FOUS11 KWBC 260823
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Mon Feb 26 2024

Valid 12Z Mon Feb 26 2024 - 12Z Thu Feb 29 2024

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

A series of winter storms will bring significant snowfall and
dangerous travel to much of the West over the next few days,
including the Pacific Northwest through the Northern and Central
Rockies, across CA, and the Great Basin.

The initial shortwave energy currently moving into northwest WA
will continue to advance southeast/east today and tonight with its
associated cold front sweeping the region through late Tuesday
while the trough axis elongates and takes on a neutral tilt by the
time it reaches the Rockies and Plains. The northern stream
impulse will push an impressive arctic cold front southward, which
will have the two-pronged effect of driving low-level convergence
for strong ascent, while also causing snow levels to crash rapidly
from around 3500 ft early, to less than 500 ft in its wake. This
will cause increasingly more significant impacts to travel as snow
spreads into the lowlands and valleys through much of the West,
with significant travel problems likely at most of the area
mountain passes from the Cascades through the Central Rockies.
Additionally, strong fgen, elevated SBCAPE, and gusty winds will
likely produce a linear feature, or features, of snow squalls
moving along or just behind it. While additional accumulations
within these squalls are likely to be modest, brief intense
snowfall rates of 1-2"/hr combined with strong winds could produce
near zero visibility at times. More general snowfall rates of
1-2"/hr is also expected in the terrain, most likely in favored
upslope regions.

For the Day 1 period, a large potion of the Pacific Northwest
mountains and Mountain West have high (>80%) probabilities of at
least 6 inches, extending across the Cascades/Olympics eastward
through the Northern Rockies including the Salmon River/Sawtooth
Ranges and the Blue Mountains in Oregon. And extending into the
Absarokas, northwest WY ranges and then through the Wasatch, CO
Rockies and finally the San Juans and Sangre de Cristos. For the
12" threshold, the Oregon Cascades, northwest WY ranges, and the
CO Rockies have the greatest (above 80%) probabilities. With long
duration of heavy snow rates of 1-2"/hr in many of these areas,
snowfall totals in the higher terrain reaching 2 feet across the
Intermountain West ranges, and as much as 5 feet in the Cascades.

By Day 3 (Wednesday/Wednesday night), another potent storm system
is set to begin impacting the Pacific Northwest where a large/deep
trough settles along the coastal region with a piece of shortwave
energy moving onshore. While ahead of the approaching front snow
levels will rise steadily, the sharp cold front pushing through at
the end of the period (early Thursday morning) will send snow
levels down to 1500-2000 ft across the Olympics and northern WA
Cascades. This system will have plenty of moisture onshore and
favorable forcing for ascent to produce widespread precipitation
across the region. As a result, the latest WPC snow probabilities
are already high (>80%) for the WA Olympics and Cascades as well
as the far northern reaches of the Rockies in northern
ID/northwest MT. The northern Cascades also already have high

80%) probabilities of at least 18 inches for Day 3.

...Northern Plains to the Great Lakes... Days 2-3...

A fast moving but amplifying shortwave will race eastward from the
Northern Rockies on Tuesday, reaching the Northern Plains Tuesday
night and then becoming more neutrally tilted as it crosses the
Great Lakes during Wednesday. Sharp but transient height falls and
impressive PVA will result in dual waves of low pressure moving
along the arctic cold front which will also traverse southeast
beneath this impulse. During this evolution, a potent subtropical
jet streak will intensify south of the amplifying trough, reaching
more than 150 kts over the Central Plains Wednesday, and get
pulled poleward to produce enhanced LFQ diffluence atop the
greatest height falls. This favorable overlap will enhance the
surface low pressure falls, and it is likely that despite the fast
overall motion, intense ascent will spread across the region late
in the Day 1 period through Day 2 (early Tuesday through Tuesday
night).

This deep layer lift will act upon increasing moisture as PWs
surge northeastward downstream of this trough and within
intensifying moist isentropic ascent in the 290-295K layer. The
dual waves will somewhat offset the meridional extension of the
highest moisture, but there is some potential for a better surge
of theta-e air and a modest TROWAL pivoting into the northern
Great Lakes. Although the antecedent airmass is marginally cold
enough for heavy snow, the isentropic ascent atop the front
combined with the rapidly cooling airmass indicates much of the
precip will be anafrontal, which will overlap with the colder air,
a deepening DGZ, and periodic impressive fgen to drive heavy snow
rates. This suggests a fast moving swath of heavy snow will move
eastward through the Northern Plains and then Great Lakes,
followed by LES downstream of Lake Superior. Due to the tightening
pressure gradient and strengthening winds, near-blizzard
conditions will possible across far eastern North Dakota and
northwest Minnesota with the combination of strong winds and
snowfall creating near zero visibilities at times. The latest
Winter Storm Severity Index is highlighting Moderate level
impacts, driven mostly by the blowing snow and ground blizzard
potential.

WPC probabilities for at least 4 inches are now mostly confined to
northern MN and peak between 40-60 percent. Once the secondary low
pressure rapidly intensifies over the Great Lakes, another band of
heavy snow will be possible on its backside, potentially clipping
parts of northern WI, the U.P. of Michigan, and northern L.P. of
Michigan. Here, the 4" probabilities are lower, generally in the
20-30 percent range. By the end of Day 3, the sweeping front will
have progressed through the East Coast and with it, a rapidly
colder airmass takes over. This will setup a favorable lake effect
snow regime, particularly downwind in the favored areas off Lake
Superior and potentially Lake Erie and Lake Ontario. In the U.P.
of Michigan, 4" probabilities for Day 3 are up to 50 percent while
in western NY, peak between 30 and 50 percent.

Weiss/Taylor


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

**A strong cold front will continue to progress through the
region, reaching the Northern Rockies today and the Central
Rockies Tuesday.

**Snowfall rates of 1-2../hr are expected over the Oregon
Cascades and Northern Rockies today, before spreading into the
Great Basin and Central Rockies Tuesday. These snow rates combined
with winds gusting 50-65 mph will produce near-blizzard conditions
with significantly reduced visibility and snow-covered roads
leading to dangerous travel.

**Snow totals greater than 2 feet are expected (>80% chance) in
the Cascades through Tuesday, with locally as much as 4 feet
possible in the highest terrain. Elsewhere across the
Intermountain West, there is a high chance (>70%) of more than 1
foot of snow in the higher elevations. Lowering snow levels will
also produce some accumulations onto the valley floors.

**Snow squalls are expected to develop along the path of the cold
front today and Tuesday. Where snow squalls occur, intense snow
rates will produce rapid drops in visibility and a flash freeze,
resulting in dangerous travel.

**Temperatures will fall into the teens and single digits by
Tuesday morning along and east of the Rockies.


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