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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2054Z Feb 25, 2024)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Sun Feb 25 2024

Valid 00Z Mon Feb 26 2024 - 00Z Thu Feb 29 2024


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

Back to back significant systems will bring widespread heavy snow
and dangerous travel to much of the West from the Pacific
Northwest through the Northern and Central Rockies, and across CA
and the Great Basin.

A potent shortwave digging out of the Gulf of Alaska will pivot
southeast along the British Columbia coast and then dig onshore WA
state Monday morning. This will produce increasingly confluent
mid-level flow downstream, driving impressive moisture onshore D1.
As this trough continues to dig southeast, it will interact with a
southern stream shortwave moving east into southern CA, especially
by D2, creating a full-latitude trough which will pivot east into
the Intermountain West Tuesday and then continue to advect to the
Central Plains Wednesday. It is unlikely that phasing of the
northern and southern streams will occur as guidance continues to
progress faster with the northern stream which leaves the CA
shortwave behind, but either way the confluent moisture streams
from each of these impulses will be acted upon by robust synoptic
ascent to drive widespread precipitation across the region.

At the same time, this 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 is 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.

WPC probabilities for more than 6 inches D1-D2 are extremely
impressive for a large portion of the West. Probabilities
exceeding 80% extend along the length of the Cascades and the
Olympics, eastward along the Northern Rockies including the Salmon
River/Sawtooth Ranges and the Blue Mountains in OR, into the
Absarokas and NW WY ranges, and then southward along the Wasatch,
the CO Rockies, and into the San Juans and Sangre de Cristos. 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.

As this first trough pivots into the Central Plains, brief
shortwave ridging will occur in its wake bringing an end to the
forcing and a brief respite to snowfall. However, by early D3, a
resumption of onshore flow will occur into the Pacific Northwest
as mid-level zonal flow gradually backs to the SW ahead of yet
another shortwave digging along the British Columbia coast. This
backing of the low to mid level flow will cause WAA and snow
levels steadily rising to around 3000 ft again. This should
prevent additional lowland/valley snow, but once again significant
accumulations are likely as reflected by WPC probabilities for
more than 6 inches once again reaching above 80% in the WA
Cascades and Northern Rockies, with locally an additional 1 foot
of snow likely in the higher elevations.


...Northern Plains to the northern 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 D2-3.

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 D3. 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 late D3. WPC
probabilities D2 for more than 4 inches reach 50-70% from eastern
ND into northern MN, and 70-90% across the U.P. of MI on D3.


Weiss

*** Key Messages for Significant Western U.S. Storm ***

--Strong winter storm and cold front
The strong cold front will move into the Northwest this evening,
progress southeastward on Monday into the Northern Rockies, and
then into the Central Rockies by Tuesday.

--Heavy mountain snow
Snowfall rates of 1-2"/hr are expected over the Cascades and
Northern Rockies through Monday, 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.

--Significant snow accumulations
More than 2 feet of snow is 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.

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

--Much colder air behind the strong cold front
Temperatures will fall into the teens and single digits by Tuesday
morning along and east of the Rockies.