FOUS11 KWBC 010925
QPFHSD

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

Valid 12Z Sat Feb 01 2025 - 12Z Tue Feb 04 2025


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

...Prolonged heavy snow Oregon, Idaho, western Wyoming tonight
through Tuesday night with locally extreme impacts...

...This heavy snow spreads south down the Sierra Nevada Monday
night into Wednesday...

A prolonged Atmospheric River (AR) and cold-core low maintain an
extreme wintry pattern over much of the western U.S. through the
middle of next week. The cold core low over the Queen Charlotte
Sound will drift south over Vancouver Island through Sunday and
linger around/west of Washington through the middle of the week.
Height falls from troughs rounding the low are bringing a sharply
colder profile western WA/northwestern OR this morning with snow
levels quickly descending to sea level over the PacNW
Lowlands/Seattle/Portland. As seen in recent years, these cold core
lows often have little QPF in global guidance with much of the low
land snowfall coming in narrow/transient bands that are just now
starting to be realized in CAMs. As of now general snow totals from
Seattle through Portland are generally around 2", but given the
three-day cold spell, localized higher totals can be expected.

A strong baroclinic zone in the AR has been shunted south to far
northern CA. Mild temperatures in the AR moisture plume axis with
snow levels over the Sierra Nevada over 8000ft with heavy precip
that lingers into Sunday night. A narrow stripe of heavier/
prolonged snow occurs on the northern side of the baroclinic zone
where sufficient moisture meets colder air. Southern OR/ID through
western WY sees pivoting bands/swaths of moderate to heavy snow in
this stalled pattern tonight through Tuesday night. Day 1 snow
probs for >12" are fairly spread out from the WA Cascades, highest
OR Cascades and High Sierra (above pass level) along the Wasatch
and western WY ranges.

The northern side of the baroclinic zone becomes the focus for
western heavy snow late tonight. Day 2 snow probs for >12" is
30-50% over much of southern OR into far northern CA, the
Sawtooths, and all western WY ranges. Similar probs merely drift
north for Day 3 making for the risk for several feet of snow in
48hrs in these areas.

As the trough over the NW becomes more positively tilted, height
falls are directed south over CA Monday night with Day 3 snow probs
for >12" 40-80% over the Shasta/Siskiyou and northern/central Sierra Nevada.

Banding under the zonal jet Sunday night spreads east over the
Dakotas with intensity forecast to weaken as they cross MI/the
Great Lakes Monday, but that should continue to be monitored. Day
2.5 snow probs for >4" are 30-70% from southeast MT east over northern SD.


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

A progressive shortwave trough shifts east over the Northern
Plains today ahead of a diffluent left-exit region of a 130kt jet
streak to provide ascent and allow snow to develop later this
morning over MN on increasing 850-700mb WAA. Northern WI and the
MI Upper Peninsula see moderate snow this afternoon/evening with
locally enhanced lift on the North Shore of Lake Superior on
southeasterly flow. Light snow spreads over much of MI early Sunday
while the heavier snowfall over northern MN/northern WI tapers off
by Sunday afternoon. Day 1 WPC snow probabilities for >6" are
40-80% from greater Duluth through the North Shore with 30-60% for
>4" for the rest of the Arrowhead of MN and over most of the U.P.

The WAA continues to shift east over the Northeast Sunday afternoon
into Monday. Day 2 snow probs for >4" are 20-30% for the Tug Hill
and higher Greens/Whites.

The next wave strengthens a bit over the Northeast Monday night
with Day 3 snow probs for >4" 20-50% over the Adirondacks, and
northern VT/NH into northern Maine.


Jackson

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