Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
454 AM EST Thu Dec 09 2021

Valid 12Z Thu Dec 09 2021 - 12Z Sun Dec 12 2021


...Sierra Nevada to the Central Rockies (Day 1)...
A low amplitude shortwave embedded within progressive
northwesterly flow over the eastern Pacific is expected to amplify
as it moves onshore across California today. Favorable upper
forcing along with the associated frontal band is expected to
support mountain snows along the Sierra Nevada.  With snow levels
dropping below 5000 ft, probabilities are high that many locations
along the central Sierra, including the passes, will see snow
accumulations exceeding 6 inches.

Organized precipitation, including mountain snow, is expected to
spread east through the Great Basin into the central Rockies.
Southwesterly flow ahead of the advancing trough is forecast to
direct deepening moisture into a low-to-mid level baroclinic zone
extending across the region -- supporting areas of heavy mountain
snow.  The areas impacted are expected to include the central and
eastern Nevada, Utah, western Colorado, and the south-central
Wyoming ranges. For some areas, including the Wasatch into the
Uintas in Utah, and the Elk Mountains northward to the Sierra
Madre and Medicine Bow mountains in western Colorado and southern
Wyoming, WPC probabilities indicate that local accumulations of 8
inches or more are likely by late Thursday.

Snows are expected to quickly end across the Sierra Nevada and
across much of the Great Basin, as the trough continues to move
progressive east late Thursday.  However, heavy snows are likely
to continue and expand across the central into the southern
Rockies through the overnight into early Friday. From the Sierra
Madre south to the San Juan Mountains, WPC probabilities indicate
that additional accumulations of 8-12+ inches are likely through
12Z Friday. Two day totals are expected to exceed two feet across
portions of the higher terrain. 


...Central High Plains to the Upper Midwest and Upper Great Lakes
(Days 1-3)...
Lee-side cyclogenesis over eastern Colorado in response to the
advancing trough is expected to support snows developing farther
to the north over southeastern Wyoming into the Nebraska Panhandle
and southwestern South Dakota beginning this afternoon and
evening. Strong low-to-mid level frontogenesis within the right
entrance region of a curved upper level jet streak will help
support a stripe of moderate to locally heavy snows shifting east
from the High Plains across southern South Dakota and northern
Nebraska overnight into early Friday.  WPC probabilities indicate
that accumulations of 6-12 inches are likely, with heavier amounts
possible, across south-central and eastern WY, southern SD and
northern NE, northwest and north-central IA, southern MN, central
and northern WI, and much of Upper Michigan. 

...Pacific Northwest (Days 1-3)...
Initial shortwave trough will pivot across the west coast early
today, giving way to a brief bout of shortwave ridging before a
more expansive trough south of the deepening Gulf of AK low
approaches the Pacific Northwest coast Friday night and Saturday.
Heavy snows during day 1 (12Z Thu-12Z Fri) will be confined to the
WA and northern OR Cascades above 1500-2000 ft. The aforementioned
shortwave ridging will lead to decreasing snowfall on Friday,
before the more robust warm conveyor belt (deep layer WAA and
isentropic ascent) brings much heavier snows Saturday-Saturday
night across the Olympics and and WA/OR Cascades above 3000-3500
ft, where accumulations of a foot or more will be likely.

For Days 1-3, the probability of significant icing is less than 10
percent.



~~~Key Messages for December 9-11 Winter Storm from the Central
Rockies to the Upper Midwest~~~

...Significant winter weather storm expected...
Heavy snow will lead to hazardous to difficult driving conditions
and blowing snow may significantly reduce visibility in some
areas.

...Heavy snow will span from the Central Rockies toward the Upper
Midwest...
A foot or more of snow is expected across the Central Rockies,
with isolated 2 feet across the highest terrain. Meanwhile, a
swath of 6-12 inches of snow (with locally higher amounts) is
anticipated from the Wyoming Front Range eastward toward the Upper
Midwest.

...Significant disruptions to travel expected...
Hazardous driving conditions are expected, as the combination of
heavy snow and blowing snow leads to slippery roads and
significantly reduced visibilities.


Hurley