Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
125 PM EST Fri Feb 23 2024

Valid 00Z Sat Feb 24 2024 - 00Z Tue Feb 27 2024

...The West...
Days 1-3...

Complex mid-level pattern days 1-2 will evolve into an amplifying
trough across the West by Sunday, resulting in an increasingly
active period of winter weather within the region.

Split flow tonight will manifest as mid-level NW winds within a
pinched gradient south of a trough over Alberta moving across the
Pacific Northwest, with SW flow expanding across the Desert
Southwest downstream of a closed low over the Pacific. A weak col
will exist in between resulting in weak shear and minimal across
across much of the West. This pattern will persist through
Saturday night before amplification begins in response to a potent
shortwave and accompanying vorticity maxima diving along the
British Columbia coast and moving onshore WA/OR Monday. This will
result in a deepening longwave trough across much of the West,
with downstream divergence and height falls producing robust
synoptic ascent to support expanding precipitation.

As this lift intensifies and expands, it will interact with
increasing moisture across the region. Two waves of modest IVT are
progged to surge into the West according to CW3E probabilities,
reaching above 250 kg/ms in the northern tier (above +1 sigma
according to NAEFS) and approaching 500 kg/ms late in the period
in the Great Basin and Desert Southwest. This enhanced moisture
will be acted upon by the deep layer ascent to produce expanding
precipitation from NW to SE, aided by a potent surface cold front
which will dig across the Pacific Northwest, reaching the Great
Basin by Monday evening.

This cold front will not only produce additional lift through
low-level convergence, but will also cause a rapid drop in snow
levels from generally 3500-5000 ft late D2, to below 500 ft in the
WA/ID/MT, and around 2000 ft just behind the front. The
combination of lowering snow levels with robust synoptic lift
(aided by periodic upslope flow) and persistent moisture advection
will result in heavy snow spreading across much of the terrain of
the west by D3, with snow possibly filling into the lowlands and
valleys as well. WPC probabilities for more than 6 inches of snow
D1 and D2 are above 70%, but confined to the higher terrain of the
WA Cascades and Northern Rockies. By D3 the footprint of high
(>70%) probabilities for more than 6 inches expands along the OR
Cascades, into much of the ID terrain including the Salmon River
and Sawtooth Ranges, and into parts of NW WY near the Grand
Tetons. Local snowfall during the 3 days will likely exceed 4 feet
in the highest terrain, and difficult to dangerous travel is
likely at many of the mountain passes by Monday.

Finally, there is some potential as well for snow squalls along
the cold front as it drops rapidly southeast D3. Along this front,
the environment appears favorable for convective snow showers and
possible snow squalls as reflected by elevated low-level RH
aligned with impressive 0-2km fgen and SBCAPE exceeding 100 J/kg
resulting in a high SnSq parameter. It is a bit too early to
discern where the greatest threat for snow squalls exists, but the
threat for snow squalls with briefly intense snowfall rates and
gusty winds leading to difficult travel is increasing for parts of
the Northwest, Great Basin, and Central Rockies.


...Ohio Valley...
A compact but potent shortwave will dig out of the Great Lakes
tonight into Saturday, resulting in a fast moving surface low
moving across the Ohio Valley. The antecedent thermal structure is
marginal for heavy snow, but impressive ascent combined with cold
advection behind this wave will result in a changeover from rain
to heavy snow for parts of IN/OH/WV. Snowfall rates may reach
1"/hr at times, which should overcome the warm ground to result in
rapid accumulation, but the fast movement and limited spatial
extent of this ascent will limit snowfall totals. WPC
probabilities have increased, and now feature a 10-20% chance for
more than 4 inches of snow in a few areas.


The probability of ice accumulations of 0.10 inch or more is less
than 10 percent.

Weiss