Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 PM EDT Sun Oct 17 2021

Valid 00Z Mon Oct 18 2021 - 00Z Thu Oct 21 2021

...Pacific Northwest and Northern California to the North-central
Rockies...
Days 1-3...
An amplifying trough currently extending south off the Pacific
Northwest coast will push ashore by this evening before closing
off into a low over northern CA tonight. Prefrontal precip is
pushing ashore this afternoon with snow levels around 6000 ft as
the precip reaches the Cascades and Sierra Nevada before dropping
to around 4000 ft before precip shifts farther inland Monday
morning. Day 1 snow probabilities for 6 or more inches are
moderate from the far southern OR Cascades to the south-central
Sierra Nevada.


Timing of the inland progression has been a bit uncertain among
guidance, but the 12Z suite today has pretty good agreement with
the upper low moving east across northern NV Monday, northern UT
Monday night, and along the WY/CO border Tuesday before crossing
western Nebraska Tuesday night. Favorable upper jet forcing will
support organized inland precipitation, including mountain snows,
from northern Nevada and southern Idaho into the north-central
Rockies Monday into Monday night including the Independence and
Ruby mountains in northeastern NV, the Wasatch and Uinta of UT and
the Wind River of WY where Day 1.5 to 2 snow probabilities for 6
or more inches are moderate.

By Tuesday morning, the upper low will be close enough to the
Rockies to allow lee-side cyclogenesis over northeast CO which
will wrap in Gulf of Mexico-sourced moisture which along with
upslope flow will enhance snowfall over the WY Rockies. Day 2.5
snow probabilities for 6 or more inches are moderate over the
highest plains/foothills of east-central WY and high over the
northern Laramie, Wind River, and southern Bighorn ranges.

As the low tracks across the north-central Plains a TROWAL
develops, but rather marginal thermals over western Neb through
central SD will greatly limit snowfall on the plains. Day 3
probabilities for 4 or more inches are 20 to 30 percent over the
Black Hills.  


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson