Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 PM EST Thu Dec 01 2022

Valid 00Z Fri Dec 02 2022 - 00Z Mon Dec 05 2022

...Pacific Northwest, California, Central and Northern Rockies...

Days 1-3...
Active period will continue for much of the West as a first storm
system crosses through the Intermountain West and central/northern
Rockies tonight into Friday morning, before separate systems begin
to impact the Pacific Northwest and Sierra Nevada late Friday. The
first area of heavy snow is associated with an upper trough
pushing into the Great Basin tonight into early Friday and weaken
as it looses favorable upper dynamics. Lingering heavy snow is
likely throughout the central and southern Sierra Nevada along the
tail end of a modest atmospheric river as well. Modest snow
amounts are also expected around Yellowstone and points south into
the northern Wasatch and Uintas in UT. Height falls and ample
moisture will complement lower-level frontogenesis and orographic
upslope over these and other higher elevations extending eastward
into CO along and behind the cold front. WPC probabilities for at
least 8 inches of snow are high over many mentioned mountainous
areas.

Another upper low will drop southward out of the eastern Gulf of
Alaska along 130W just offshore WA/OR late Friday into early
Saturday. Trend has been for a bit less QPF over the Pac NW, and
mostly confined to the Olympics where significant accumulations
are still possible. That results in high WPC probabilities for at
least 8 inches across the Olympics. By late Day 2 and into Day 3,
that upper low will wobble toward the coast but the tail-end of
the lead system off California may support another wave of
precipitation into the Golden State, focused primarily over the
Sierra but also into the Shasta Siskiyous and Klamath Mountains.
WPC probabilities for at least 8 inches of snow are high (>70%)
over these areas through Day 3, and moderate (>40%) for at least
12 inches of snow.

...Northern Minnesota and the U.P. of Michigan...

Days 1-2...
A quickly-moving positively-titled trough will race through the
Northern Plains/Upper Midwest/western Great Lakes late Friday into
Saturday. Trend continues for a quicker progression and less QPF
overall since yesterday. However, modest upper divergence in
association with the retreating northern jet coupled with
cyclogenesis over the Plains lifting the surface low through the
Corn Belt will support an area of accumulating snow to the
northwest of the low. Ratios will rise as colder air moves in but
at the same time as QPF exits/diminishes. WPC probabilities for 4
inches or more of snow are low across much of northern MN, with
slightly higher probabilities over the western U.P. of Michigan
due to additional lake effect once cold air sweeps in behind the
system on Saturday. Gusty winds may accompany this system and
increase potential travel hazards within any localized areas
experiencing modest snowfall rates.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.


Snell


***Key Messages for Nov 29 to Dec 2 Winter Storm Affecting Western
U.S. into Friday***

- Mountain road travel disruptions across the Sierra Nevada into
tonight. Peak snow rates in excess of 3 inches per hour will
continue to leading to dangerous or impossible travel. Backcountry
avalanches are also possible.

- If you must travel across the Sierra Nevada, be prepared for
rapidly changing conditions, road closures, and have winter
driving supplies.

- Locally heavy mountain snow will occur tonight into Friday for
the Intermountain West and North-Central Rockies.