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Probabilistic Heavy Snow and Icing Discussion...UPDATED
NWS Weather Prediction Center College Park MD
405 PM EST Thu Dec 25 2025

Valid 00Z Fri Dec 26 2025 - 00Z Mon Dec 29 2025

*** Continued Heavy Snow on the Sierra Nevada through Friday ***

*** Heavy Wintry Mix tonight through Friday night from the Great
Lakes through the Northeast and Mid-Atlantic ***

...Great Lakes to Northeast... Days 1-2...
Maximum impact level from WSSI: Moderate

Quasi-zonal flow continues to direct a plume of moisture
originating from the West along the US/Canadian border
leading to WAA- driven snow over the MN Arrowhead into the U.P. of
Michigan. With cold sub-32F sfc temps over parts of MN/WI/Lower MI
at precip onset tonight, some freezing rain is likely with minor
accumulations over MN/WI. WPC probabilities for at least 4 inches
of snow are low to moderate (20-60%) over the Keweenaw Peninsula
into the eastern U.P.

Into D2, shortwave will push through northern Lower MI with its
sfc reflection over IN/OH and the 850mb low moving through BUF. The
sfc low will weaken as it approaches the Appalachians, getting too
far behind the mid-level shortwave, and ultimately jumping into
the Atlantic by 12Z Sat. From Friday to Saturday morning, a variety
of ptypes are expected over the region (central Appalachians/Mid-
Atlantic to the Northeast). The stubborn cold surface air will hold
tight over Lower MI and especially into the Laurel Highlands in
western PA where significant freezing rain is possible. WPC
probabilities of at least 0.10" icing are high (>70%) from central
to southeast MI, including Detroit, as well as into western
PA/western MD. 40 to 70% probabilities of at least 0.25" icing
exist over southeast MI down through northwest PA into the Laurel
Highlands. Light icing is likely on either side of this zone of
maximum probabilities -- on the south side along the I-66 corridor
(DC) and into eastern WV. To the north/northeast, deeper cold air
at the surface will allow for sleet to be a dominant ptype over
central/eastern PA with up to an inch of sleet possible.

To the north, from central NYS through northeast PA into the
Catskills/Poconos and to the NYC Metro area, an axis of moderate to
locally heavy snow is increasingly likely, driven by the stronger
height falls at the nose of the incoming 130kt jet. Beginning to
see numerical guidance fall in line on the axis of greatest
snowfall potential with the NAM shifting further south compared to
its placement at 00z. Rest of the dynamical guidance is keying on
the area from southern NY state down through the Lower Hudson to
NYC and 50-75 miles on either side seeing at least 4" with >6"
becoming a greater likelihood around the Catskills into the Lower
Hudson Valley to the north of NYC.

WPC probabilities for at least 4 inches of snow are still >50% in
a zone bounded between I-890/Rt 17/I-476 to the west, the southern
Adirondacks on the north side, Berkshires/I-91 to the east, and
into the NYC Metro/Long Island to the south. Probabilities for at
least 6" are now between 50-70% for that area of central NY state
down through southern NY, including the Catskills/Lower Hudson/NYC
corridor south and east of I-88. Though the system will be
progressive, combination of higher SLRs and upslope could yield
more than 8 inches of snow into the southern Catskills where areal
average maxima is currently positioned with some deterministic
indicating >10" on their outputs.

Fracasso/Kleebauer

...California/Sierra Nevada... Day 1...
Maximum impact level from WSSI: Extreme

*** Heavy Snow Continues on the Sierra Nevada through Tonight ***

The final round of widespread heavy snow arrives early this evening
to the northern Sierra Nevada then works its way down the length
of the Sierra Nevada through the overnight. Expect a few hours of
2-3"/hr rates (per the 12Z HREF) above the snow level of 5000ft
(north/central Sierra) to 6000ft (southern Sierra) with an
additional 12-18" overnight.

Moderate snow showers continue into Friday evening with snow
levels dropping to 4500ft. The upper trough which had been offshore
finally shifts east to the Great Basin Friday night, cutting off
Sierra snow by 12Z Saturday.

Far northern CA ranges (Shasta Siskiyou/Klamath/etc) can expect
two more rounds of heavy snow: this evening above 4000ft and early
Friday down to 3500ft. Snow impacts to I-5 near Mt Shasta are
likely Friday. Some moderate snow linger Friday night before
ending by 12Z Saturday.

For SoCal, snow levels remain around 7000ft through Friday morning
and most of the additional precip. Snow levels do drop below
6000ft Friday afternoon with some 1"/hr rate potential around Big
Bear Lake into the evening before tapering off.

Jackson

...Pacific Northwest, Great Basin and Rockies... Days 1-3...
Maximum impact level from WSSI: Moderate to Major

Deep low pressure off CA will continue to infuse moisture across
the West with a separate trough pushing down the BC coast Friday
and over the northern Rockies Saturday providing a final focus for precipitation and ending the active pattern.

Ongoing snow over the Cascades tapers off this evening with
moderate to locally heavy snow for the length of the Cascades
Friday and Friday night with snow levels dropping on the WA
Cascades from 3000ft to 1000ft and over the OR Cascades to 2000ft.
Day 1.5 snow probs for >8" are 50-90% on the WA/OR Cascades
including for the Snoqualmie and Santiam passes.

The next round of moisture from CA arrives into the northern
Rockies this evening with prolonged moderate to heavy snow until
the cold frontal passage Friday night. Snow levels drop from
6000/7000ft to 3000/4000ft through this time leading to increasing
travel impacts. Day 1.5 snow probs for >8" are 50-80% over the
Sawtooths, northern ID/western MT ranges, western WY ranges, the
higher central NV ranges, and the Wasatch.

The progression of the cold front is notable with the Day 2.5 snow
probs for >8" limited to the Wasatch and 50-80% for northern and
western CO Ranges.

Jackson

...Winter Storm Key Messages are in effect for California (1) and
the Great Lakes/Northeast (2) as linked below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

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