FOUS11 KWBC 131838
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 PM EDT Mon Apr 13 2026
Valid 00Z Tue Apr 14 2026 - 00Z Fri Apr 17 2026
...Central Rockies...
Day 1...
Closed mid-level low emerging from the Great Basin will open into
an amplified, but positively tilted, trough as it ejects into the
Central Rockies by Tuesday evening. This will lead to a period of
enhanced lift through divergence and height falls, with the LFQ of
an accompanying upper jet streak helping to produce cyclogenesis in
the lee of the CO Rockies Tuesday aftn. Mid-level Pacific moisture
on the 700-500mb SW flow will increase column PWs to +1 to +2 sigma
across CO, with some additional low-level moisture becoming
available late D1 as cyclonic flow around the developing cyclone
creates E/NE winds from the Central Plains to advect additional
moisture westward from the Gulf moisture return. This moisture
being acted upon by the increased synoptic lift (and some upslope
flow on the low-level E/NE winds) will create periods of moderate
precipitation Tuesday, with significant snow accumulations expected
above generally 7000-8000 ft. WPC probabilities D1 are moderate to
high (50-90%) for 6+ inches across the Wasatch, San Juans, and CO
Rockies, with locally 12+ inches possible (30-50%) across the San
Juans before precip winds down very late Tuesday night as the
surface low pulls away to the east.
...Pacific Northwest through the Northern Rockies...
Days 1-3...
An increasingly active week of winter weather will begin today,
with widespread snowfall expected across most of the terrain from
the Pacific Northwest through the Northern Rockies.
The period begins with an amplified/closed low over southeast
Alaska dropping steadily southward to reach the British Columbia
coast by 00Z/Wed, and then pivot onshore WA/OR before 00Z/Thu. This
evolution will not only produce impressive height falls for ascent,
but also pinch the downstream flow to help surge Pacific moisture
into the region. This is reflected by IVT for which both the ECENS
and GEFS indicate has a >90% chance for exceeding 250 kg/m/s, with
the West-WRF indicating a narrow corridor of 500+ IVT surging
onshore as well. This IVT will be driven rapidly eastward beneath a
zonally oriented jet streak amplifying south of the upper low,
combined with intensifying SW low-level flow driving WAA ahead of a
surface cold front. Together, this will push PWs to +1-+2 sigma,
although the coverage of these highest PWs will be somewhat
confined to a narrow channel ahead of the cold front.
As this moisture spreads east and is acted upon by the robust
ascent, the result will be an expanding shield of precipitation,
with locally heavier precipitation expected in the higher terrain
where upslope enhancement is likely. Snow levels will vary widely
through the event, but are expected to generally fall D2/D3 behind
the cold front and the parent trough driving it southeast.
On D1, the heaviest accumulations are likely confined to the
Cascades as the event begins, which is reflected by WPC
probabilities for 6+ inches of snow above 50% focused in the WA
Cascades and Olympics, and generally above 4000 ft. During D2, the
coverage of heavy snow expands considerably as the front and
accompanying synoptic ascent shift east in tandem with the core of
the IVT moving inland. This results in WPC probabilities for 6+
inches being above 50% from the Olympics, along the WA and OR
Cascades, and eastward into the Blue Mountain, Sawtooth/Salmon
River, and much of the Northern Rockies. Snow levels in the Rockies
will fall from around 6000 ft to 4000 ft, but will crash well below
pass levels, as low as 2500 ft, in the Cascades, leading to
hazardous pass-travel. Finally, during D3 the snowfall will wane
across the Cascades but continue to expand eastward as far as the
Absarokas, Tetons, and Wind Rivers, while continuing across much
of the rest of the Northern Rockies. Snow levels D3 fall to
3000-4000 ft in these areas, with WPC probabilities suggesting a
70+% chance of at least another 6 inches of snow. Storm total
snowfall during this event will be considerable, forecast to reach
1-3 feet in the higher elevations.
Additionally, as the cold front sweeps southeast Wednesday night
and Thursday, it will be accompanied by a line of heavier
precipitation as low-level RH, 0-2km fgen, and 0-2km CAPE
maximize. The simulated reflectivity from the available guidance
suggests there will be a line of precip with embedded heavier
rates, and the snow squall parameter rises above 1. While the setup
is not ideal, and we are past the climatological favored period for
snow squalls, this setup does suggest the potential for convective
snow showers or snow squalls across the area. Additional snowfall
from any showers/squalls will be limited, but briefly heavy rates
and gusty winds could cause hazardous travel. This will need to be
monitored as we approach the high-res guidance windows to get a
better understanding of the timing, placement, and risk, of any
snow squalls.
The probability of significant icing for days 1-3 across the CONUS
is less than 10 percent.
Weiss
$$
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