Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Fri Nov 04 2022

Valid 12Z Fri Nov 04 2022 - 12Z Mon Nov 07 2022


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

Strong atmospheric river (AR) with IVT progged to reach 1000
kg/m/s, nearly +5 sigma above the climo mean according to the
NAEFS ensemble tables, will swing onshore today and tonight
driving impressive precipitation into the Pacific Northwest. The
most intense precipitation will occur with WAA today and tonight
ahead of a shortwave which will move onto the WA coast by Saturday
morning, with a secondary surge of ascent Sunday evening as a
stronger closed mid-level low approaches the WA/OR coast in
continued broad cyclonic flow across the west. Persistent
confluence of the mid-level flow will combine with a long duration
zonally oriented jet streaking across the Pacific to keep
anomalous moisture across much of the west. While snow levels
ahead of the first shortwave will rise as high as 8000 ft, they
will drop quickly behind a cold front D2 in the Pacific NW and D3
across other parts of the west, falling to as low as 1500-2000 ft
near the Canadian border. This will likely result in a multi-day
heavy precipitation and heavy snow event from the Olympics of
Washington through the Northern Rockies, to as far south as the
Sierra and Colorado Rockies/Laramie range of WY by D3.

With a generally warm column, at least during the period of most
robust overlap of moisture and ascent, SLRs will likely be
limited, and expect that the NBM SLRs are too high even across the
Intermountain West region. Across the coastal ranges of CA/OR/WA
and into the Cascades, a very moist column suggests SLRs will be
severely compromised, and could fall below the Baxter climatology
25th percentile, suggesting a very heavy and wet snow. This is
also reflected by pWSSI that is driven significantly by snow load.
Still, with tremendous moisture surging onshore, D1 WPC
probabilities for 8+ inches of snow are above 80% in the WA
Cascades, the Bitterroots, Blue Mountains, Northern Rockies, and
Tetons, with locally more than 2 ft of snow likely in the highest
terrain. For D2 /Saturday and Saturday night/ the heaviest
snowfall continues across many of these same ranges, with the
secondary forcing re-enhancing snowfall in the Cascades, and while
total snowfall D2 will likely be less than D1, WPC probabilities
feature a high risk for more than 8 inches from the Olympics
through the Cascades and as far southeast as the Tetons. During
D3, the best forcing begins to expand southward, while snow levels
crash along the Canadian border. This will bring heavy snow into
the Sierra where WPC probabilities are moderate for 8 inches, with
heavy snow also expanding into the Okanogan Highlands. The lower
snow levels by D3 should allow for heavy snow to begin to
accumulate more efficiently at pass level, with a high chance for
more than 4 inches of snow as some of the important passes
including Stevens, Sherman, Santiam, and Willamette.


...Central Plains and Upper Midwest...
Days 1-2...

A shortwave rotating through a deep trough across the Four Corners
will pivot northeastward this evening and deepen, becoming
negatively tilted and closing off Saturday morning near Iowa. This
feature will then continue to lift quickly off to the northeast,
exiting into Ontario Saturday night. The amplification of this
trough will help drive downstream jet streak intensification, with
a speed max progged to reach 170 kts over Ontario tonight, leaving
robust upper diffluence within its RRQ. This diffluence combined
with height falls and divergence will produce strong deep layer
ascent, resulting in a rapidly deepening surface low as it moves
from Oklahoma this morning to Ontario Sunday morning.

As this low shifts northeastward, there appears to be an
increasing threat for a band of heavy snowfall within an
impressive overlap of fgen/deformation to drive intense UVVs into
an elevated but saturated DGZ. Isentropic ascent along the
295-300K surfaces will ascend into a TROWAL with mixing ratios
around 4g/kg, suggesting a very moist column with some elevated
instability. While the column will be marginally conducive for
snowfall due to a modestly cool low levels, this intense ascent
should result in enough dynamic cooling that rain changing to snow
could accumulate from Kansas through Nebraska, Iowa, and
Minnesota. The guidance has become a bit more aggressive with this
potential this evening, and this is reflected by the WPC snow band
tool suggesting a high probability for 1-2"/hr snowfall rates and
even some 3"/hr rates possible should convection occur. While
confidence is modest and ground temperatures are warm which should
result in very low SLRs (just around 5:1 according to the
experimental NBM) the latest WPC probabilities indicate a 10-20%
chance for 2 inches of snow, centered near southeast Nebraska,
with a secondary maxima across the Arrowhead of MN. This event has
at least some potential to over-perform should changeover occur
more rapidly, with the 90th percentile in both the NBM and WSE
suggesting the potential for isolated 3-4" of snowfall.

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


Weiss