FOUS11 KWBC 160830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Mon Dec 16 2024

Valid 12Z Mon Dec 16 2024 - 12Z Thu Dec 19 2024

...Pacific Northwest through the Northern Rockies... Days 1-3...

An active period of weather continues across the West as a trough
persists offshore shedding impulses and accompanying IVT onto the
coast. Despite this, the general trends will be for bulging
thicknesses across the West with increasingly quiet weather
forecast through the period.

On D1, a strung out vorticity lobe accompanying a shortwave trough
will lift northeast towards the WA coast, driving a frontal system
eastward towards the shore. As this feature moves to the east, it
will weaken and elongate, but still push increasing moisture into
the region within confluent mid-level flow. The highest
accompanying IVT will move across CA and into the Great Basin, but
a broader surge of elevated PWs will shift into most of the Pacific
and interior Northwest, resulting in a shield of precipitation
lifting across the area. Snow levels west of the Cascade crest will
rise to around 4000 ft, but remain much lower to the east,
especially across interior WA state at just 1500-2500 ft, but then
rise again to around 3000 ft in the Northern Rockies. Where the
best overlap of synoptic lift (through height falls, downstream
divergence, and upper level diffluence) and upslope flow combine,
likely in the Shasta/Siskiyou region, the OR Cascades, and as far
east as the Salmon Rivers and Tetons. In these ranges and above the
aforementioned snow levels, WPC probabilities D1 for more than 8
inches are generally 70-90%, with locally as much as 2 feet possible.

As this first wave sheds inland, it will be quickly followed by
another, more impressive surge of IVT (GEFS and ECENS probabilities
for >500 kg/m/s as high as 80%). This secondary plume will traverse
more quickly north as a low-level trough axis and following warm-
front extending from another approaching shortwave pushes from
northern CA into WA state. This surge will drive snow levels
rapidly upward, reaching as high as 8000 ft by the end of D2 west
of the Cascades, but even 5000-7000 ft as far east as the northern
Rockies and the Great Basin. This will cause a rapid decrease in
snow coverage while SLRs also fall to become much heavier (also
reflected by increasing snow load probabilities in the WSSI-P). WPC
probabilities D2 are highest in the WA Cascades and Northern
Rockies where they reach 70-90% for an additional 6+ inches, with
lighter additional accumulations expected across the Salmon Rivers,
Sawtooth, and Tetons.

During D3 most of the ascent and accompanying moisture shifts
eastward into the High Plains, bringing a brief reprieve to
snowfall in the Northwest. However, early in the period /Wednesday
morning/ some light snow is likely in the higher terrain of the WA
Cascades, Okanogan Highlands, and Northern Rockies, where WPC
probabilities are moderate (50-70%) for 4+ inches of additional
snowfall. Potentially more problematic early D3 will be the threat
of some freezing rain along and east of the Cascades, including
some of the passes. The deterministic icing amounts from the
guidance are quite low, as are WPC probabilities for even 0.01
inches of ice (just 10-30%), but any light icing could be
problematic for the Wednesday morning commute.

...Northern Plains into the Upper Midwest... Day 2...

A modest mid-level impulse will traverse southeast from the
Northern Rockies to the Western Great Lakes Tuesday, producing
sharp but temporally short height falls, combined with increasing
LFQ diffluence as a jet streak pivots across the region. The trends
in the guidance have been for this jet streak to be more intense
and more strongly poleward arcing, while also being a bit
displaced to the south. Although moisture downstream of this
shortwave will be modest, it will likely be effectively wrung out
as fgen intensifies directly into the deepening DGZ, supporting
snowfall rates in a narrow channel that may exceed 1"/hr. The
system is progressive, but the briefly heavy snow should cause some
moderate accumulations and modest impacts. WPC probabilities are
currently less than 10% for 4+ inches (but 50-70% for 2+ inches),
but it is possible some higher snowfall will occur with this band.

...Northeast & Eastern Great Lakes... Day 3...

Guidance starting to converge on a solution which will bring a
progressive but impactful winter storm to the interior Northeast
Wednesday night and Thursday. Dual shortwaves, one lifting out of
the Missouri Valley Wednesday, and a second northern stream impulse
diving across the Great Lakes may interact or phase near 12Z
Thursday (end of D3) over Upstate New York. This will occur along
an eastward racing cold front, with downstream and intensifying
warm advection spreading moisture northward as an expanding
precipitation shield from the eastern OH VLY into New England.
It appears through the OH VLY and Mid-Atlantic, most of the
precipitation will be rain as the column dries before cold air can
catch the moisture.

However, across interior PA, NY, and northern New England, the
risk for heavy snow is increasing as the two shortwaves phase and
produce secondary cyclogenesis off the coast. At this time there
continues to be considerable timing and spatial spread of this
secondary low, leading to wide fluctuations in snowfall potential.
However, the setup should result in a pronounced band of heavy snow
through WAA/Fgen, resulting in an axis of heavy accumulations.
Where this is going to occur, and with what intensity, remains very
uncertain as reflected by DESI LREF plumes and very low WSSI-P
probabilities for moderate impacts. However, current WPC
probabilities have increased, and feature a 50-70% chance of at
least 4 inches of snow, greatest across the high terrain of the
Adirondacks, Greens, and Whites.

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

Zonal flow across Western Canada will become suppressed in response
to a sharpening shortwave which will amplify as it digs east of the
Northern Rockies and reaches the Dakotas before the end of the
forecast period. This shortwave is progged to remain progressive,
but should deepen substantially and will work in tandem with a
digging jet streak to drive pronounced ascent Wednesday night into
eastern MT and the Dakotas. This synoptic lift moving atop a
baroclinic gradient along a weak front will help launch
cyclogenesis, and a strong clipper type low is likely to dive into
the region D3. Impressive WAA downstream of this feature will help
deepen the DGZ (SREF probabilities for > 50mb of depth increase to
70%), which should work together with the ageostrophic response to
the jet streak to produce a band of impressive fgen. The column is
quite cold at this time and the best fgen may reside above the
DGZ, but a band of heavy snow is becoming more likely Wednesday
night. A lot of details will still need to be ironed out, but
current WPC probabilities are moderate (50-70%) for 4+ inches of
snow across northern ND, with additional moderate to heavy snow
likely to continue downstream through D4.

Weiss
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