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

Valid 00Z Thu Dec 07 2023 - 00Z Sun Dec 10 2023

...Pacific Northwest ...
Days 1-3...

The forecast remains on track for an active Pacific jet to
maintain heavy mountain snows across northwest quadrant of the
Lower 48. At the beginning of the Day 1 period (Thursday evening),
a potent 250 mb jet streak is forecast to dig into the base of a
shortwave trough across the Pacific Northwest, resulting in an
amplifying upper-pattern across the Four Corners going into the
weekend. Strong height falls forced by the digging jet streak will
lower snow levels to below 3000 feet, and maintain bouts of heavy
mountain snowfall across the Cascades, Sierra, and Bitterroots,
where current WPC probabilities indicate a high (>80%) of snowfall
exceeding 4".

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 shortwave ejecting out of the Northern Rockies will move into
the Northern Plains and amplify into a closed low near ND Friday
morning. This will drive downstream height falls and periods of
impressive PVA as spokes of vorticity rotate around the closed
feature. Aloft, a modest jet streak will pivot northeast
downstream of a longwave trough centered over the Rockies, with
the favorable LFQ diffluent region overlapping the best height
falls to help strengthen a surface low late Thursday night through
Friday.
This surface low will track northeast from ND Thursday night into
Ontario, Canada by late Friday night, and will combine with at
least modest upstream theta-e advection to produce a swath of
precipitation, generally on the north side of this low. This
precipitation will be wrung out by additional ascent as a modest
deformation axis pivots west of the low as it pulls away, with
this likely falling as snow as it backs southward into ND on D2.
The DGZ deepens considerably during this period and will support
some higher SLR, but total forcing is still expected to be modest
and there is some uncertainty as to how far south into the CONUS
the heavy snow will fall. At this time, WPC probabilities for more
than 4 inches are localized to extreme northern ND and peak at
20-40%.


...Central Rockies & High Plains...
Day 2...

By Friday morning, lee-cyclonegenesis is well underway over the
High Plains as the digging Four Corners jet-streak couples with
the right entrance region of a Northern Plains jet. The resulting
enhanced region of deformation and frontogenesis in the 700-550 mb
layers (possibly maximized in the DGZ) will result in a burst of
moderate to locally heavy snowfall beginning Friday morning over
the Colorado Rockies. This activity is expected to spread
southeastward with time toward the Sangre De Cristos as
east-northeasterly surface flow ascends the terrain behind a
strong cold-frontal passage as snow levels plummet to below 3000
feet. The latest WPC probabilities maintain high (>80%)
probabilities of snowfall exceeding 6 inches in the Peaks of the
Rockies, while maintaining modest (40-50%) odds of 6 inches over
the Sangre De Cristo.

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 visibility for motorists.

Asherman/Weiss/Mullinax