FOUS11 KWBC 261945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Thu Dec 26 2024

Valid 00Z Fri Dec 27 2024 - 00Z Mon Dec 30 2024

...The West... Days 1-3...

Widespread active weather to impact the Pacific and Interior
Northwest through the weekend.

Broad troughing centered south of Alaska will shed periodic
shortwave energy eastward towards the Pacific Coast through the
weekend, although accompanying surface lows are progged to remain
north into Canada. This will result in a pattern which features
impressively convergent flow coming eastward from the Pacific,
leading to rounds of elevated IVT as period atmospheric rivers (AR)
surge onshore. This moisture combined with strong jet streaks aloft
will result in widespread heavy precipitation across much of the
Pacific and Interior Northwest, with precipitation spreading as far
south as the Sierra, and as far east as the Central Rockies, at times.

The first wave will move onshore late Thursday night into Friday
morning as the low pressure lifting into British Columbia pushes a
warm front eastward and into OR/WA. The accompanying moist
advection will push IVT above the 99th percentile according to the
CFRS climatology, highest into the Great Basin, but the northern
periphery of >90th percentile IVT will lift into OR/ID, and GEFS
IVT probabilities for 500 kg/m/s peak above 50% even as far east as
the Foothills. The overlap of moisture with ascent (aided by
transient LFQ jet level diffluence and periods of upslope in N-S or
NW/SE terrain features) will result in widespread precipitation D1
from the Sierra and coastal OR through the Great Basin and into the
Central Rockies. Snow levels in the highest IVT core will reach
5000-6000 ft, but remain around 3000-4000 ft farther north, leading
to at least modest winter impacts at the Passes. WPC probabilities
D1 for more than 8 inches of snow are high (70-90%) for parts of
the Sierra, OR Cascades, Blue Mountains, Salmon River/Sawtooth
region, Uintas, Tetons, and into the Park Range of CO. Locally 1-2
feet is likely in the highest terrain.

A second, somewhat weaker, impulse will follow immediately in the
wake of this first wave and lift northeast into British Columbia
once again, with the attendant warm front and accompanying
warm/moist advection lifting into WA/OR late Friday night into
Saturday. Once again, enhanced ascent through jet streak will help
expand moisture, with upslope flow driving the most intense
precipitation into the Cascades and farther east in the Salmon
River/Sawtooth Ranges. Snow levels with this second wave will be
slightly higher than the first, around 8000 ft across CA/Great
Basin, and 4000-6000 ft as far north as the WA Cascades. Heavy snow
is likely once again above these levels, and in some places farther
east into ID/MT/WY snow intensity may decrease between the two
waves, but will never really shut off. Current D2 WPC probabilities
are high (70-90%) for more than 8 inches again from the WA
Cascades, into the Northern Rockies, and across the Tetons.

Yet a third wave in this persistent confluent onshore flow will
push onshore Saturday night and Sunday as a more pronounced
shortwave trough digs eastward and comes onshore near the OR/WA
coast at the end of the forecast period. This will again be
accompanied by onshore flow and enhanced IVT, but mesoscale forcing
for ascent may be a bit more robust Sunday as a warm front stalls
in the vicinity beneath the favorable LFQ of a more potent stream.
The axis of higher moisture is more restrictive this day as NAEFS
PWs above the 97.5 percentile are in a narrow channel from OR to
UT, but this is also where some enhanced fgen may occur through the
favorable overlap of low-level WAA and upper level diffluence.
Uncertainty is considerable in the placement of this corridor, but
another round of heavy snow is likely above generally 4000-6000 ft
on Sunday. WPC probabilities D3 are above 70% for 8 inches D3
across the spine of the Cascades, into the Salmon River/Sawtooth
region, Blue Mountains, and continue in the vicinity of
Yellowstone NP. 3-day total snowfall of 3-5 feet is possible in the
hied terrain of the Cascades and Tetons.

...Interior Mid-Atlantic and Northeast... Days 2-3...

A shortwave rotating through the base of an elongated trough
positioned from the Northern Rockies through the Ohio Valley will
lift northeast into the Great Lakes Friday night into Saturday
morning while a surface high pressure sits south of New England.
The confluent flow between these two features will surge a plume of
moisture northward on 300K isentropic ascent leading to PW
anomalies as high as +2 sigma according to NAEFS from the Central
Appalachians through the northern Mid-Atlantic. The surface high
will slowly retreat during this period, but cold air at the surface
will at least initially be trapped leading to some light freezing
rain accumulations. WPC probabilities for 0.1" or more are 10-30%,
focused in the Catskills and southern Adirondacks, but light icing
accumulating to above 0.01" may impact much of PA, Upstate NY, and
southern New England.

After a brief break in precipitation, a more significant wave of
low pressure will develop across the Ozarks Saturday evening in
response to a potent shortwave trough digging through the primary
trough axis from the Central Plains. This shortwave will gradually
weaken into D3 as it encounters mid-level ridging across the
eastern CONUS, but a favorably placed jet streak will leave
sufficient diffluence through the RRQ to allow slow deepening of
the surface wave as it moves into the eastern Great Lakes and then
Ontario Province by the end of the forecast period. To the east of
this low, precipitation will expand on renewed WAA, while subtle
wedging of the retreating high clings across northern PA, Upstate
NY, and northern New England. Eventually all of the precipitation
should turn to rain as WAA overwhelms the column, but a period of
icing is likely before that time, leading to at least modest
accumulations of ice. Some of the recent WSE plumes are quite
aggressive across NH/ME, so there is potential for heavier icing,
but at this time the WPC probabilities for 0.1" or more are capped
at 30-50% in central ME.

Weiss
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