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/La...

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