FOUS11 KWBC 020840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Thu Jan 2 2025

Valid 12Z Thu Jan 02 2025 - 12Z Sun Jan 05 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. 

Periodic impulses shedding through the flow will move into a region
of greater mid-level divergence D1, leading to broad ascent and 
increasing precipitation coverage. For today, this will be most 
pronounced along a low-level warm front/baroclinic gradient lifting
east/northeast from OR into the Central and Northern Rockies. 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.

Heavier snowfall is likely at the very end of D1 into much of D2 
as more substantial moisture and ascent spread onshore Friday into 
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 moving across eastern Maine early this morning
will usher 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, which eventually swings northwestward towards the Hudson
Bay as a larger upper level low and stalls. This allows for strong
and persistent cold northwesterly flow over the currently ice-free
Great Lakes and substantial snowfall likely associated with the 
LES, especially 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 through Friday, 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 and Mid-Atlantic... 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
today. 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, while 925-850mb fgen 
increases, although only modestly. 

This will result in a stripe of moderate snow from Iowa this
morning through the Ohio Valley, where at least brief snowfall 
rates to 1"/hr are possible, leading to a general 1-3" of snowfall
accumulation, 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%) for 6+ 
inches of snow, leading to local maxima around 12 inches by the end
of the forecast period.

Finally, although total accumulations are expected to be minimal, some
snow showers and snow squalls may crest the Appalachians and move 
across the Mid- Atlantic states Friday afternoon as the vorticity 
max dives southeastward. The late afternoon timing of this vort max allows
for steep lapse rates from the sfc all the way up to 500 mb and the
potential for snow squalls to develop and potentially contain
1-2"/hr snowfall rates. Surface temperatures in the Washington 
D.C. to Baltimore region will start out above freezing, but areas 
just north and west may be right around the freezing mark. Either 
way 850 mb temps of -5 to -6C will allow for the heavier precip 
rates to quickly fall as snow and surface temperatures to quickly 
crash as well. This may lead to potentially hazardous travel and 
rapidly deteriorating road conditions for the Friday evening 
commute depending on the strength of the snow showers/squalls and 
where exactly they impact in the Mid-Atlantic.

 ... continued ...
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