FOUS11 KWBC 262021
QPFHSD

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

Valid 00Z Tue Feb 27 2024 - 00Z Fri Mar 01 2024

...Northern and Central Rockies... Day 1...

A potent shortwave will dig out of the Pacific Northwest this
evening and then slide towards the Northern Plains by Wednesday
morning. Although the sharpness of this feature is likely to
weaken with time, it will become embedded within a larger
wavelength, full latitude trough across as it sheds to the east.
This will result in expansive deep layer ascent through height
falls and PVA, with the resultant strengthening of the subtropical
jet streak (reaching 190 kts) downstream driving additionally
enhanced synoptic lift. This shortwave will drive an arctic cold
front southeast through the Rockies and into the High Plains by
Wednesday, which will result in locally enhanced ascent through
surface convergence, while also leading to increasing upslope flow
into the terrain on the post-frontal wind shift.

Although this northern stream trough will outrun a southern stream
impulse, this secondary feature will play an important role in the precipitation as well. Southwest flow downstream of this secondary
feature will advect deeper moisture northeastward into the Desert
Southwest and Central Rockies, with PW anomalies according to
NAEFS exceeding +2 sigma in some areas driven by IVT of more than
250 kg/ms. This moisture will align ahead of the front to be wrung
out effectively by the approaching lift, causing widespread
precipitation from the Northern Rockies, through the eastern Great
Basin, and most impressively into the Central Rockies. Although
much drier air will follow the front, an extended duration of
heavy snowfall is likely in these areas before drying occurs late
D1 into D2. Snow levels ahead of the front are forecast to be
3000-5000 ft, but will fall extremely quickly behind the front to
below 500 ft. While this indicates the heaviest snow will occur
above 3000 ft which will experience the longest duration of snow,
before precip wanes entirely even the lower valleys could see
modest accumulations. This is reflected by WPC probabilities for
6+ inches reaching above 80% from the Absarokas and NW WY ranges,
eastward through the Big Horns, and south into the Wasatch, CO
Rockies, San Juans, and Sangre de Cristos. 1-2 feet of snow is
likely in the higher terrain, and the lowering snow levels will
also allow for some light accumulations down into the valleys
across much of the region before snow winds down into D2.

Additionally, the strong cold front moving southeast will create a
favorable environment for linear snow squalls along and just
behind the front. The overlap of 100-200 J/kg of SBCAPE with
impressive 0-2km fgen and strong winds drives a high SnSq
parameter across the Rockies/Great Basin with the frontal passes,
resulting in sow squalls that will likely have snowfall rates of
1-2"/hr and strong winds to produce periods of near-zero
visibility. Although additional snow accumulations within any
squalls will likely be light, the rapid changes in conditions and
resultant icy roads will create dangerous travel through Tuesday.

...Pacific Coast into the Northern Rockies... Days 1-3...

In the immediate wake of the lead shortwave moving across the
Intermountain West on D1, brief shortwave ridging will bulge
across the Pacific Northwest before rapidly transitioning to zonal
flow from the Pacific once again. The cause of this rapid change
is yet another shortwave digging out of the Gulf of Alaska and
traversing along or just offshore the British Columbia coast,
resulting in downstream backing of the flow from W to SW by
Thursday morning. The 500mb pattern will additionally respond by
becoming increasingly pinched downstream of this shortwave and
accompanying strung out vorticity lobes, driving more intense
moist advection onshore, especially D2 and D3 from WA through
central CA. This mid-level confluence will be directly topped by
an upper level jet streak reaching 110 kts and arcing from SW to
NE, enhancing onshore moisture advection while also providing
additional ascent through LFQ diffluence.

This moisture advection will manifest as an impressive atmospheric
river (AR), with both the GEFS and ECENS ensembles indicating a
greater than 70% chance of 500 kg/ms IVT shifting onshore late D2
into D3. Although the core of this IVT is expected to impact
coastal OR and northern CA, the accompanying PW plume is progged
to reach as far inland as the Northern Rockies and Great Basin as
reflected by NAEFS IVT anomalies of +2 and PW anomalies nearing +1
well inland. The overlap of the impressive synoptic lift and this
increasing moisture will result in yet another round of heavy
precipitation spreading across the West, initially in the Pacific
Northwest late D1/early D2 and then expanding south and east as
far as the Sierra to the south and Absarokas to the east as a
surface cold front sags southeast with time beneath the mid-level
trough.

Snow levels during precipitation onset will be much higher than
with the previous event, likely starting around 4000-5000 ft,
possibly as high as 6000 ft in the Sierra. The heaviest
precipitation is expected during these higher snow levels due to
the accompanying WAA which will keep these snow levels elevated
while driving the more intense ascent for expanding precipitation.
However, the cold front sagging southeast will result in a slow
decline in snow levels, falling to around 1500 ft in the Cascades
by the end of D3, but still remaining elevated above 5000 ft in
the Sierra at that time. Impactful snow is still possible as these
snow levels crash, but regional soundings and ensemble clusters
show decaying column moisture during this time, so lower-elevation
snowfall is likely to be much less than at higher elevations. This
is reflected by WPC probabilities for more than 6 inches of snow
reaching above 80% in the Olympics, WA Cascades, OR Cascades, and
Northern Rockies D2, and then spreading rapidly southeast during
D3 into the Shasta/Trinity/Siskiyou region and the Sierra, as well
as into central ID and NW WY terrain. D3 snowfall will continue
across the Cascades as well, and by D3, storm total snowfall above
3000 ft will likely exceed 4 feet in the higher elevations of the
Cascades, and 1-2 ft elsewhere. Heavy snow rates of 1-2"/hr
combined with strong winds will result in major impacts to travel
across these areas as well.

Although beyond this current forecast period, additional heavy
snow is likely to continue across this area through D4 and D5,
finally waning later in the upcoming weekend.

...Northern Plains to the Great Lakes... Days 1-3...

A multi-stream shortwave will move out of the northern High Plains
early Tuesday, with a northern vort max along the ND/Canadian
border and a southern one moving eastward out of SD into the Corn
Belt. These two will coalesce into a negatively-tilted trough over
the Great Lakes that will continue to strengthen into Ontario and
then Quebec as an upper low by early Thursday. Aloft, the northern
stream jet will dip into the lifting southern stream jet across
the Southwest early Tue, with broad upper divergence in the LFQ of
the 180kt jet favoring surface cyclogenesis. This will take low
pressure from SD across MN into MI by Tue evening/overnight.
Despite the lack of high pressure over Canada out ahead of the
system and only marginal temperatures today, increased northerly
flow around the deepening cyclone will draw in colder air around
the system, favoring all or nearly all snow from ND into northern
MN. The best moisture flux will be into the Great Lakes with
strong W to WSW flow at 850mb across the central Plains (~50kts)
that curves northward, but dynamics support at least some higher
theta-e air wrapping back into the low via a TROWAL (albeit,
briefly) as the system maintains a progressive pace. Combination
of favorable FGEN, lift into the DGZ, and timing (overnight into
early morning hours) could support a narrow heavier axis of
snowfall within a broader swath of light to modest snowfall across
eastern ND into northern MN. In addition, due to the tightening pressure gradient and strengthening winds, near-blizzard
conditions will be possible across far eastern North Dakota and
northwestern Minnesota with the combination of strong winds and
snowfall creating near-zero visibility 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 of snow are
moderate (40-70%) from eastern ND into northern MN, and high

70%) over northern MN around International Falls. Within this

area, WPC probabilities of much higher snowfall (>12 inches) is
low (10-20%), suggesting the potential for enhanced banding within
this broader zone. 12Z HREF shows probs of >1"/hr rates up to 70%
around 16Z Tue over northern MN as well.

As the low wraps up into Quebec by D3, the strong cold front will
bring in much colder air to the Great Lakes region, favoring some
lake effect snow over especially the Michigan U.P. starting
Wednesday on northerly flow then into the eastern Great Lakes
(western NY) as the flow backs to NW by early Thursday. Heights
will be fairly quick to rise from NW to SE across the Great Lakes,
so the LES may be short-lived and largely end by the end of this
period (00Z Thu). WPC probabilities for at least 4 inches of snow
on D3 are highest (30-50%) over central NYS around SYR on WNW flow
and also into the Chautauqua ridge. In addition, with colder air
helping to change rain to snow, WPC probabilities for at least 4
inches are low (10-30%) over northern New England with wrap around
snow and also into the central Appalachians via upslope


Weiss/Fracasso


Key Messages for the Intermountain West Winter Storm

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

--Near-blizzard conditions and dangerous travel
Snowfall rates of 1-2"/hr will move into the Great Basin and
Central Rockies on 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
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 along cold front passage
Snow squalls are likely along the path of the cold front from
Montana to Idaho today and Wyoming to Utah and Nevada on 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 throughout the Intermountain West.

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