Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
456 AM EST Sun Dec 11 2022

Valid 12Z Sun Dec 11 2022 - 12Z Wed Dec 14 2022

...Northeast...
Day 1...

A weak wave of low pressure moving out of the Great Lakes this
morning will drop southeastward across Upstate New York and
Southern New England beneath a potent but filling shortwave. As
this shortwave moves towards Long Island, it will overlap with the
weak LFQ of a modest jet streak arcing off the Mid-Atlantic coast
to induce secondary surface low development skirting east into the
Atlantic Ocean. Between these two features, an inverted trough
will likely pivot southward across the region, leading to enhanced
omega, and turning winds more to the E/NE to drive some enhanced
upslope ascent into the terrain from the Berkshires through the
Catskills. While overall this feature is transient, enough moist
ascent and a pocket of -EPV aloft to support CSI could yield
snowfall rates which briefly touch 1"/hr as shown by the WPC snow
band tool, and modest accumulations are likely across the area.
WPC probabilities for 4 inches have come up, and are above 70% for
the Berkshires, southern Adirondacks, northern Poconos, and
Catskills where locally 6+" of snow is possible.

While the low level thermal structure is marginal for areas near
the south coast, the aforementioned CSI potential could result in
dynamic cooling to produce a burst of moderate to heavy snow into
Long Island, including New York City, as well which is reflected
in the latest high res guidance. After coordination with WFO OKX,
it is likely that accumulations would be modest due to hostile
antecedent conditions, but brief travel impacts could be possible
Sunday night.


...Pacific Northwest and California through the Great Basin &
Rockies...
Days 1-2...

Large amplified mid-level low will continue to progress slowly
across the West, bringing pronounced height falls, periods of PVA
through vorticity lobes shedding around the low, and upper
diffluence as the persistent subtropical jet rotates around the
base of the trough. This will result in widespread impressive
synoptic ascent, with moisture continuing to funnel across the
region as the atmospheric river progresses eastward. Waves of low
pressure will drive a cold front eastward across the region as
well, combining with the cold pool aloft to lower snow levels
allowing precipitation to expand as snow across much of the area.

For D1, the heaviest snow is likely in the Sierra where orthogonal
mid-level flow will efficiently wring out moisture within the AR
as heavy snow, with snowfall rates continuing 1-3"/hr early before
slowly waning as the best forcing shifts eastward. This will
likely accumulate to another 1-2 feet, leading to storm total snow
of more than 5 feet in this range. Other heavy snow areas on D1
include the mountains of the Great Basin in Nevada, the southern
Wasatch of UT, and up into the Sawtooth of ID and towards the
Absarokas where mid-level fgen or upslope enhancement will drive
more impressive snowfall rates. WPC probabilities for more than 12
inches are 30-50% in these ranges, with a WPC probabilities for
more than 4 inches encompassing nearly all of the Great Basin,
eastern OR, Mogollon Rim, and into the Northern Rockies D1. By D2
the best overlap of moisture and forcing shifts eastward, but
heavy snow is again expected along the Mogollon Rim and White
Mountains, as well as the San Juans, northern Wasatch, and CO
Rockies where WPC probabilities for more than 6 inches are 40-80%.


...Central High Plains through the Upper Midwest...
Days 2-3...

...Blizzard conditions becoming more likely for parts of the
Plains...

A large and impressive winter storm is set to spread heavy snow,
significant freezing rain, and strong winds from the Central High
Plains northeast through the Upper Midwest. The driver of this
system will initially be the large upper low pivoting across the
Intermountain West Monday night, and this is progged to deepen
into an anomalous closed upper low over the Central Plains by
Wednesday. Downstream of this feature, broad but robust mid-level
divergence will combine with rapid height falls and strong upper
diffluence along the LFQ of a poleward streaking subtropical jet
streak to induce a surface low tracking from eastern CO through
eastern SD. While this low is not progged to intensify rapidly as
it rapidly becomes vertically stacked, impressive downstream WAA
surging meridionally out of the Gulf of Mexico will transport
excessive moisture noted by NAEFS IVT of +6 sigma, with little
longitudinal gain thanks to the highly amplified mid-level flow.
This theta-e ridge will wrap cyclonically within the WCB into an
impressive TROWAL, and this isentropic ascent will pivot into the
Plains to spread an expansive area of heavy precipitation from the
High Plains of CO through ND and eventually downstream into MN/WI
by the end of the forecast period.

While there is high confidence in a region of heavy snowfall NW of
the low, there remains uncertainty both into timing of the
evolution, and how the warm sector/dry slot will evolve to the
east. The GFS/GEFS/NAM have all become a bit more wrapped up
tonight spreading stronger WAA well north towards the
international border of MN, while the non-NCEP guidance is
generally a bit weaker with the WAA and a little farther displaced
west. Overall, the placement of features is well agreed upon, but
the thermal structure varies greatly, especially for eastern SD,
IA, ND, and MN. Where the WAA is most intense and where the dry
slot wraps northeastward, precipitation will likely fall as
freezing rain, with significant accretions forecast. Still a lot
of uncertainty as to where the greatest freezing rain will fall,
but WPC probabilities for more than 0.1" are greatest along the
Coteau des Prairies and the Buffalo Ridge where they exceed 50%,
and locally more than 0.2" of freezing rain is possible. Even in
these areas though, as the low occludes to the east, the dry slot
will likely fill in and cut off the warm air, turning these areas
back to snow later in the event with modest accumulations possible.

Farther to the north and west, an impressive deformation axis is
likely to pivot, producing bands of heavy snow rates which almost
certainly will exceed 1"/hr in the presence of theta-e lapse rates
below 0C/km. A deepening DGZ as cold air funnels into the low will
yield a near iso-thermal layer within and just beneath the DGZ,
suggesting good aggregate maintenance, and with elevated fgen
driving additional ascent, SLRs could be quite impressive,
approaching 20:1 as shown by Cobb methodology. That SLR may not be
fully realized due to potential fracturing as winds exceed 40kts
below the DGZ, but still a fluffy snow should accumulate
efficiently, especially across SD where WPC probabilities for more
than 8 inches are above 80%, and locally 18 inches of snow is
possible. Widespread 6+ inches is progged by the WPC probabilities
from the high plains of northeast CO through far eastern MT and
into north-central MN, generally north of Minneapolis. The
greatest uncertainty is across eastern SD where marginal thermal
structure and the advancing dry slot may limit snowfall initially,
but as column cools again during occlusion some heavy wet snow
could still accumulate efficiently here noted by pWSSI for snow
load maximizing near Aberdeen, SD.

The combination of heavy falling snow and strong winds will likely
produce blizzard conditions across at least western portions of
the central and northern High Plains.


Key Messages for December 9-15 winter storm from the Pacific Coast
through the Upper Midwest:
--A major winter storm with high winds, heavy precipitation, and
extreme mountain snow will continue across California today then
traverse the Great Basin and Desert Southwest through Monday.

--Snowfall rates of 1-3"/hr will continue in the Sierra Nevada
today, resulting in nearly impossible travel due to near zero
visibility and snow covered roads, especially across mountain
passes.  1-2"/hr snowfall spreading into the Great Basin and
Desert Southwest will result in difficult travel as well.

--Snowfall totals of 1-3 feet are forecast for many of the
mountain ranges of the Intermountain West, with locally more than
5 feet of storm total snow likely in the Sierra Nevada.

--This system will deepen into a major winter storm from the
Central High Plains through the Upper Midwest Monday through
Wednesday.

--Multiple days of significant impacts to travel and
infrastructure due to snow, blowing snow, and freezing rain are
expected next week. Blizzard conditions are likely for parts of
the Central and Northern Plains, and travel may become impossible.


Weiss