FOUS11 KWBC 040840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Sat Jan 4 2025

Valid 12Z Sat Jan 04 2025 - 12Z Tue Jan 07 2025

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

An amplified shortwave trough will continue to move progressively
east from the Intermountain West into the Rockies today, bringing
mountain snows to portions of the northern and central Rockies. The
heaviest amounts are expected to fall along the northern to central
Utah and the northwestern Colorado ranges. Snowfall rates
increasing up to 2-3 inches an hour later this morning, are expected
to result in snowfall totals of 6-12 inches, with locally heavier
amounts across portions Wasatch and Uinta Mountains.

Meanwhile, unsettled weather will continue in the Northwest, as a
warm front associated with a shortwave trough moving over the top of
an upstream ridge approaches the region today. This is expected to
produce snowfall accumulations of 4-8 inches for many Cascade
locations above 4000 ft, with heavier totals across the higher
peaks. This system will slide southeast across the northern Rockies,
bringing some additional locally heavy snows to the northern Idaho
and western Montana ranges as another wave approaches the Pacific
Northwest on Sunday.

Followed by a sharp, amplifying ridge, moisture and the threat for
additional heavy snow will be limited with this third system as it
drops south from the Pacific Northwest into California and the Great
late Monday into Tuesday.

...Great Lakes LES... Days 1-2...

The strengthening -NAO block over Greenland and the North Atlantic
will keep a large gyre-like 500mb upper low parked over southeast
Canada and the northwest Atlantic over the next few days. On its
western flank, it will work in tandem with strong 1040mb+ high
pressure over central Canada to keep a tight pressure gradient over
the Great Lakes, resulting in cold NWrly flow over the Lakes. This
will prompt favorable LES on northwesterly flow along with a few
upper level disturbances traversing the Great Lakes through Sunday.
The Great Lakes will gradually cool, but lapse rates will remain
steep enough over and downwind of the lakes to where lake- induced
instability may exceed 500 J/kg on occasion. The snow belts of the
Michigan U.P. and from the shores of eastern Lake Ontario to the
Tug Hill will be most favored for heavy snow through this weekend.
A prolonged WNW- NWrly fetch connecting a single band from lakes
Superior, Huron, and Ontario will help produce the more significant
amounts (1-2ft+) in Upstate NY between Syracuse and the Tug Hill.
The WSSI shows Major to Extreme impacts for the Syracuse metro
region and points northwest to Lake Ontario. Latest WPC
probabilities are high (>70%) for at least 12 inches across this
narrow LES band downwind of Lake Ontario.

...Northern High Plains... Day 1...

The ongoing winter storm in the Northern Plains is a great example
of mesoscale banding via strong frontogenetical forcing. Snow is
underway from northwest MT to western SD as healthy mid-level
divergence exists associated with an elongated sharp trough axis
forecast to cross Alberta and the Intermountain West today. Closer
to the surface, lower pressure over the western U.S. and strong
1040mb+ high pressure over Canada is leading to winds near the
surface to be unusually strong out of the SE that is directing
moisture, with origins out of the Gulf of Mexico, into the Northern
High Plains. This is leading to exceptional 850mb WAA emanating
out of the Southern High Plains at low level. In addition, this
mean SErly flow at low- levels favors upslope enhancement through
Saturday as the region becomes favorably placed on the northern
flank of the 700mb low by 18Z today. Lastly, SWrly 700mb winds out
of the southwestern U.S. are causing their own WAA and FGEN
forcing as warm 700mb temps collide with much colder air in the
Northern Plains and create a stripe of moderate snowfall.

The end result is a setup for bands of heavy snowfall setting up
from the heart of "Big Sky country" on south and east into western
South Dakota. The WPC Snowband Probability Tracker still shows
HREF guidance sporting 1-2"/hr rates early this morning across
southwest MT. Areas as far south as northeast Wyoming also could
see these heavy snow rates, primarily east of the Big Horns where
upslope flow can enhance snowfall rates. As the best upper level
dynamics shift southeast this evening, some lingering easterly
flow may keep periods of snow in the forecast through the overnight
hours, but snow should finally taper off by Sunday morning. WPC
probabilities show a wide swatch of high chance probabilities

70%) for at least 4" of snow from southwest Montana to northeast

South Dakota. The WSSI shows widespread Minor Impacts through the
Northern High Plains with some spotty Moderate Impact areas in
north-central Montana (combined with snow occurring before 12z this morning).


... Continued ...
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