952
FOUS11 KWBC 072021
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 PM EST Sun Dec 7 2025

Valid 00Z Mon Dec 08 2025 - 00Z Thu Dec 11 2025


...Pacific Northwest & Northern Rockies...
Days 1-3...

The Pacific Northwest and Northern Rockies will face a steady
barrage of Pacific moisture over the next few days thanks to a
multi-day strong atmospheric river that is pinned between a strong
subtropical ridge off the coast of CA and a stalled, positively-
tilted upper low in the Gulf of AK. Tonight, PWATs start out
around the 90th climatological percentile throughout the Northwest,
before a more impressive plume of PWATs above the 99th
climatological percentile stream in on Monday. Even on Tuesday and
Wednesday, as the subtropical ridge expands a little farther
north, a fire- hose of Pacific moisture will continue into the
Pacific Northwest that spills into the Northern Rockies as well.
Add in steady synoptically-forced ascent aloft from a strong 250mb
jet stream and persistent upslope flow into many of these mountains
ranges, and the recipe is for heavy snowfall from the Olympics and
Cascades to the western WY Rockies above the wavering high snow
levels.

Tonight, snow levels largely look to start out around 5,000ft. As
the strong AR and its associated WAA aloft approaches on Monday,
these values increase to above 6,500 ft in the northern WA
Cascades and above 7,000 ft from Mt. Rainer and points south.
Farther east, snow levels will rise across the northern Rockies as
well, topping 6,000ft in many cases before falling on Tuesday as
the AR orients itself farther south as a shortwave trough ejects
into the northern Plains. Then, snow levels once again rise on
Wednesday as levels increase above 6,000 ft from southern
Washington to west-central MT. Starting this evening and lasting
through early Wednesday evening, 72-hr WPC probabilities for more
than 12 inches are high (70-90%) across the WA Cascades above 5,000
ft, as well as parts of the northern Rockies in ID, western MT,
and western WY above 6,000-7,000 ft. WPC probabilities even show
speckled areas of low- to-moderate chances (20-50%) for localized
snowfall totals >30" in the more remote reaches of these regions
through Wednesday.


...Southern Appalachians & Mid-Atlantic...
Days 1...

A quick moving 500mb trough will track from the TN Valley Sunday
night to the Southeast Coast by Monday afternoon. 500mb PVA ahead
of the upper trough will be paired with easterly low-level flow
that allows for some upslope enhancement along the southern
Appalachians. Snow will be at its heaviest during the day on Monday
but taper off by Monday evening. As the 500mb vorticity maximum
reaches NC Monday afternoon, a weak 850mb low development off the
coast of NC`s Outer Banks will allow for easterly low- level winds
to increase. This combined with sufficient low-level WAA and
moisture advection may create a band of snow that results in a
quick hitting 1-3" snow event in southern VA and northern NC as
far north as the Richmond metro area and south along I-95 and I-85
towards the Raleigh/Durham area. WPC probabilities currently show
low chances (20-40%) for snowfall totals >4" in portions of the
VA/NC Blue Ridge and Potomac Highlands of WV. Probabilities for
>2" are 40-60% east of the Blue Ridge across southern VA and could
lead to slippery road conditions to start the workweek.


...Northern Plains/Upper Midwest/Great Lakes...
Days 2-3...

By Monday night, as high pressure builds in over the Northeast, the
first in a parade of "clipper" systems makes its way toward the
Upper Great Lakes with more light-to-moderate snow expected.
Guidance continues to highlight that strong SWrly flow ahead of
the Clipper will produce a band of heavy lake effect snow oriented
from Lake Michigan and far northwest Michigan aimed northward into
the eastern U.P.. WPC probabilities through Tuesday show moderate-
to- high chances (50-80%) for snowfall totals >6" in over the
eastern Michigan U.P. The eastern Michigan U.P. As the clipper`s
warm front moves east, sufficient 290K isentropic ascent and
850-700mb WAA will give rise to snow over the eastern Great Lakes
and northern New England by Tuesday afternoon. Immediately
downwind of Lakes Erie and Ontario, snowfall totals will generally
range between 1-4". However, the Tug Hill has moderate chances
(40-60%) for snowfall totals >4" through early Wednesday morning.

By Tuesday morning, a vigorous 500mb shortwave trough with origins
from the large Gulf of AK longwave trough will race towards the
Northern Plains. Given its origins, this "Alberta Clipper" will be
more intense than most clippers. This is because it will have
exceptional synoptic-scale forcing aloft in the form of a roaring
150kt 250mb jet streak, plus anomalous moisture associated with the
strong AR over the Pacific Northwest. By 18Z Tuesday, the
clipper`s MSLP looks to be sub-990mb, which ranks below the 1st
climatological percentile for this time of year. As the storm works
its way across the Dakotas Tuesday morning, impressive 290K
isentropic lift and corresponding 850-700mb FGEN will not only help
to sustain this potent clipper, but foster a potent band of snow
northeast of the 850mb low track. There is the potential for an icy
wintry mix just along and north of the surface warm front as a
protruding >0C warm nose forces snow to melt into the form of sleet
or freezing rain. But the storm`s fast motion should limit the
impacts from freezing rain to around minor impacts (ice accretions
less than a tenth of an inch most likely).

By Tuesday night, the storm will remain in the mid-980s for
pressure as it races through WI and into MI by Wednesday morning.
WAA out ahead of the storm is likely to cause periods of snow to
envelope the eastern Great Lakes that include areas such as
northwest PA and western NY. From the Northern Plains the the Great
Lakes, not only will snow be problematic for traveling, but gusty
winds are likely to cause blowing and drifting across roads and
cause reduced visibilities. WPC probabilities show a swath of
moderate chance probabilities (40-70%) for snowfall totals >4"
from northeast ND on south and east through the Upper Midwest and
the northern half of Michigan`s Mitten. Note there are also low-to-
moderate chance probabilities (20-40%) for totals >6", which does
show up on several deterministic guidance members. There has
continued to be some latitudinal spread in guidance with AIFS and
AIGFS guidance along with the operational GFS leaning towards a
more southern track that gets central MN and southern WI into
chances for heavy snow.

While the storm`s speedy forward motion should help to reduce the
chances for more prolific snowfall totals, hourly snowfall rates of
>1"/hr combined with gusty winds will allow for minor to moderate
impacts at the very least. This is supported by the WSSI-P which
depicts high chances (60-90%) for Minor Impacts on Tuesday from
northern ND all the way to northern MI. Residents in the Upper
Midwest and Great Lakes should continue to monitor this storm
closely, as should residents in the Northeast as probabilistic
guidance shows measurable snowfall from this system is possible on
Wednesday.


Mullinax/Snell




$$