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

Valid 00Z Fri Dec 10 2021 - 00Z Mon Dec 13 2021


...Central Rockies to the Upper Midwest...
Days 1-2...
As an amplifying trough shifts east across the western U.S., snow
will continue to develop through the evening and persist through
the overnight across portions of the Southwest and the central to
southern Rockies, with heavy accumulations likely for portions of
the central Rockies.  Deep southwesterly flow ahead of the trough
will continue to feed moisture into a region of broad scale
ascent, bolstering the potential for heavy snows across the
western Colorado and the adjacent far-southern Wyoming and
far-northern New Mexico ranges.  WPC probabilities indicate that
accumulations of a 8 inches or more are likely from the Sierra
Madre south to the San Juans.  Locally heavier totals well
exceeding a foot are likely across some of the higher peaks.

Meanwhile, snow will also continue to develop and spread east of
the Rockies from southeastern Wyoming into southwestern South
Dakota and northwestern Nebraska.  Low pressure developing over
Colorado is expected to direct deepening moisture into a
low-to-mid level baroclinic zone, fostering the development of
moderate to heavy snow bands.  After a few latitudinal shifts over
the past few days, the past few model runs have been generally
consistent showing a band of heavier accumulations extending from
southeastern Wyoming along the South Dakota-Nebraska border. 
Consensus of the 12Z guidance was slightly heavier, which is
reflected in the higher probabilities for heavy snow the region. 
The latest WPC probabilities indicate that accumulations of 8
inches or more are very likely, with a significant threat for
localized amounts of 12 inches or more.

Moderate to heavy snows are forecast to shift east across southern
South Dakota and northern Nebraska during the day of Friday.  By
Friday night, multiple streams phasing over the central Plains and
mid Mississippi Valley will support additional intensification of
the surface low pressure center as it turns northeast from the mid
Mississippi Valley.  Favorable upper forcing along with low-to-mid
level frontogenesis is expected to support moderate to heavy snow
bands developing on the northwest side of low.  Latest HREF
guidance is showing significant potential for snowfall rates of
over inch/hr within these developing bands as they translate
northeast across southern Minnesota into Wisconsin and the U.P. of
Michigan.  Overall, the guidance has shifted upward with amounts
here as well, with WPC probabilities indicating that accumulations
of 8 inches or more likely from the western Minnesota-Iowa border
to the U.P. of Michigan.

By late Saturday, snows are expected to quickly diminish as the
low tracks rapidly north of the Great Lakes.

...Pacific Northwest to the Northern Rockies and the Sierra
Nevada...
Days 1-3...
A strong frontal system associated with an amplifying trough
digging south from the Gulf of Alaska into the northeastern
Pacific is expected to bring heavy precipitation back into western
Washington and Oregon, with relatively lighter amounts spilling
east into the northern Rockies on Saturday.  Models show an
increase in snow levels late Friday into early Saturday with the
associated warm front moving into the region, before dropping
later in the day as the trailing cold front moves onshore later in
the day.  The trough will continue to amplify offshore as energy
continues to dive south from the Gulf of Alaska on Sunday.  This
will shift the focus for strong moisture advection farther south
into northern California, where heavy precipitation, including
mountain snow, is expected on Sunday.  WPC probabilities indicate
that three day totals of 18 inches or more is likely for the
Olympics and along much of the Washington and Oregon Cascades,
including most of the primary passes.  Heavy accumulations of a
foot or more are likely for the Mt Shasta region, as well as the
northern Sierra Nevada.  For areas farther east, accumulations of
a foot or more are also likely for the parts Blue Mountains, and
portions of the northern Rockies from northern and central Idaho
to northwestern Montana.  

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...
A winter storm will track from the Utah and Colorado ranges toward
the Central Plains Friday and the Upper Midwest and Upper Great
Lakes Friday night into Saturday.

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


Pereira