Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 PM EST Fri Dec 27 2024

Valid 12Z Fri Dec 27 2024 - 12Z Mon Dec 30 2024


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

Nearly continuous precipitation will persist across the Pacific and
Interior Northwest through the weekend, but a lull finally appears
late Monday.

Broad troughing centered south of Alaska begins the period
/Saturday 00Z/ driving impressively confluent and zonal flow to its
south. Embedded within this flow, moisture will stream across the
Pacific as an atmospheric river (AR), funneling into the Pacific
Northwest from far northern CA into southern WA. IVT within this
plume of moisture is progged by both GEFS and ECENS to have a high
probability (>80%) of exceeding 500 kg/m/s, with the axis most
directly impacting the OR/CA border D1, just south of a powerful
Pacific jet streak also angling onshore. Together, this will spread
elevated PWs, above the 97th percentile according to NAEFS, as far
east as UT by Saturday night. As is typical with ARs, the
accompanying WAA will raise snow levels, and the NBM is aggressive
showing a sharp gradient in snow level along and south of this
directed moisture. Snow levels across CA and the Great Basin into
UT will reach as high as 8000 ft, while remaining generally
4000-6000 ft farther north. This suggests the heaviest
precipitation will occur above most passes, but WPC probabilities
are high (70-90%) for more than 12 inches of snow across the WA
Cascades, parts of the Northern Rockies and Salmon River/Sawtooth
area, and into the NW WY ranges.

Late D1 into D2, the trough south of Alaska will re-orient itself,
becoming deeper as a closed mid-level low moving towards British
Columbia Sunday morning. This will cause downstream ridging, at
least briefly, causing a temporary reprieve from precipitation
across the West. However, by early D2 /Saturday night/ renewed
moisture will surge onshore with a now SW oriented AR (once again
high probabilities for IVT exceeding 500 kg/m/s) moving into OR.
Embedded within this trough, a more pronounced shortwave and
accompanying vorticity maxima will shed eastward into CA/OR Sunday
aftn, and it is likely a wave of low pressure will move onshore
during this time, shifting east/northeast along a warm
front/baroclinic gradient.

Initially the most widespread precipitation will occur along this
warm front, with locally enhanced fgen occurring as the LFQ of a
potent jet streak interacts with the low-level warm front leading
to strong ascent and heavy precipitation rates. However, with time
as this boundary lifts north, more widespread precipitation will
spread across the West as the accompanying WAA lifts isentropically
(most pronounced at 290K) into the interior NW. This will cause
impressive deep layer lift, aided in terrain by strong upslope flow
especially in N-S or NW-SE oriented mountain ranges. During this
time, snow levels will warm gradually as Pacific air floods
eastward and WAA expands, leading to snow levels that will be as
high as 5000-6000 ft in many areas as far north as the WA/OR border
and into the Absarokas. This will limit snow accumulations at many
of the area passes (lower snow levels will be accompanied by
lesser precipitation), but impactful winter weather is still
expected across many areas Sunday, reflected by WPC probabilities
for more than 12 inches that are again above 70% across many of the
same areas on D1, with the exception being across the Cascades
where the highest potential shifts from WA to OR.

During D3 the greatest moisture will get shunted eastward into the
Northern Rockies and eventually the northern High Plains as a
surface wave develops over WY, leading to more pronounced wrap-
around moisture into MT/WY, and causing a flip in wind direction
leading to upslope flow on eastern slopes across MT/WY. Heavy
snowfall is likely again during D3, but with a gradual decrease in
coverage and intensity. Still, WPC probabilities D3 are high (>70%)
for 8+ inches in the Cascades, Northern Rockies, NW WY ranges, and
into the Park Range of CO. In some areas, 3 consecutive days of
snow could accumulate to as much as 5 feet, with the highest
snowfall probable in the Tetons.


...Mid-Atlantic and Northeast...
Days 1-3...

Dual low pressures, one that is relatively weak on Saturday, and
another much stronger Sunday into Monday, will lift northeast
through the Ohio Valley and west of the Appalachians, spreading
moisture into the Mid- Atlantic and Northeast.

The first of these waves, a weak low along a triple point, will
lift from the lower MS VLY into Michigan D1, with an attendant warm
front pivoting as far north as Upstate NY by Saturday night. As
this warm front lifts northward, it will spread greater moisture
into the Mid-Atlantic and Northeast, especially along the 300K
surface where mixing ratios rise to 4-6 g/kg, driving PWs to above
the 90th climatological percentile as far north as northern New
England by 00Z/Sunday. High pressure to the north will gradually
retreat, and the antecedent airmass is quite dry so this will
somewhat eat into the available moisture for precipitation.
However, it will also keep wet-bulb temperatures below 0C as this
channel of moisture lifts northward, resulting in a narrow corridor
of freezing rain from PA into New England. Amounts should be light,
but any icing can cause impacts, and current WPC probabilities D1
are as high as 10-30% for 0.1" of ice across the Poconos and
Catskills.

After this first wave exits, a brief respite will occur before the
more significant low develops and lifts northward from the lower MS
VLY into MI once again, but with a more pronounced occlusion
driving greater moisture northward. Strong warm/moist advection
downstream of this low will surge PWs to above the 99.5th
percentile within the core, surrounded by a broader axis of PWs
above the 97.5th percentile encompassing much of the Northeast
according to NAEFS. The aforementioned high pressure will retreat
more significantly Sunday in response to impressive mid-level
divergence as the primary upper low closes off over MI, and the
guidance has backed off on the low-level wedging across New
England. Cold antecedent temperatures at at least some wet-bulb
affects will permit precip to begin as freezing rain in many areas
from PA and Upstate NY trough central/northern New England, but
confluent southerly flow will quickly turn most p-type to rain by
the end of D2. The exception will be confined to northern NH and
ME, but even here ice accretion forecasts have come down as
reflected by WSE plume trends. Still, some icing is likely which
could create slippery travel, and WPC probabilities for 0.0.1" are
10-30% D2, and around 10% D3 across central and northern portions
of ME.


...Central Plains...
Day 3...

A fast moving shortwave trough emerging from the Central Rockies
will pivot into the Central Plains Monday morning concurrent with a
zonally oriented jet streak shifting across the Southern Plains.
This will produce an efficient overlap of height falls and
diffluence atop a warm front to drive cyclogenesis, and a deepening
low pressure is progged to move from WY to MO D3. Downstream of
this deepening low, warm and moist advection, especially along the
285K-290K surfaces, will lift isentropically into a modest TROWAL,
providing additional support for ascent and the guidance is
trending towards a more robust swath of snow, especially near the
SD/NE border, and before subsequent low pressure develops near the
Ozarks late in the period. Ensemble clusters suggest still a lot of
spread in low placement and intensity, but confidence is increasing
in a swath of heavy snow despite a modest thermal environment as
dynamic cooling helps to increase snow accumulation potential. At
this time, WPC probabilities are modest, but feature a 10-30%
chance for 2" of snow across southern SD, with more snow possible
into D4 and beyond this forecast period.


Weiss