Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EST Sat Feb 24 2024

Valid 00Z Sun Feb 25 2024 - 00Z Wed Feb 28 2024


...The West...
Days 1-3... 
An exceptionally unsettled pattern will develop on Sunday and
persist through the forecast period. Widespread heavy snow, gusty
winds, and even snow squalls are likely across much of the West.

Split flow 500mb pattern to start the period will gradually
transition into a full latitude trough encompassing the western
CONUS by Tuesday. The driver of this evolution will be a potent
shortwave trough and accompanying strong vorticity maxima which
will rotate down the British Columbia coast and then onshore WA
state Monday morning. This feature will then continue to deepen as
it advects eastward, forcing a potent but positively tilted
longwave trough across the Great Basin by Tuesday as it interacts
with a southern stream shortwave moving along the CA coast. By the
end of this period, this deep trough will be positioned NE to SW
from the Northern High Plains through the Desert Southwest,
producing widespread ascent through height falls/PVA, enhanced by
an intensifying subtropical jet streak south of this amplifying
trough.

The impressive synoptic lift will act upon an increasingly moist
environment as confluent mid-level flow and the jet streak aloft
surge Pacific moisture eastward noted by IVT exceeding +2 sigma in
two waves across the West, leading to above normal PWs gradually
dropping E/SE through the forecast period. At the surface, a
strong cold front will drive additional ascent, but more
importantly cause a rapid reduction in snow levels which will
start around 4000-5000 ft D1, to below 500 ft late D2 in the
Northwest, and then continue to fall into D3 across the Great
Basin and Central Rockies. This suggests that the heaviest
snowfall, which will be most impressive in the favored upslope
terrain, will be above 4000 ft, but significant accumulations are
likely at many of the area passes, and some light accumulations
are likely even into the lower elevations and valleys as snow
levels crash before precip wanes. ***WPC probabilities...***
Snowfall rates in many areas, but especially in the terrain, will
likely exceed 1"/hr at times, which when combined with strong
gusty winds will create dangerous travel across many areas.

WPC probabilities on D1 for more than 6 inches of snow are above
80% in the WA Cascades and Northern Rockies, generally above 3500
ft. These probabilities expand rapidly southeast D2 to encompass
much of the terrain from the Olympics eastward through the
Northern Rockies and southward into the northern Sierra, Salmon
Rivers, and ranges of NW WY. With snow levels falling late D2,
light accumulations may also begin to spread into the lower
elevations and valleys of the Intermountain West. By D3 the most
impressive overlap of ascent and moisture pivots east, shifting
the highest WPC probabilities for more than 6 inches into the
Wasatch, Big Horns, and much of the CO Rockies where they exceed
80% for 6 inches. During this period, much heavier snowfall is
likely in the higher terrain with 2-4 feet likely above 4000 ft in
the Cascades, and 1-2 feet in the other higher terrain.

Additionally, the potent cold front will cause an increasing
threat for convective snow showers and snow squalls in a linear
fashion along and just behind the boundary. The guidance continues
to look favorable for snow squalls as strong winds mix down along
the front, combining with SBCAPE of 100-200 J/kg and impressive
0-2km fgen, driving widespread high values of the SnSq parameter.
Where squalls occur, short-duration near white-out conditions are
likely despite the modest temporal duration of any squalls. There
is still uncertainty as to the most likely placement of any snow
squalls Monday and Tuesday, but the area from the interior Pacific
Northwest southeast through the Great Basin and into the Central
Rockies appears most favorable at this time. Extremely dangerous
travel is likely during any of these snow squalls.


...Northern Plains into Upper Midwest...
A lobe of vorticity demarcating the leading edge of a positively
tilted longwave trough will move east from Alberta/Montana and
shift into the Dakotas by Tuesday evening /late D3/. Impressive
height falls and modest downstream divergence ahead of this
feature will merge with increasing diffluence within the LFQ of a
potent subtropical jet streak arcing out of the Southwest to drive
deep layer ascent across the region. At the surface, this will
manifest as a potent cold front surging southeast, with a surface
wave moving eastward along it. This results in increasing 290K
moist isentropic ascent pushing greater moisture northward,
characterized by PW anomalies around +1 sigma according to NAEFS.
The column ahead of this front will be marginally cold for
snowfall, but it appears most of the precipitation will be wrung
out within post-frontal overrunning as the column cools rapidly
and flow backs more to the E/SE, resulting in a corridor of
moderate to heavy snow, especially for northern MN. There remains
considerable uncertainty in the timing of this front and
associated available moisture, but current WPC probabilities for
more than 4 inches of snow reach 40-50% from eastern ND through
northern MN.

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

~~~Key Messages for the Significant Western U.S. Storm~~~

--A strong winter storm and cold front will move into the
Northwest on Sunday and progress southeastward on Monday into the
Northern Rockies.

--Heavy mountain snow over the Cascades will impact the passes by
late Sunday, where there is >80% chance of at least a foot of snow
above 1500ft through early Tuesday.

--Snowfall will become heavy at times with rates of 1-2"/hr along
with windy conditions, creating areas of blowing and drifting snow
as well as significantly reduced visibility.

--Snow squalls are likely along the path of the cold front on
Monday over the northern Great Basin/Rockies, which could create a
rapid drop in visibility and a flash freeze on roadways leading to
dangerous travel.

--Much colder air behind the strong cold front will drop
temperatures into the teens and colder by Tuesday morning.

 
Weiss