FOUS11 KWBC 120907
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
507 AM EDT Wed Mar 12 2025
Valid 12Z Wed Mar 12 2025 - 12Z Sat Mar 15 2025
...The West...
Days 1-3...
...Impactful heavy snow for most California mountain ranges
this afternoon into Thursday, mostly the southern Intermountain
West terrain and Rockies late week, then blizzard potential on the
northern Plains Saturday. Please see the latest Key Messages
linked at the bottom...
Potent upper-level trough continues to dig as it tracks south with
the parent low drifting south from the Gulf of Alaska. The base of
the trough approaches northern CA this afternoon, then digs south
to the northern Baja through Thursday. Enhanced moisture transport
has raised snow levels to around 3500ft in WA, 4500ft in OR,
5000ft in northern CA and 6000ft on the Sierra Nevada. However, the
cold frontal passage and falling heights aloft will result in snow
levels generally around 3500ft along the Cascade Range on south to
the Shasta/Siskiyou this evening and 2500ft late tonight as snow
rates decrease after the upper trough passage.
The Sierra Nevada will be the focus of the moisture plume which
features >400 kg/m/s SWrly IVT (near the 99th climatological
percentile via ECMWF SATs) just ahead of the cold frontal passage
this evening. This when snowfall rates will be at their heaviest...
2-3"/hr will be common per 00Z HREF for several hours between 20Z
and 10Z Thursday down the length of the Sierra Nevada above about
6,000ft which is where Day 1 PWPF for >18" snow is categorical. Some
upslope flow will linger into the Sierra Nevada Thursday morning,
before a brief lull in the action arrives Thursday afternoon.
By Friday morning, the next Pacific storm system arrives, producing
more moderate snow rates for the Cascades and Sierra Nevada.
However, snow levels now progged to remain low through this time,
around 1500ft in western WA, and 2500 to 3000ft OR through the
Sierra Nevada. Day 3 PWPF for >8" is 40-80% for the OR Cascades and Shasta/Siskiyou, peaking around 60% for the WA Cascades and Sierra Nevada.
Farther south, the Transverse Ranges as well as the higher peaks
of the Peninsular Ranges will see heavy snow late tonight through
Thursday morning as the snow levels drop from 5500ft to 4000ft. Day
2 PWPF for >8" are 40-80% in the higher terrain.
Overall, this is an ideal pattern for somewhat brief, but heavy
snow for the Shasta/Siskiyou and along the entire length of the
Sierra Nevada. Snowfall will be measured in feet with as much as
4-5 feet forecast in the peaks of the Sierra Nevada and 3 feet at
the higher pass levels such as on I-80. The WSSI retains shows
Major to Extreme Impacts in most of these ranges with Snow Amount
the primary impact, but some impacts as a result of Snow Load and
Blowing Snow are expected as well. Dangerous to impossible travel
are anticipated with the potential for some instances of power
outages and tree damage as well.
The highly anomalous IVT responsible for the barrage of heavy snow
in the California mountain ranges will advect moisture well inland
across much of the Intermountain West as well. Heavy mountain snow
is expected in the Blue, Sawtooth, and Absaroka of the northern
Rockies. The high moisture plume will coincide with the added help
of strong diffluent flow downstream of the upper trough to
generate heavy snow from the Tetons on south through the Wasatch,
Uinta, central Nevada ridges, the Kaibab Plateau and Mogollon Rim,
and western Colorado Rockies. Day 1.5 and 2 WPC probabilities show
high chances (>70%) for snowfall totals >8" in all of these
mountain ranges, but it is the Wasatch, Uinta, and Mogollon Rim
most notably that could see snowfall totals between 1-2 feet
through Friday.
...Northern Plains...
Day 3...
The trough takes on a negative-tilt Thursday night over NM and
closes in to a mid-level low over KS Friday morning. This low
further deepens/strengthens as it tracks northeast over MN into
Saturday. The southern stream wave crossing the southern Plains
today limits Gulf moisture from streaming up the Plains until
Thursday night, limiting the the moisture available moreso than if
the wave weren't crossing today. While enough Pacific- sourced
will be present to allow some wrap around banding on the low over
the High Plains Friday that banding should be fairly sparse until
Friday night when the activity should be over central Neb/SD. This
is notable as the area is in drought and will be experiencing
strong winds ahead of the system - in some areas of the High Plains
there may be just a dry wind shift. Once those bands do develop as
they head for the eastern Dakotas and MN, heavy snow banding is
possible. Powerful winds with the system will immediately result in
blowing snow and a potential blizzard. Marginal thermals at the
onset should result in a rain to wintry mix to snow progression
with some ice accretion potential most likely over the Red River of
the North Basin along the ND/MN border.
Jackson
...Winter Storm Key Messages are in effect as linked below...
https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png
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