Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Wed Dec 06 2023

Valid 12Z Wed Dec 06 2023 - 12Z Sat Dec 09 2023

...Northwest & Northern Rockies...
Days 1-3...

An active jet stream pattern across the North Pacific will direct
a pair of storm systems at the northwestern U.S., resulting in
heavy mountain snow from the Cascades and northern Sierra Nevada
to the Tetons and Wasatch. The first storm system arrives when an
upper level disturbance ejecting out of the base of a longwave
trough in the northeast Pacific delivers another round of Pacific
moisture to the region. As the cold front clears the region and
700-500mb height falls ensue, snow will pick up in intensity first
along the Cascade Range and as far south as the northern Sierra
Nevada of northern California, then over the Blue Mountains of
northeast Oregon and both the Boise and Sawtooth of central Idaho
Wednesday night. By Thursday morning, the divergent left-exit
region of a 250mb jet streak combined with steady 700mb moisture
flux and topographically-favored ascent leads to heavy snow in the
Northern Rockies. This includes mountains such as the Bitterroots,
Lewis Range, Absaroka, Tetons, and northern Wasatch. Thursday
appears to be a rather snowy day across all these mountain ranges,
while at the same time, a second upper level disturbance tracking
into northern California brings another surge of 850-700mb
moisture flux. By 00Z Friday, just about all snow levels will be
somewhere between 3,000-4,000ft on average. The second storm
system will work its way east into the heart of the Intermountain
West Friday morning with strong 850-700mb CAA over the northern
Great Basin and into the Wasatch. Upslope flow will maximize
snowfall rates along the Wasatch and Uinta during the day Friday
with snow gradually tapering off by early Saturday morning.

WPC probabilities for 48-hr snowfall totals >8" feature a high
chance (>80%) in the Oregon Cascades, the Salmon/Shasta and Sierra
Nevada of northern California, the Boise/Sawtooth/Bitterroots of
Idaho, the Lewis Range (including Glacier NPS), the Tetons of
western Wyoming, and the northern Wasatch of Utah. The WSSI
depicts Moderate to even Major Impacts in the Oregon Cascades, the
Salmon/Shasta and Sierra Nevada of northern California, the Blue
Mountains of northeast Oregon, portions of the Boise/Sawtooth of
central Idaho, and the Sessions Mountains north of Salt Lake City.
The primary driver in the WSSI is Snow Amount although some
ranges, such as the northern Sierra Nevada and the Wind River
Range of western Wyoming, are likely to encounter Moderate impacts
due to Blowing Snow. Travel along or over these aforementioned
mountains (particularly their passes) is likely to be treacherous
with some road closures possible. Motorists should exercise
caution when driving in these areas Thursday into Friday.

...Northern Plains...
Days 2-3...

A strengthening storm system over North Dakota is likely to
produce periods of snow from northern Montana to northern North
Dakota Thursday night into Friday. The key factors for this storm
are its track, the progressive nature of the storm system, and
placement of its deformation axis. Latest 00Z guidance drifted the
heaviest snowfall over southern Canada closer to the US/Canada
border, but not enough to lead to an increase in probabilities for
>4" of snowfall. Latest WPC probabilities for >4" continue to top
out around the low chance of 10% along the North Dakota/Manitoba
border. That said, these trends of heavier snowfall totals
approaching the International border suggests higher snowfall
totals can not be ruled out just yet. Regardless of amounts, the
real impacts may be courtesy of strong wind gusts. Between 06-18Z
Friday, NAEFS shows 850mb winds are above the 90th climatological
percentile. There still remains some uncertainty in the storm
track, but even with just a couple inches, whipping wind gusts
could lead to significantly reduced visibilities and drifting snow
in parts of northeast Montana and northern North Dakota. The
latest WSSI does show Minor Impacts for far northeast Montana and
much of northern North Dakota with its Blowing Snow component the
primary driver in the WSSI algorithm.

...Central Rockies & High Plains...
Day 3...

The second upper level shortwave trough responsible for the heavy
snow in the Wasatch and Uinta on Friday is forecast to amplify and
deepen over the For Corners region late Friday. The shortwave
amplification ultimately leads to 200-500mb heights that become
lower than the 10th climatological percentile according to NAEFS
Friday afternoon and Friday night. As 700-500mb frontogenesis take
shape over the Colorado Rockies, 850-700mb moisture flux streams
in from the south and west, and lee cyclogenesis ensues over the
central High Plains, periods of heavy snow are expected to develop
over the Colorado and northern New Mexico Rockies by Friday
afternoon and persist into the overnight hours. Meanwhile,
northeasterly surface winds over the central High Plains allows
for upslope enhancement into the Front Range of the Colorado
Rockies and as far south as the Sangre De Cristo. Latest WPC 24-hr
probabilities showed high chances (>70%) for snowfall totals >6"
for most elevations at and above 8,000ft in the Colorado Rockies
and Sangre De Cristo. However, some of the lower elevations also
sport moderate odds (40-60%) for >6" of snow, which includes the
Palmer Divide. Lastly, even the Denver/Boulder metro areas have
moderate chances (40-60%) for snowfall totals >4". The latest
experimental Probabilistic WSSI suggests moderate risks (50-60%)
for Minor impacts in the Denver/Boulder metro area on Friday and
into Friday night. This also includes a stretch of I-25 from
Denver and Colorado Springs to the section of I-25 south of
Pueblo. This suggests moderate chances for winter driving
conditions may occur, which may include a combination of snow
covered roads and reduced visibilities for motorists.

Mullinax