Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Sat Dec 09 2023

Valid 12Z Sat Dec 09 2023 - 12Z Tue Dec 12 2023

...Upper Midwest...
Day 1...

Mid-level low over north-central Minnesota this morning will drift
east over northern Wisconsin today. A well formed swath of light
to moderate snow wrapping around this low will shift east over MN
today with continued favorable forcing aloft overlapping
low-to-mid level frontogenesis. This deformation zone shifts east
over Lake Superior tonight with lake enhanced snow over the
western U.P. with moderate probs for >4" over the Porcupine and
Huron Mtns.


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

Moderate atmospheric river PW anomalies of +1 to +2 sigma arrives
into western WA this morning with low initial snow levels (around
1500ft) that steadily rise over 6000ft late this afternoon in this
warm core flow. However, before then, snow rates quickly rise to
1-2"/hr while snow is at or below Cascade pass level. Day 1 snow
probs are high for >8" for the WA Cascades. The whole system will
be stretched out into Monday as the dynamics weaken at the expense
of the developing Eastern US system, but the onshore flow will
continue to produce light to modest snow over the ID Rockies with
moderate probs for >6" on Day 2 for the Blue Mountains, Salmon
River, Sawtooth, Bitterroots, and western WY ranges.


...Central Appalachians through the Interior Northeast...
Days 2/3...

A reinforcing shortwave trough over the northern Rockies and a
digging trough over the southern Rockies interact Sunday night
over the Midwest/Mid-Atlantic, forming a negatively-tilted trough
that quickly lifts up an interior coastal track up the Eastern
Seaboard Sunday into Monday. Rapid development of a surface low
tracking north along the sharp cold front will result in a
powerful cyclone tracking north across New England Sunday night
into Monday. The deepening low will direct ample moisture onto the
cold side of the front, supporting rain changing to snow beginning
Sunday evening in the south-central Appalachians, before moving
north through the interior Northeast Sunday night and Monday. The
heaviest amounts are expected to center across northern Upstate
New York and northern Vermont where a coupled upper jet and strong
low-to-mid level frontogenesis are forecast to support heavy rates
on the backside of the low. Variability in the low track and
intensity of the snow bands is still present with the 00Z GFS
farther east than consensus and the 00Z NAM the most dynamic (the
06Z NAM just came in much farther east like the 00Z GFS). The 00Z
EC/CMC solutions offer similarly terrain based snowfall solutions
over the central Appalachians of WV and more so over northern PA,
Upstate NY including th Catskills and Adirondacks, then for the
Greens and Whites of northern New England. Day 2.5 snow probs for
>6" are moderate (30-70%) for these areas with low probs for >12"
over the northern Adirondacks.

This is expected to be a fast-moving storm, with the surface low
moving quickly into eastern Canada by Monday evening.  However,
strong northwesterly to westerly flow in the wake of the system
will support lake effect and orographic snows in the lee of the
eastern Great Lakes and over northern New England late Sunday
night into Monday night. Day 2.5 snow probs in typical NW to W
flow snow belts east of Lakes Erie and Ontario are moderate for
>6".


Jackson