FOUS11 KWBC 030900
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EST Fri Jan 3 2025

Valid 12Z Fri Jan 03 2025 - 12Z Mon Jan 06 2025

...Western U.S.... Days 1-3...

Precipitation will continue to spread inland across western Oregon
and northwestern California ahead of an occluded low approaching the
coast this morning. Deep, moist southwesterly flow ahead of the
boundary, along with strong forcing will support an uptick in
precipitation, with orographically focused moderate to heavy amounts
forecast to develop along the coastal ranges into the southern
Cascades and northern Sierra Nevada. However, high snow levels are
expected to confine any heavy snow accumulations to the highest
peaks early on. Then, as the upper trough and associated cold front
begin to move inland, snow levels will begin to lower, expanding the
potential for heavy snow along the southern Cascades, the
northwestern California ranges, and the northern Sierra later today.
Meanwhile, a well-defined warm front, lifting across the northern
Cascades, Intermountain West, and Rockies, will be a focus for
organized precipitation and locally heavy mountain snow as well today.

Meanwhile to the east, moisture focused by low-to-mid level
convergence along the western periphery of strong high pressure
extending from central Canada into the Northern Plains will support
a swath of light to moderate snows extending from central Nebraska northwestward through the High Plains and into the northern Rockies.
Upslope flow is expected to help enhance amounts across the central
to the northwestern Montana ranges.

By early Saturday, the upper trough is expected to bring high
elevation snow into the central Rockies as it moves across the Great
Basin into the region. This will be followed by a second system,
which is forecast to bring additional orographically focused heavier
amounts into the Olympics and the northern Cascades Saturday into
early Sunday.

Conditions will remain unsettled across the Northwest into Monday,
as a third shortwave impacts the region. However, a building
upstream ridge will preclude any long fetch of deep moisture and the
threat for widespread heavy amounts across the region.

WPC probabilities indicate that three day totals exceeding 8 inches
will be mostly confined to the higher elevations of the Cascades,
northern Sierra Nevada, and portions of the Rockies from
northwestern Montana to northern Utah and Colorado. Widespread, but
lighter amounts are expected across central and southeastern
Montana, with high probabilities for accumulations greater than 4
inches covering much of the region.


...Great Lakes and Northeast... Days 1-2...

The strengthening -NAO block over Greenland and the North Atlantic
is aiding a mature surface cyclone to stall over Quebec and help
produce persistent cold northwest flow between it and a strong
surface high over Saskatchewan. This will prompt favorable LES on
northwesterly flow along with a few upper level disturbances
traversing the Great Lakes over the next few days. The Great Lakes
will gradually cool as a result, but the lapse rates will remain
steep enough over and downwind of the lakes to where lake-induced
instability could top 500 J/kg on occasion. The snow belts of the
Michigan U.P., northern portion of Michigan's Mitten, from
northeast Ohio on north east along the Chautauqua Ridge, and from
the shores of eastern Lake Ontario to the Tug Hill will be most
favored for heavy snow through early Sunday AM. Favorable fetch
connecting a band from lakes Superior, Huron, and Ontario will help
produce the most significant amounts (1-2ft+) in Upstate NY between
Syracuse and the Tug Hill. Snow could also be measured in feet
from northeast OH, northwest PA, and western NY (south of Buffalo).
Snowfall rates in these areas of 1-2"/hr are expected and could
come in waves over the next few days. The WSSI shows Major Impacts
for these areas with even a small Extreme area (extremely
dangerous to impossible travel and widespread closures) in the Tug
Hill through Sunday AM.


...Central Appalachians and Mid-Atlantic... Day 1...

A compact wave of low pressure moving across the Ohio Valley early
this morning will move progressively eastward beneath a modest
shortwave racing southeast within the broad trough amplifying
across the eastern CONUS. Immediately in the wake of this
shortwave, a potent jet streak reaching 150kts will dig rapidly out
of the Northern Plains, producing a narrow but intense corridor of
overlapping ascent through height falls, PVA, and LFQ diffluence.
This will help at least subtly deepen the low and the accompanying baroclinicity/attendant fronts as it moves eastward to off the VA
coast Friday night.

The system is compact and progressive, so snowfall in general is
expected to be low outside of the Central Appalachians. Here,
despite the modest forecast depth of the DGZ, appreciable upslope
flow will drive ascent favorably into the DGZ, while some moisture
enhancement occurs on post-frontal NW flow from the Great Lakes.
This suggests an extended period of moderate to heavy snow,
especially D1, with the higher elevations receiving the most
significant snow accumulations. Event total snow here could reach
double- digits in the higher terrain.

Finally, although total accumulations are expected to be minimal, some
snow showers or convective snow showers moving along the cold front
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 into the DGZ, which could promote brief
snowfall rates above 1"/hr as shown by low probabilities in the WPC
snowband tool. Low-level temperatures will be slightly above
freezing from Richmond, VA to Philadelphia, PA, but wet-bulb
temperatures below 0C could allow any heavier rates to accumulate
on roads and elevated surfaces, causing hazardous travel despite
minimal accumulations, during the Friday aftn/eve commute.
Additionally, where low-level instability is the greatest,
especially overlapping the higher moisture content focused along
the MD/PA line, isolated snow squalls are also possible.


...Northern High Plains... Days 1-2...

The forecast remains on track and very similar to the previous
issuance as an amplified upper level trough moving into the
Pacific Northwest this evening will spread downstream ascent
through increasing divergence into the Northern High Plains by
Saturday morning. This feature is progged to split into dual
vorticity lobes, with the primary one swinging into the Central
Rockies, while a secondary impulse lifts into Alberta/Saskatchewan.
Downstream of this trough, weak shortwave ridging will initially
be placed over the region, but this will gradually become displaced
by the approaching trough, leading to increasingly strong deep-
layer ascent. This forcing will act upon a saturating column as
290K isentropic ascent maximizes in a narrow corridor from NW to SE
from the foothills of central MT east into the High Plains of SD,
which will effectively overlap with a stripe of robust 700-600mb
fgen. The overall setup is not ideal for extremely heavy snow, but
an expanding footprint of snowfall is likely, and rates will be
enhanced by a cold column with very fluffy SLRs expected.
Additionally, the guidance has become more excited about snowfall
potential as reflected by an EFI of 0.8 to 0.9, primarily due to an
extended period of moderate to at times heavy snow. WPC
probabilities have increased as well, and now feature a 70-90%
chance for 4+ inches across central MT, with locally up to 10
inches possible in some of the higher terrain, and a 30-50 chance
for 4+ inches across far NW SD.

...Continued...

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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