FOUS11 KWBC 170746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Tue Dec 17 2024

Valid 12Z Tue Dec 17 2024 - 12Z Fri Dec 20 2024

...Pacific Northwest through the Northern Rockies... Days 1-2...

Shortwave ridging over the Pacific Northwest will quickly be
displaced by an approaching trough moving eastward from the
Pacific leading to height falls along the coast. While the core of
this feature will lift into British Columbia, a secondary vorticity
maxima will pivot onshore NW WA state Tuesday night with enhanced
ascent through PVA, height falls, and concurrent upper level
diffluence. This impulse will move quickly eastward, but will be
accompanied by a lead warm front and trailing cold front, causing
fluctuations in snow levels.

The heaviest precipitation is likely to accompany the warm front,
generally 18Z Tuesday through 12Z Wednesday, with a secondary surge
along the trailing cold front Wednesday morning. During the warm
frontal passage and accompanying WAA/moisture surge, snow levels
are progged to climb to as high as 8000 ft along and west of the
Cascades. This will limit significant snowfall to just the higher
terrain, with rain the primary p-type at the passes. However,
during the transition from cold to warm, a period of freezing rain
is possible even as low as Snoqualmie and Stevens Passes, creating
hazardous travel this evening before changing to all-rain. East of
the Crest, cold air locked in within Canadian high pressure will
allow for more substantial freezing rain/ice accretion through
early Wednesday. The subsequent cold front will produce primarily
rain, as snow levels crash behind it but occur with rapid drying as
well.

For the areas that get snow and ice accumulations, moderate impacts
are expected as reflected by the WSSI-P showing a high chance
(>80%) for moderate impacts in the WA Cascades and into the
foothills. Here, WPC probabilities for 0.1 inches of ice are
generally 10-30%, and 70-90+% for 6 or more inches of snow. By D2
as the cold front shifts east, additional moderate snowfall
accumulations are likely in the Northern Rockies where WPC
probabilities are 30-50% for 6+ inches.


...Northern Plains into the Upper Midwest... Day 1...

A compact and fast moving shortwave will eject from the Northern
Rockies early Tuesday and then race eastward towards the Great
Lakes. This feature will remain of low amplitude, but be
accompaniedby a potent vorticity streamer to enhance otherwise
modest mid- level ascent. More impressive will be a strengthening
jet streak beginning to arc poleward immediately downstream of the
shortwave trough axis. Together this will produce an narrow
corridor of intense ascent which can support a heavy snow band
moving generally west to east from SD through southern MN and into
WI. The most intense ascent should occur during the daylight hours,
but a deepening DGZ (SREF 100mb of depth probabilities reaching
50%) which is aligned with the greatest ascent through 700-600mb
fgen, will support heavy snow rates that could exceed 1"/hr at
times. The progressive nature of this will limit snowfall totals,
but WPC probabilities for 2+ inches are above 70% in a stripe
across eastern SD into SW MN, with locally 4-6" probable as
reflected by HREF max ensemble output in the most intense snow
banding.

Days 2-3...
After this first wave exits, a brief respite will occur before a
more pronounced system digs out of Canada and dives SW into the
Northern Plains. This will be driven by a potent shortwave dropping
from the Canadian Rockies and into North Dakota by Thursday
morning, with the primary ascent efficiently overlapped with the
LFQ of a strengthening jet streak also digging into the region.
This deep layer ascent will impinge into a low-level baroclinic
boundary as a warm front drapes eastward, resulting in rapid
cyclogenesis in eastern MT, with this low then moving into ND and
then into the Great Lakes by the end of D3.

As this wave moves eastward and deepens, WAA/isentropic ascent will
begin to intensify between 280K-285K on Thursday surging moisture
into and downstream of the system, reflected by an axis of PWs
exceeding the 90th percentile according to NAEFS. At the same time,
this WAA will occur favorably into an extremely cold column to
deepen the DGZ to more than 100mb, and intensifying fgen will drive
pronounced ascent into this DGZ. The deep DGZ, a sub-DGZ isothermal
layer, and modest winds in a very cold column suggest SLRs will be
quite high, and this will rapidly accumulate despite the general
progressive nature of this system. At this time, WPC probabilities
D2 are high (70-90%) for more than 4 inches across much of northern
ND, with a stripe of 30-50% probabilities for 4+ inches extending
as far east as the Door Peninsula of WI. Locally, 6-8" of snow is
possible across ND during the event.


...Northeast & Eastern Great Lakes... Days 2-3...

Surface low pressure will develop near the TN VLY Wednesday morning
in response to an elongated shortwave diving from the Central
Plains across the region. This shortwave will be accompanied by
modest height falls/PVA to drive ascent, and interact with the RRQ
of a distant but still noteworthy jet streak pivoting over the
eastern Great Lakes. This low will then lift northeast along a cold
front, while a secondary, and more intense, northern stream
shortwave move over the Great Lakes and into the Northeast. These
features are likely to interact across the Northeast, leading to
secondary low pressure developing off the New England coast and
deepening as it moves into Canada.

Moisture associated with these waves will surge northward on rich
theta-e advection, resulting in PWs which are progged by NAEFS to
exceed the 90th percentile in the CFSR database. This will allow
for widespread precipitation to become heavy beginning around 00Z
Thursday, with wintry precipitation spreading across interior
portions of the northeast. While there still remains some
longitudinal spread in the placement of this low as it strengthens
near New England, the ensemble clusters have begun to focus a bit
farther to the east. This will create an environment that is colder
and more supportive to wintry precipitation, especially as the low
pulls away Thursday morning. The passes of the front combined with
the isallobaric flow into the surface low will help enhance fgen as
well, which when overlapping the increased deformation NW of the
deepening low/interacting shortwaves will likely lead to some heavy
snow rates across interior New England. However, the column as a
whole still appears generally marginal for snow, so a heavy, wet
snow, with low SLRs is likely except in the highest terrain.

The progressive nature and low SLR nature of this system will limit
total snowfall amounts, but heavy accumulations are still possible,
especially in the higher terrain which could result in at least
modest impacts due to snow load. This is reflected by WPC
probabilities that feature a moderate risk (50-70% chance) of more
than 4 inches in the highest terrain of the Adirondacks, Greens,
and Whites, with lesser accumulations extending through much of
northern New England except in the lowest valleys. Locally, 12" of
snow is possible in the highest terrain of the Whites near Mt.
Washington.

Weiss
$$
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