Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Wed Jan 1 2025

Valid 12Z Wed Jan 01 2025 - 12Z Sat Jan 04 2025

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

Persistent troughing across the Pacific will maintain broad SW flow
into a modest ridge along the coast until Saturday when a
pronounced closed low digs eastward towards OR forcing a trough to
move inland to the Great Basin by the end of the forecast period.

Periodic impulses shedding through the flow will move into a region
of greater mid-level divergence D1 and the first half of D2,
leading to broad ascent and increasing precipitation coverage. For
D1, this will be most pronounced along a low-level warm
front/baroclinic gradient lifting east/northeast from OR into the
Central and Northern Rockies tonight and Thursday. This will
result in a swath of precipitation, primarily as snow, but with
snow levels rising to as high as 6000 ft in southern OR/northern
Great Basin, with a sharp gradient to less than 1000 ft into the
Northern Rockies and Columbia Basin. The heaviest snow D1 is
expected in the Central Rockies from the Tetons through the Park
Range where higher SLR and strong upslope flow will enhance
accumulations. Here, WPC probabilities are above 70% for 8+ inches
of snow in the higher terrain, with 1-2 feet of snow possible.

A brief respite in heavier snowfall is likely the first half of D2
before more substantial moisture and ascent spread onshore
late Friday and through Saturday. This more enhanced
forcing/moisture is associated with the closed low moving onshore
and causing the entire trough axis to pivot eastward. Impressive
lift will be aided by the LFQ of a modest, but well placed, upper
jet streak, and there is likely to be strong deep layer lift
expanding across much of the Pacific and Interior Northwest Friday
night, shifting as far as the Northern High Plains by Saturday
night. The most impressive ascent will again be collocated with a
secondary warm front lifting northward from OR to Canada on Friday,
leading to enhanced WAA/isentropic ascent within the broad synoptic
lift regime. While overall omega is modest, a swath of moderate to
heavy snow is likely from the mountainous areas of northern CA
through OR, WA, and into the Northern Rockies, with snow levels
generally rising to 4000-6000 ft. This is reflected by WPC
probabilities which suggest a high risk (70-90%) for more than 6
inches of snow in the Cascades and Northern Rockies, with more
general onshore flow leading to upslope snow across the typical
terrain features D3 from the Sierra, Cascades, and into the
Northern Rockies again.

Additionally, the warm advection overrunning cold high pressure
centered over Saskatchewan is progged to leave low-level cold
easterly flow from the Columbia basin into the Columbia Gorge.
Although eventually this will be scoured out, the setup appears
favorable for a prolonged period of moderate freezing rain in the
eastern foothills of the Cascades and into the Columbia
Gorge/Basin. The heaviest icing is expected along the Hood River
Valley and into the Gorge which will take a longer time to scour
out the cold air, and this is reflected by WPC probabilities that
reach as high as 30% for 0.1" of ice accretion.


...Great Lakes and Northeast...
Days 1-3...

Surface low pressure will be moving across eastern Maine and into
the Canadian maritimes to begin to the period, ushering in a long
duration lake effect snow (LES) event into the weekend. This
surface low will be pushed northeast in response to a potent
vorticity maxima rotating through the negatively tilted trough, and
although this low could deepen rapidly in response to favorable
overlap of PVA, height falls, and LFQ diffluence beneath a zonally
aligned jet streak to the south, the best moisture transport will
shift away from Maine early D1. Still, some WAA snowfall will
blanket eastern and northern Maine through Thursday morning with
modest accumulations.

The more significant snow associated with this system will occur in
its wake as N/NW flow develops across the Great Lakes and
Northeast, leading to increased coverage of both LES and upslope
flow into the Upstate NY/New England terrain. This will be in
response to a persistent but amplifying mid-level trough expanding
across the east, with weak vorticity lobes swinging through the
flow to help sharpen the trough and enhanced CAA. This will lead to
enhanced upslope flow and heavy snow, especially in the
Adirondacks, Greens, and mountains of NW Maine through D1 and into
D2, with a deep DGZ and impressive lift leading to WPC
probabilities that feature a high risk (>70% chance) for 8+ inches
of snow in the NW Adirondacks and northern Greens.

More substantial accumulations are likely, however, within the LES,
especially late D1 and through D2 as lake-induced instability
climbs above 500 J/kg. The pattern will support heavy LES in the
favored W/NW snow belts from the U.P., across the L.P., and then
downstream to east of Lakes Erie and Ontario, with a favorable
upstream moisture connection from Lakes Superior and Huron
enhancing LES off of Lake Ontario. LES is expected to be most
widespread and heavy D1 and D2, when 1-2"/hr snows, possibly
greater at times, cause WPC probabilities to be high (>70%) for 6+
inches downstream of most Lakes, and reach above 90% for 8+ inches
east of Lake Ontario D2. Total snowfall of multiple feet is likely,
with the highest along the Tug Hill Plateau. LES will continue
through D3 with just slightly lesser intensity and coverage as
reflected by WPC probabilities for an additional 6+ inches being
above 70% in just narrow channels near the Chautauqua Ridge and the
Tug Hill Plateau.


...Midwest through Central Appalachians...
Days 1-3...

An inverted surface trough sandwiched between a retreating high
pressure over the Southeast and an incoming strong Canadian high
pressure from Alberta will become a focus for a developing wave of
low pressure and an associated stripe of snowfall from west to
east. The sharpening of this trough will initially be in response
to a subtle vorticity lobe swinging through the broad trough across
the east, ejecting from the High Plains into the Upper Midwest
Thursday. Immediately following this impulse, a potent jet streak
will drop southeast into the Northern/Central Plains while
amplifying to 150 kts, providing favorable overlap for deep layer
ascent. This will stem cyclogenesis along the inverted trough, with
increasing downstream warm/moist advection surging moisture along
the 280-285K isentropic surfaces into the Upper Midwest. This WAA
will help deepen the DGZ to the east (SREF probabilities for 100mb
of DGZ depth climbing to 10-30% from the Upper Midwest into the OH
VLY), while 925-850mb fgen increases, although only modestly.

This will result in a stripe of moderate snow from Iowa through the
Ohio Valley, where at least brief snowfall rates to 1"/hr are
possible, leading to WPC probabilities for more than 2 inches that
reach 70-90% in central IA, with locally up to 4" possible.

Farther downstream, as the low coalesces to be more pronounced and
has stronger frontal features accompanying it, a period of strong
upslope flow will occur in its wake across the Central
Appalachians. The DGZ is shallow and thin, but pronounced lift into
it will result in a period of heavy snow, especially late Friday
into Saturday, from the Laurel Highlands through southern WV.
During this time, moisture will be enhanced as well from upstream
Great Lakes connection, enhancing the potential for heavy
accumulations, and WPC probabilities D2 are high (>70%) and
moderate (30-50%) D3 for 4+ inches of snow, leading to local maxima
around 12 inches by the end of the forecast period.

Finally, although accumulations are expected to be minimal, at
worst, some light snow may crest the Appalachians and move across
the Mid-Atlantic states Friday afternoon bringing at least
conversational snow to the areas around Washington, D.C.


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

A stripe of heavy snowfall is becoming more likely Saturday from
central MT through western SD. This snowfall will be associated
with a slowly advancing warm front topped by increasing divergence
downstream of a potent shortwave digging into the Four Corners
region. While jet dynamics are weak during this time, the
downstream moist advection on impressive 280-290K isentropic ascent
should expand precipitation in a NW to SE fashion late Friday night
and through Saturday. Robust 850-700mb fgen into this moistening
column will help intensify ascent resulting in heavier snow rates,
but at this time the intensity of this snow is still uncertain due
to modest DGZ depth probabilities and a lack of ideal overlap
between the best ascent and the DGZ. Still, a band of moderate to
heavy snow is becoming more likely, and WPC probabilities have
increased to 30-50% for 4+ inches across north-central MT.
Additional increases are possible with later model runs.


Weiss