Sierra. SLRs in the Sierra will be generally quite low,
likely near the Baxter 25th percentile, but should climb D3 as
700mb temps crash on strong CAA. This will allow snow to pile up
more efficiently later in the period, and could result in 2-4 feet
of total accumulation even at pass level. Finally, also on D3,
snow levels fall to just a few hundred feet in the Pacific
Northwest, so if any moisture can remain, some light snow could
occur in the lowlands surrounding Seattle and Portland by Tuesday
morning.


...Upper Midwest...
Day 1...

A negatively tilted upper low will pivot across Wisconsin and into
Ontario today and tonight, with the associated surface low
tracking over the U.P. this evening. WAA downstream of this trough
combined with robust deep layer ascent through height falls,
mid-level divergence, and upper diffluence in the RRQ of a
departing jet streak will drive impressive lift and widespread
precipitation across the Upper Midwest. Much of this will be rain.
However, as the low begins to pulls away, cold air will wrap back
into it from the NW while isentropic lift into a modest TROWAL
pivots towards the Arrowhead of MN. This will support a changeover
from rain to snow, with sharp deformation leading to at least
briefly heavy snowfall rates progged by the WPC snow band tool of
1"/hr or more. Overall, the guidance has backed off a bit on the
accumulations of snowfall that may result from this briefly heavy
snow wrapping around the back side of the departing low, but a
deepening and moist DGZ with aggregates falling through a near
isothermal layer below could still result in light accumulations,
especially in the higher terrain around the Arrowhead. WPC
probabilities for 2 inches are 20-40%, highest across the Iron
Ranges.


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

A longwave trough aligned along the Pacific Coast will continue to
amplify and dig southward, with spokes of vorticity rotating
cyclonically around it lifting into the High Plains. One of these
spokes will lift northeast Monday into Tuesday, providing ascent
aided by downstream divergence and the diffluent LFQ of a
secondary jet streak arcing across the Great Basin. This will
result in a modest wave of low pressure developing across WY, with
pronounced WAA to its northeast driving ascent and a progressive
band of moderate to heavy snowfall. The guidance is insistent that
this band will move quickly northward, but along and just west of
the track of the low, some enhanced isentropic upglide will lift
westward into the Northern Rockies foothills to produce locally
heavier snowfall of longer duration. Overall the snow amounts
should be modest except across central MT where a pivoting band of
heavier snow is possible, and both the NBM and WSE percentiles
have increased this morning. Current WPC probabilities for 4+
inches are 10-20% in western ND and eastern MT, but as high as 70%
north of Great Falls, MT.


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

Weiss