Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Wed Jan 31 2024

Valid 00Z Thu Feb 01 2024 - 00Z Sun Feb 04 2024

...The West...
Days 1-3...
Moderate to heavy precipitation, fueled by an atmospheric river
channeled by strong southerly flow preceding a deep upper low,
will continue to spread inland across Northern California this
evening.  The ongoing south-north moisture transport will
transition to a southwest-northeast orientation as a strong
subtropical Pacific jet/mid level shortwave nears the Southern
California coast this evening.  This will help split the system
into two parts -- with the northern section remaining tied to the
upper low that is forecast to linger along the Northwest coast
through the period, while a negatively-titled shortwave,
accompanied the bulk of the deeper moisture, moves across the
Southwest into the southern Rockies.   

The Sierra Nevada are most likely to face the heaviest amounts
through Day 1 (ending 00Z Fri).  While mild Pacific air and
moisture will support high snow levels at the onset, snow levels
are forecast to drop overnight into Thursday with the advancing
cold front.  With IVT values reaching around 400 kg/m-s into the
Sierra foothills, heavy snow (2-4"/hr) will fall over higher
elevations initially, but then lower to around 5000-6000ft.
Multiple feet of snow are likely in the Sierra above 7000-8000ft,
with WPC probabilities for at least 4 inches around 50% for areas
as low as about 5000ft. With snow levels lowering through Southern
California, the higher terrain east of L.A. will see accumulating
snow as well.  While the heaviest amounts are forecast on Day 1,
persistent onshore flow will support additional snow showers as
snow levels continue to drop through Friday.  This is expected to
produce several more inches of snow along the northern and central
Sierra.  In the northwestern California coastal ranges, the threat
for locally snow will extend closer to the coast as snow levels
dip below 3000ft on Friday. 

Meanwhile east of the Sierra, southwest flow ahead of the
advancing southern stream trough will favor the southwest-facing
terrain in southern/central Nevada, southern Utah, and across
Arizona (Mogollon Rim to the White Mountains) then into the San
Juans late Thursday into early Friday. For many of these areas,
storm total amounts exceeding 8 inches is expected in the higher
terrain.

Heavier snow will continue through Saturday, with the focus
shifting further north along the central Rockies as a mid-to-upper
level center develops and begins to lift north.  Model spread and
forecast uncertainty increases late in the period.  With impacts
on where the better upslope flow and heavier QPF will develop over
the High Plains late in the period, the GFS is displaced further
to the north than most of the other guidance.  Most guidance
however offers a good signal for locally heavy amounts developing
over the north-central Colorado ranges, with the WPC PWPF
indicating high probabilities for accumulations of 8 inches or
more across much of the area on Day 3.

Back to the north, even though the deeper moisture will be
directed to the south, moisture will be sufficient enough to
produce some locally heavy snow accumulations across portions of
northern Intermountain West and Rockies as it interacts with the
northern stream trough.  WPC PWPF continues to show high
probabilities for accumulations of 8 inches or more across the
Blue Mountains in northwestern Oregon and across the central Idaho
ranges.  


...Northern Maine...
Days 1-2...

A Canadian cold front and a wave of low pressure developing along
the boundary will push southeastward out of Quebec and into the
Northeast late Thursday.  Light snow is expected to spread across
portions of northern New York and New England ahead of the
approaching front on Thursday, continuing into the evening as the
system progresses across the region.  Suggesting the better
forcing and heavier QPF will focus mostly to the north,
probabilities for snow accumulations exceeding 4 inches have
receded further north across northern Maine in the latest run,
confining the higher probabilities to far northern Aroostook
County.

The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.

Pereira