Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
202 PM EST Sat Feb 1 2025

Valid 00Z Sun Feb 02 2025 - 00Z Wed Feb 05 2025


...Western U.S. across the Northern Plains...
Days 1-3...

...Atmospheric River (AR) event to bring prolonged heavy mountain
snow to parts of the West and Rockies...

Combination of a strong upper low over coastal British Columbia
and another upper low north of Hawaii moving eastward will maintain
onshore flow into the West over the next several days, focusing a
plume of moisture into NorCal and the northern Great Basin to the
northern Rockies. The moisture axis will ebb and flow north then
south as the two players jockey for prominence but the net result
will be lighter snow in the lower elevations north of NorCal,
moderate to heavy snow in the mountains (especially from NorCal
across southeastern Oregon into central Idaho and western WY) with
very heavy snow at the highest peaks. Much of the Sierra will
remain on the warmer side of the AR event which will retain higher
snow levels.

For Day 1, the focus will be over central Idaho into western WY
with moderate to heavy snow in the mountains, generally above
5000-7000ft depending on location. Snow will gradually lessen
across the WA and northern OR Cascades but not shut off
completely. Into Day 2, the approach of the upper lows will help
raise heights a bit over the interior, helping to nudge the
moisture axis northward and increase QPF over NorCal into southern
OR. Downstream, extended jet and an approaching arctic front from
the north will help spread snow into southern/southeastern MT and
into the High Plains/northern Plains in the fast flow. Guidance
continues to waver on the north-south QPF axis across ND/SD, but
potential remains for several inches of snow where modest FGEN sets
up (albeit briefly). WPC probabilities for at least 4 inches of
snow D2 are moderate (40-70%) along the ND/SD border.

Into D3, the northern upper low will meander just west of
Vancouver Island as the southern ex-upper low moves into NorCal
with another surge of moisture that will favor the northern to
central Sierra. The moisture plume will move inland and sink a bit
southward, allowing snow levels to fall a bit but still promoting
heavy snow above 7000ft where WPC probabilities for at least a foot
of snow just D3 are >50%. Over the northern Great Basin to the
northern Rockies, the moisture plume will continue to lift
northward as well in response to the upstream digging heights,
favoring central Idaho into western Montana atop the cold air mass
in place from D2.

Three-day snowfall totals will exceed 1-3 feet in the higher
mountain elevations with lighter snow of an inch or two (or a few)
on the valley floors in most locations across OR/ID. Much more snow
is likely over most of Montana due to the increased low-level
convergence via the arctic front beneath the moisture plumes,
supporting light and fluffy snow. Over western WA/OR, precipitation
will be light and mostly snow or a mix of some rain/snow during
the day with marginal temperatures. This should lead to light
accumulations through the I-5 corridor where WPC probabilities of
at least two inches of snow during the next three days are >20-50%
(especially away from the water and with at least some elevation).


...Upper Great Lakes and Northeast...
Days 1-3...

A progressive shortwave trough slides east over the Northern
Plains through tonight ahead of a diffluent left-exit region of a
130kt jet streak. This will continue to provide ascent along with
modest 850-700mb WAA and allow for an area of light to moderate
snow initially across MN, northwest WI, and the MI Upper Peninsula
to eventually shift east across the Upper Great Lakes into Sunday.
The heaviest snow on D1 is forecast on the North Shore of Lake
Superior across far northeast MN on southeasterly flow, where WPC
probabilities for at least 6 inches of snow are greater than 70%.
Otherwise, light snowfall spreading over northern WI and MI is
expected to remain under 4 inches, with scattered 20-40% WPC
probabilities for more than 4 inches in the MI Upper Peninsula.

The WAA continues to shift east over the Northeast Sunday afternoon
into Monday, with largely light snowfall and the potential for
light icing (less than 0.1") in the Allegheny Mts of PA and
locations east of the Continental Divide where low-level cold air
damming is more favorable. Day 2 snow probs for >4" are 20-30% for
the Tug Hill and higher Greens/Whites.

The next wave enters the Upper Great Lakes on D2 (Monday) as a
subtle shortwave riding along mostly zonal, but fast flow across
the north- central United States creates the potential for a stripe
of moderate to locally heavy snowfall stretching eastward out of
the Dakotas. The first system on Sunday is expected to push a cold
front well south across the central Plains and Midwest on Monday,
with briefly strong 850-700mb fgen and a strengthening upper jet
streak over the Great Lakes. This will prompt an east-west oriented
band of snowfall across central MN, northern WI, and northern MI
on Monday. WPC probabilities on D2 of at least 4 inches of snow are
between 20-40% across this region. By D3 (Tuesday) this 850-700mb
fgen axis slides into the Interior Northeast and along with
trailing CAA and upslope flow into northern NY, VT, NH, and ME,
locally heavy snow is possible. WPC probabilities for at least 4
inches of snow are high (70-80%) from the Adirondacks to the
northern White Mts.


Fracasso/Snell