Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 PM EST Wed Jan 24 2024

Valid 00Z Thu Jan 25 2024 - 00Z Sun Jan 28 2024

...Western U.S....
Days 1-2...

An upper level trough slamming into the West Coast is responsible
for infusing Pacific moisture into the western half of the U.S.
today and into the second half of the work-week. A steady diet of
850-700mb moisture being channeled into the Pacific Northwest
keeps heavy snow in the forecast for elevations >4,000ft in the
Olympics and Cascades. WPC PWPF tops out with high chances (>70%)
for snowfall totals >8" through Thursday in these ranges. The
upper trough will shear-off into two distinct features: one will
track north and east through the northern Rockies tonight and into
the Canadian Prairies by Thursday while the other dives south and
east into the Four Corners states Thursday afternoon. The latter
will escort the residual plume of Pacific moisture into the
Intermountain West and eventually into the southern Rockies both
Thursday and into Friday. Due to the upper trough's steady
porogression, lack of deep moisture aloft, and marginal boundary
layer temps, most of the potentially heavy snowfall will be
confined to elevations >8,000ft. WPC PWPF sports moderate-to-high
chances (40-70%) for snowfall >6" in the Wasatch and San Juans of
Colorado on Thursday. As the upper low tracks through the Four
Corners region Thursday night, the upper trough will deepen and
eventually spawn a 500-700mb low over northeast New Mexico by
early Friday morning. As the 700mb low strengthens, 850-700mb
northeasterly flow will advect moisture from south Texas into the
front range of the Sangre De Cristo in southern Colorado and
northern New Mexico. Latest WPC PWPF shows low chances (20-40%)
for snowfall totals >6" over Raton Mesa along I-25 at the moment,
but this upper level evolution can foster heavy snowfall amounts
should the upper low tracks slowly east on Friday. Portions of
northeast New Mexico and southwest Colorado should keep an eye on
forecast updates throughout the next day or so given the lingering
uncertainty in snowfall amounts.

Kansas could also see some wintry impacts, but the marginal
thermals and high uncertainty in storm evolution make for a low
confidence forecast. WPC PWPF shows 10-30% chances for >0.1" of
ice accumulation in west central Kansas on Saturday at the moment,
but this could just as easy become more geared to snow or rain
depending upon storm track and the strength of the upper low.

...Northeast...
Days 1-2...

A sliver of 500mb vorticity and a strengthening 500mb jet streak
will place northern New England in a favorable position for strong
vertical ascent atop the atmosphere. This coincides with a surge
in 850-500mb moisture overrunning a weak frontal boundary that
also sports a burgeoning warm nose of >0C temps between 850-750mb
according to sampled area soundings along the ME/NH border. As a
result, freezing rain will be the primary precipitation type, thus
making ice accumulations the most impactful hazard tonight into
Thursday morning. WPC PWPF shows moderate-to-high chances (50-80%)
for ice accumulations >0.1", but less than <5% for amounts >0.25".
This suggests the primary impacts are likely to be confined to
travel with icy roads and walkways that are untreated. The
Probabilistic WSSI (WSSI-P) shows as moderate chances (40-60%) for
Minor Impacts from east-central Vermont and central New Hampshire
to the Kennebec Valley of central Maine. Light snow is possible
across northern Maine but totals should generally be <2". Drier
conditions will return by mid-morning and remain dry through
Thursday evening.

Around midday Thursday, a dying cold front will bring about a
brief injection of cold Canadian air that becomes anchored yet
again by high pressure over Quebec. Meanwhile,  a potent upper
level shortwave trough over the Middle Mississippi Valley will
escort an IVT over the Northeast topping the 90th climatological
percentile according to NAEFS. Strong 850-700mb warm air advection
and 290K isentropic glide over the Northeast will give rise to
precipitation once again overrunning the sub-freezing boundary
layer will result in freezing rain over northern New Hampshire and
in the Kennebec Valley of Maine on Friday. WPC PWPF shows moderate
chances (40-60%) for ice accumulations >0.1" with the White
Mountains more likely to see those >0.1" amounts. Farther north,
with a colder air-mass more firmly in place thanks to a weak area
of high pressure to the north, this will allow for heavier
snowfall rates to transpire across northern Maine. Given the
strong divergence aloft and strengthening FGEN, hourly snowfall
rates of 1"/hr would be possible. The WSSI-P shows moderate
chances (40-60%) for Minor Impacts across northern Maine with WPC
PWPF depicting similar probabilities for >4" of snow on Friday.
Hazardous travel conditions are possible across northern Maine on
Friday. Snow will taper off Friday evening with high pressure
building in overhead for Saturday.

Mullinax