FOUS11 KWBC 210630
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
230 AM EDT Tue Apr 21 2026
Valid 12Z Tue Apr 21 2026 - 12Z Fri Apr 24 2026
...California...
Days 1-2...
The heavily discussed and anticipated mid-level closed low
positioned off the CA/OR coast tonight will gradually lift E/NE,
with the core of the low progged to come onshore northern CA late
tonight/very early Wednesday morning. This evolution will result in
impressive height falls downstream into CA, with additional ascent
supplied by secondary shortwave energy rotating through this
trough and lifting northeast through the Great Basin. Impressive
height falls and PVA, working together with the LFQ of an upper
Pacific jet streak working onshore, will drive large scale ascent,
with lift becoming robust today and tonight.
This dep layer lift will act upon an increasingly moist column to
support widespread moderate to heavy precipitation. This moisture
will be channeled onshore via confluent flow E/SE of the upper low
and directly beneath the Pacific jet streak to drive IVT to around
500 kg/m/s as supported by the West-WRF and elevated probabilities
from the ECENS and GEFS (up to 60% chance). This steady stream
onshore of moisture will be wrung out by the aforementioned
synoptic lift, with additional ascent through upslope flow into the
terrain of northern CA and the Sierra providing locally enhanced
lift. ALthough snow levels will begin elevated at 6000-7000 ft,
they will crash rapidly beneath the upper trough, becoming as low
as 4500 ft, or even potentially lower (NBM 10th percentile is 3500
ft in the Sierra) due to steep lapse rates allowing for some precip-loading/dynamic cooling. This will result in heavy snow
accumulations in the terrain, especially above 6000 ft, but with
significant snowfall above 4500 ft also possible.
The heaviest snow is expected across the Sierra where D1 WPC
probabilities are high (>70%) for at least 12 inches of snow, and
1-2 feet is possible in the higher terrain. This will be in
response to snowfall rates that could reach 1-2+"/hr as reflected
by the WPC prototype snowband tool as the upslope flow into the
steep lapse rates aloft drives potential convective rates. With
this snow falling below many of the Sierra passes, treacherous to
impossible travel is likely (WSSI-P indicates a 40-50% chance of
major impacts due to heavy snow and wind). Farther north across the Shasta/Siskiyou/Trinity region, snow levels should generally remain
above the important travel passes, but in the higher elevations WPC probabilities are high (>70%) for at least 8 inches of
accumulation D1.
As the system ejects into the Great Basin and Northern Rockies D2,
the accompanying ascent will shift away from CA, bringing an end to
the heavy snow early Wednesday leaving only light/additional
accumulations across these areas.
...Northern Rockies...
Days 2-3...
The same system that will bring heavy snow to California will
continue its trek into the northern Rockies Wednesday into Thursday
as several pieces of shortwave energy pinwheel around and within a negatively-titled upper trough. Although moisture will gradually
decay (IVT less than 150 kg/m/s spilling into the Intermountain
West), a strengthening surface low across the northern High Plains
will work together with continued mid-level southwest flow
(funneling the Pacific moisture) to produce widespread
precipitation across the region beginning Wednesday morning.
Initially, snow levels are expected to be around 8000 ft, keeping
any significant accumulations confined to the highest elevations.
However, as the upper trough swings northeast, snow levels are
expected to fall steadily, becoming 4000-5000 ft by Thursday
morning, and falling even further to around 3000-4000 ft (possibly
as low as 2000 ft in the Northern High Plains) by the end of the
forecast period. While this still suggest that the heaviest
accumulations will occur in the higher elevations from the Blue
Mountains of OR through the Sawtooth/Salmon River area of ID and
into the Northern Rockies from near Glacier NP through Yellowstone
NP, some lower elevation accumulations are also becoming likely.
Confidence is high that heavy snow will accumulate in the terrain,
and this is reflected by 48-hr WPC probabilities (between 12Z Wed -
12Z Fri) that are high (>70%) for 8 inches in much of the higher
terrain between Glacier NP, Yellowstone NP, and west to the Blue
Mountains. Locally 18 inches of snow is possible (30% chance).
However, the most challenging aspect of this forecast involves what
happens on D3 as the surface low deepens across eastern MT and into
Canada. There continues to be diverging solutions of the various
global deterministic models with both the intensity of this low and
the accompanying upper level trough evolution. The GFS/GEFS
continue to look like the outlier/amplified scenario which is also
reflected in the D3/D4 clusters, but there has been a notable shift
to increase dispersion of the GEFS with more overlap from the
ECENS/CMCE from prior runs. However, 74% of the GEFS members still
make up the most amplified solutions, with nearly 1/2 of the GEFS
indicating the deepest trough and resulting heaviest snowfall into
the High Plains. While there is increasing confidence that
sufficient moisture lifting out of the Gulf will wrap into a TROWAL
and pivot SW around the low back into MT, the weaker solutions
supported by the ECENS/CMCE are still favored, especially noting
that the recent NAM has also trended away from the GEFS. Still,
there is potential for lower elevation snow from central to eastern
MT and eventually into ND, but at this time confidence is low in
any significant accumulations. It will need to be watched in
future model cycles however, for any potentially hazardous winter
weather outside of the terrain Wednesday into Thursday.
The probability for significant freezing rain exceeding 0.1 inches
across the CONUS is less than 10 percent.
Weiss
$$
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