Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Sun Dec 15 2024

Valid 00Z Mon Dec 16 2024 - 00Z Thu Dec 19 2024


...Pacific Northwest through the Northern Rockies...
Days 1-3...

As the first storm system that brought periods of heavy snow to the
mountainous terrain of the Pacific Northwest and Northern Rockies
winds down this afternoon, the next frontal system associated with
a shortwave trough will track towards the region tonight. As the
warm front approaches, a slug of 850-700mb moisture will slam into
the Pacific Northwest coast that leads to periods of mountain snow
in the usual suspects (Olympics, Cascades, Siskiyou,
Trinity/Shasta) tonight and into Monday morning. This is no slouch
of a moisture stream either, as evident by an IVT surpassing 500
kg/m/s just off the West Coast that is above the 90th
climatological percentile per NAEFS.

This moisture will spill over into the interior Northwest and
eventually the Northern Rockies by Monday afternoon, thanks to
aforementioned IVT that by Monday morning is above the 97.5
climatological percentile as it takes aim at northern California.
In fact, 500mb mean specific humidity levels are also above the
90th climatological percentile by 00Z Tuesday across northern UT
and into the Tetons of WY. The region also lies beneath the
divergent left-exit region of a >120kt 250mb jet streak Monday
evening. Add in westerly 700-500mb winds that are 40-50 kts and
this is a good setup for upslope enhanced snowfall rates in N-S
oriented ranges (such as the Boise, Sawtooth, and Teton Mountains).
Westerlies advecting Pacific moisture into the northern Rockies
will keep snow in the forecast through Tuesday afternoon. By the
time snow concludes Tuesday night the Blue, Boise, Sawtooth, and
Teton Mountains are likely to see anywhere from 10-20 inches of
snow with totals approaching 30" in the peaks of the Tetons.

Meanwhile, the Pacific Northwest will remain mired in the same
moist westerly flow Monday night and into Tuesday as the initial
upper low opens up and tracks over head Monday night. As it heads
east on Tuesday, upper level ridging and WAA aloft ensues ahead of
the next approaching PAcific shortwave trough. The warm front
lifting north through the region Tuesday afternoon will coincide
with the arrival of strong high pressure over southwest Canada.
Sub-freezing air racing south into the Columbia Basin will make for
a classic overrunning setup that gives rise to an icy wintry mix
Tuesday afternoon and linger into Tuesday night. While WAA
increases on Wednesday ahead of the next PAcific shortwave trough,
snow levels will rise to the point where even elevations >6,000ft
will struggle to support snow. However, frozen precipitation may
linger along the northern Washington/Canada border. WSSI-P shows
low chance probabilities for Minor Impacts in parts of the Columbia
Basin due to ice Tuesday night and into Wednesday morning. Over
this prolonged stretch of heavy snow, the Cascade Range and
Olympics above 4,000ft are likely to see 12 inches at least with
the elevations above 5,000ft seeing as much as 2 feet of snow. This
includes elevations farther east concluding the Blue Mountains,
the Bitterroots in northern Idaho, and the Lewis Range in western
Montana. even the Salmon/Trinity/Shasta Mountains of northern
California above 5,000ft could pick up as much as a foot of snow.


...Central Appalachians...
Day 1...

An icy wintry mix has enveloped much of the northern Mid-Atlantic
as a potent upper low tracking across the Lower Great Lakes is
producing WAA at low-mid levels. This WAA and plume of moisture
aloft is overrunning a sub-freezing air-mass anchored by
exceptionally strong high pressure to the northeast. The upper
trough will weaken as it heads northeast this evening with
generally minor snowfall accumulations (1-3", locally up to 4") in
northern PA on east through the Poconos and Catskills tonight. The
freezing rain/sleet mix remains the most problematic when it comes
to hazardous travel from the central Appalachians this afternoon,
to
portions of the Lehigh Valley, Delaware Valley, and through the
Tri-State region north of NYC tonight and into Monday morning. This
could make for a slick morning commute in parts of the Tri-State
area. Most additional ice accumulations will be <0.1" with the
Laurel Highlands featuring the better chances (30-50%) for
additional ice accumulations >0.1" through tonight. The WSSI does
show Minor Impacts in parts of central PA, the Catskills, and
northern NJ with lingering Moderate Impacts from the Laurel
Highlands on south through Garrett County, MD and the Appalachians
of West Virginia.


...Northeast & eastern Great Lakes...
Day 3...

There remains a fair amount of spread in solutions for the next
developing wave of low pressure set to track through the Ohio
Valley on Wednesday and towards the Northeast by Wednesday
evening. While track and intensity of the storm are unclear, areas
from northeast OH and northwest PA through the northern
Appalachians are currently favored for some measurable snowfall
that may lead to hazardous travel impacts Wednesday night. The
latest WPC probabilistic guidance shows low chances (10-30%) for
snowfall >4" in the Adirondacks through Wednesday evening. Guidance
trends will be closely monitored in subsequent forecasts as it does
have the potential to be a disruptive winter storm for residents in
the interior Northeast.


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

Low pressure in North Dakota will continue to produce minor
snowfall accumulations through northern North Dakota and into
northern Minnesota tonight. As the storm strengthens overnight,
gusty winds will promote some blowing and drifting snow, but
outside of some reductions to visibility, impacts should be very
minor for these regions through Monday. WPC probabilities show
low-to-moderate chances (30-50%) for >2" of snowfall through Monday
afternoon in far northern North Dakota and northwest Minnesota.


Mullinax