Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Sun Apr 14 2024

Valid 00Z Mon Apr 15 2024 - 00Z Thu Apr 18 2024


...Northern California across Great Basin and Colorado Rockies...
Days 1-3...

Current WV satellite depicts our closed upper low entering into
the
southern CA coast with expected progression eastward as it
traverses
into the Great Basin. Snowfall continues in-of of the central and
northern Sierra with snow levels generally around 5000' expected
thanks to the falling height pattern. Highest probabilities for
>6"
of snow remain over the Siskiyou Mountains and Mt Shasta with PWPF
outputs between 50-70% with upwards of 90% in the tallest peaks. A
secondary maxima will be within the central Sierra due to primed
mid-
level ascent and a feed of Pacific moisture into the terrain as
PWPF
peaks at 50-70%. Probabilities for >8" are much lower outside the
taller peaks, but generally within the 10-30% range with over 50%
located within locations above 10000' in both regions.

Snow will subside in the Sierra and shift focus eastward into UT
on
D2 as large scale forcing increases under the diffluent pattern of
the upper low. Probabilities for >6" are greatest within the
Wasatch
down into the ranges of southern and central UT. PWPF values for
at
least 6" are between 50-80%, confined to areas above 6500' with
the
higher elevations above 8000' running closer to the upper
percentile
probabilistically.

By D3, attention shifts into the central Rockies within CO as our
upper low continues its progression to the east. Totals within the
the central Rockies are forecasted to be highest thanks to a
combination of the favored upper ascent, as well as aid from lee
cyclogenesis over the Front Range. Solid low to mid-level moisture
transport from the Gulf will funnel into the lee of the Rockies
with
primed upslope pattern within the terrain over northern CO. PWPF
for
>6" encompasses much of the terrain above 7500' with the highest
probabilities running between 50-80% with upwards of 90% in the
tallest peaks of the Medicine Bow and Park Ranges. The prospects
for
>12" are within the aforementioned area as well with probabilities
of exceedance running between 30-60% with best chance located into
the central and southern Medicine Bow mountains.

...Northern Rockies...
Day 3...

A potent mid-level shortwave will progress out of southeast BC and
make headway into the northern Rockies and adjacent Saskatchewan
by
the end of D2 into D3. A combination of strong surface ridging
providing prominent cold air advection for the season along with a
inverted trough located over the northern plains to the southern
prairies of Canada will create a primed upslope enhancement over
the
northern Rockies by Tuesday night into Wednesday. PWPF for >6" is
highest within the mountains of southwest MT into northwestern WY
over to the Big Horns, along with Glacier National as snow levels
plummet (Falling 3500' to 1500' by Wednesday AM) due to the cold
air
advection regime and falling heights aloft. Probabilities for at
least 6" are between 40-60% for Glacier National Park, and
generally
between 50-90% across those areas over into Yellowstone over to
the
Big Horn Range by the end of the period.

Kleebauer