Clifton, Virginia Weather

395
FOUS11 KWBC 140655
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Mar 14 2025

Valid 12Z Fri Mar 14 2025 - 12Z Mon Mar 17 2025

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

The active pattern continues across the West with several days of
widespread wintry precipitation impacting much of the area through
the upcoming weekend.

The period begins with a potent closed mid-level low moving across
the Southern High Plains of CO/NM. This feature is quite intense,
reflected by NAEFS 850-500mb height percentiles that are below the
minimum in the CFSR climatology across a large, and expanding,
section of the country centered along the High Plains. As this
feature shifts E/NE this afternoon, it will continue to deepen and
produce near-blizzard conditions across the Northern Plains (more
on that in the section below). Behind this departing system, heavy
snow will continue across the Southern and Central Rockies through
D1 as a combination of mid-level deformation and a trailing cold
front overlap to enhance ascent in a region of still elevated 700mb
RH across the Four Corners and into Wyoming. While this snow
should wane quickly by 00Z Saturday, and snow levels will generally
be in the 5000-6000 ft range, snow is expected to accumulate
several additional inches D1 as reflected by WPC probabilities that
are high (>70%) for 4+ inches across the CO Rockies, the northern
Sangre de Cristos, and as far north as the Big Horn Mountains of
WY.

Any break in precipitation behind this significant, but departing,
system will be short lived as the active pattern sheds another
amplifying shortwave across the Pacific and into CA/OR by 00Z
Saturday /late D1/. This next impulse will be accompanied by an
impressive lobe of vorticity, with the resultant trough development
driving height falls once again from CA through the Four Corners
into D2. Available moisture with this next impulse is progged to
be much less than with the first shortwave, reflected by NAEFS PW
anomalies that are near normal east of AZ as the maximum IVT
downstream of this trough axis is modest (GEFS and ECENS
probabilities for 500 kg/m/s less than 10%) and focused generally
south into Mexico. Of course, this is pretty typical with ARs that
are oriented more W/NW than W/SW, but enough moisture and continued
lowered snow levels of generally 3000-5000 ft will allow for at
least modest snowfall accumulations. The heaviest snowfall
accumulations D1 into D2 are expected from the Sierra into the
Mogollon Rim and into the Four Corners, where WPC probabilities are
high once again for 8+ inches, and locally 1-2 feet is possible in
the Sierra and northern CA terrain near the Shasta/Trinity region.

Interestingly, as the second shortwave digs across the Four Corners
into D2, the upstream flow becomes more favorable for a surge of
moisture and heavy precipitation farther north into the Pacific and
Interior Northwest. A trough over British Columbia/Alberta
interacts with the trough moving across the Four Corners to squeeze
the flow over the Pacific, resulting in fast and zonal flow surging
into WA/OR. Impulses within this flow will help push two distinct
surface lows onshore, the first Saturday morning and the second
early Sunday morning, helping to enhance ascent. Regardless of
those surface lows, deep layer lift becomes impressive and moisture
increases as a strengthening Pacific jet streak pivots onshore and
begins to buckle, lifting meridionally on Sunday downstream of an
even more potent trough approaching the coast.

Both ECENS and GEFS probabilities exceed 70% for 500 kg/m/s IVT
reaching the coast D3, funneling moisture along a cold front which
will elongate into the region. Although the accompanying WAA with
this atmospheric river (AR) will drive snow levels to as high as
6000 ft D3 south of the cold front, considerable moisture and a
sharp gradient in snow levels will likely result in impactful
pass-level snow from the Cascades through the Northern Rockies,
with still impressive snow but generally above pass-levels farther
south from the Great Basin into the Central Rockies, as much of the
West is again covered in elevation-based snowfall. High WPC
probabilities (>70%) late D1 through D3 become widespread for 6+
inches each day from the OR Cascades northward into the Olympics
and WA Cascades, and as far east as the Salmon River/Sawtooth area,
north into the Northern Rockies, and even as far as the Tetons.
Event total snowfall is likely to be extreme in the higher terrain
of the OR Cascades where 4-6 feet of snow is possible, with
widespread 1-3 feet in the other higher elevations regions of the
Northwest.


...Northern Plains...
Days 1-2...

Rapidly strengthening low pressure in the lee of the Rockies this
morning will approach or exceed March sea-level pressure records
across the Northern Plains as it tracks northeast through Saturday
aftn. Not only will SLP records be approached, but 850-500mb height
anomalies are progged to fall below the all-time minimums within
the CFSR climatology according to NAEFS, further indicative of the
intensity of this system. The result of this evolution will be a
major winter storm impacting the Northern Plains late Friday
through Saturday.

As the surface low tracks northeast from the Central High Plains,
it will be driven by an overlap of impressive height falls
downstream of the primary closed low and at least modest upper
diffluence in the LFQ of a poleward arcing jet streak east of the
mid-level center. Together, these will drive intense deep layer
ascent aiding in the storm strengthening. At the same time,
confluent low-level flow east of the low will draw impressive
moisture northward on 300-305K isentropic ascent. This moisture
will surge am impressive theta-e ridge northward, which will then
rotate cyclonically and lift into a robust TROWAL, pivoting over
eastern NE, SD, ND, and western/central MN Saturday. At the same
time, an impressive deformation axis will overlap with this TROWAL,
supporting an intense axis of maximized ascent to drive heavy
precipitation rates.

Initially, the theta-e advection will surge warm air into the
region such that the column will support primarily rain. However,
as the low continues to deepen and move northeast, and mesoscale
ascent intensifies, the combination of dynamic cooling and
ageostrophic flow on NW winds into the low center will rapidly cool
the column. The guidance continues to suggest a period of mixed
precip, including sleet and freezing rain, will occur, but this
again appears to be a conceptual setup where the transition period
is quite rapid. This indicates that while some light icing is
possible, the predominant wintry p-type will be snow, which will
almost certainly become heavy at times (70% probabilities for 1"/hr
according to the HREF). This is supported as well by NAM cross-
sections indicating a region of theta-es lapse rates