Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Thu Jan 30 2025

Valid 12Z Thu Jan 30 2025 - 12Z Sun Feb 02 2025

...Northeast & Northern Mid-Atlantic...
Days 1-3...


Deep low pressure over eastern CO this morning will eject east to
the Middle Mississippi Valley tonight with copious Gulf moisture
available will have directed an exceptional IVT over the eastern
third of the U.S. with values topping 1,000 kg/m/s over the TN
Valley. Pockets of freezing rain are likely late tonight into
Friday over northeast PA and southern NY with Day 1.5 ice probs for
>0.1" in the 20-30% range. Farther north, boundary layer
temperatures will remain below freezing and allow for snow to be
the primary precipitation type Friday/Friday night from south of
Lake Ontario across the Adirondacks on east through the Green and
White Mountains and southern/eastern Maine.

As the primary low weakens Friday evening across NY, a new coastal
low will form east of the MA Capes Friday night. The low will be a
fast mover with periods of snow Friday night concluding by Saturday
morning. The EC remains more suppressed/quicker with the solution
resulting in less snow than the stronger/slower/snowier GFS. Day 2
WPC probabilities for >4" are 10-30% for lower elevations from
east of Buffalo and through eastern Maine and 30-60% in terrain of
the Adirondacks, Green and White Mountains. However, should banding
develop on the northern end of the precip shield, the motion would
be along the west-east orientation and lead to localized heavy
snow of several inches. Hopefully the variation in track and
magnitude improves in the next model suite.


...Southern Rockies and Central High Plains...
Day 1...

Mid-level is now over the CO High Plains where it will continue to
develop through this morning. Strong ascent around and under this
low will persist into the afternoon with moisture aiding snow,
heavy at times, for the southern Front Range and Sangre de Cristos
and Raton Mesa. Banding extends northeast from the Palmer Divide
with snow accum to the eastern CO border. Day 1 snow probs for >4"
after 12Z are 40-70% for these areas. Snow should conclude
this afternoon as the low tracks east.


...Pacific and Interior Northwest...
Days 1-3...

An atmospheric river (AR) crosses the WA/OR coasts tonight as an
approaching trough gets drawn into a deep low drifting south from
the southern AK coast. IVT within this AR ranges between 300-500
kg/m/s Friday morning, which is as high as the 99th climatological
percentile per NAEFS. WAA increases with the AR, causing snow
levels to rise tonight through Friday morning to 4000-5000ft for
the Cascades and west. However, height falls from both the trough
crossing and the approaching cold-core low causes snow levels to
plummet Friday night through Saturday. Snow levels reach sea level
in western WA on Saturday with a strong baroclinic zone over OR
and into the northern Rockies. Upslope flow into the
Cascades/Olympics will continue through Friday night before
diminishing to more moderate rates under the colder trough Saturday
night. Heavy snow persists through Saturday for the northern
Rockies. Farther south, the westerly IVT will continue to pump
copious Pacific moisture into the West with the northern California
ranges seeing the heaviest precipitation Saturday and continuing
into next week in a prolonged onshore flow. Snow levels will be
notably higher on that side of the baroclinic zone with snow levels
>7,000ft in the Sierra Nevada, and >5,000ft in the Trinity/Shasta.

Day 1.5 WPC snow probabilities for >6" are high (60-90%) in the
WA Cascades and 50-80% in the OR/CA Cascades and northern
Bitterroots. These probabilities greatly expand Friday night with
Day 2.5 snow probs for >6" high (50-90% for the length of the
Cascades/High Sierra, Sawtooths, Bitterroots, and Absarokas/Wind
River around and south of Yellowstone.
There is concern for ice accretion at lower elevations of eastern
WA and northern ID, and western MT where surface temps have rarely
gotten above freezing in recent days. Should snow transition over
to a sleet/rain mix, ground conditions are so cold that freezing
rain would occur on these surfaces even with air temperatures above
freezing. Expect wintry conditions to linger through Monday as the
IVT to the south persists and a more frigid air- mass inches its
way south from southwest Canada, resulting in more mountain snow
and possibly icy conditions in some valleys of the Pacific
Northwest and Intermountain West.

Jackson