Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
220 PM EDT Thu Mar 20 2025

Valid 12Z Thu Mar 20 2025 - 12Z Sun Mar 23 2025


...Pacific/Interior Northwest to Central Rockies...
Days 1-3...

An active Pacific jet stream pattern will keep periods of mountain
snow in the forecast over the next few days. This is largely due to
a persistent troughing pattern over the Gulf of Alaska that
continues to reload this active pattern with storm systems racing
across the North Pacific that eventually reach the northwestern
U.S.. The Washington Cascades and Olympics above 3,500ft will be
the epicenter for the heaviest snowfall where they are more ideally
placed with the best surge of Pacific moisture, favorable upslope
enhancement, and their placement beneath a strong 130kt 250mb jet
streak aloft. The Oregon Cascades will also be on the receiving
end of accumulating snow, but with higher snow levels farther
south, elevations above 4,500ft will be most favored for heavy
snowfall through Friday. Following a brief break in the active
pattern Saturday morning, an approaching warm front from the next
storm system in line over the Gulf of Alaska will cause noticeably
higher snow levels over the Olympics and Washington Cascades
Saturday night and Sunday. Snow accumulations will likely range
between 1-3 feet in these ranges with some of the tallest peaks of
the Olympics and Washington Cascades topping 4 feet of snowfall
through Sunday.

This same plume of Pacific moisture and the aforementioned jet
streak aloft will advance inland across mountain ranges that
include the Blue, Sawtooth, Bitterroots, Lewis Range, Absaroka, and
Tetons tonight, Friday, and into Saturday morning. Elevations above
5,000ft in the Blue, Sawtooth, Bitterroots, and Lewis Range are
favored to receive as much as 1-2 feet of snow through Saturday
morning. Farther south, elevations above 7,000ft are favored in the
Absaroka and Tetons, as well as other neighboring ranges that
include the Big Snowy, Little Belt, Big Horns, and Bear River
ranges. Snowfall totals of 8-12" will be most common in these
mountain ranges, with the Tetons having better odds of seeing some
snowfall totals topping 2 feet. Lastly, some of this Pacific
moisture will bleed far enough south to reach the Colorado
Rockies and southern Wyoming Rockies Friday night and into
Saturday. Elevations above 9,000ft will be most likely to see
snowfall accumulations >6" with the tallest peaks of the Colorado
Rockies receiving over a foot before the snow tapers off Saturday
night.


...Northern Appalachians & Northeast...
Day 1...

This afternoon, a lobe of 500mb vorticity tracking towards the
Southeast will foster increasing upper-level ascent over the
Northeast coast. At the same time, a frontal boundary approaching
from the west will align itself along a more baroclinically favored
area along the Northeast coast that will give rise to a developing
wave of low pressure Thursday night near Long Island. With the
developing low positioned beneath the diffluent left-exit region of
a 250mb jet streak, low pressure will organize quickly and
southerly flow off the Gulf Stream will advect rich Atlantic
moisture around the northern and western flanks of the low. After
initially starting out as rain, a transition to snow will ensue
from the Catskills and Adirondacks on east through the Berkshires,
Green, and White Mountains on Friday. The storm is a quick mover
thanks to the next upper trough within the active jet stream
pattern over the Great Lakes. Any lingering snow in parts of
northern Maine should conclude Friday night. WPC probabilities
depict low-to-moderate chances (20-50%) for snowfall >2" in the
higher elevations of the Green and White Mountains. The tallest
peaks, including Mount Washington, could see some snowfall totals
>4" through Friday afternoon.


...Upper Midwest & Great Lakes...
Day 3...

A steepening upper-level trough over the Northern Plains will
provide modest ascent aloft and strengthen low pressure over the
Dakotas. Out ahead of this upper-level trough is increasing
850-700mb WAA that gives rise to low-mid level FGEN over the Upper
Midwest and into the northern Great Lakes Sunday morning. As Sunday
unfolds, a more organized 850mb low is likely to form over
northwest Wisconsin with a much of the atmospheric column well
saturated over northern Minnesota, northern Wisconsin, and the
Michigan U.P.. Based on the track of the 850mb low and the pivoting
deformation axis on the storm's northwest flank, it is northern
Minnesota that presently features the best odds (50-70% moderate
chances) for >4" of snowfall. The Minnesota Arrowhead, with the
added benefit of easterly flow that provides additional lake-
enhancement, has low-to-moderate chances (30-50%) for snowfall
totals >8". Northern Wisconsin and the Michigan U.P. may mix with
rain or stay a wintry mix longer, which combined with generally
lower SLRs may keep snowfall totals a bit more capped compared to
northern Minnesota. However, it will be the CAA on the back side of
the storm and across Lake Superior Sunday night when these areas
see their best chances for measurable snow.


For days 1-3, the probability of significant icing of at least
0.10" is less than 10 percent across the CONUS.


Mullinax