Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EST Thu Nov 30 2023

Valid 00Z Fri Dec 01 2023 - 00Z Mon Dec 04 2023

...Cascades through the Northern Rockies...
Days 1-3...
A prolonged period of wet and unsettled weather, that is likely to
continue into next week, is now developing across of the Pacific
Northwest with the arrival of the first in a series of shortwaves.
 Modest moisture and the progressive nature of this system will
limit amounts. However, snow levels are low, impacting the Cascade
passes at the onset.  While the leading wave moves into the
northern Rockies on Friday, an upstream wave will quickly follow
with an uptick in moisture and better forcing expected to support
heavier amounts across western Washington and Oregon while snow
levels remain relatively low.  This second wave will be
progressive as well, however a long fetch of onshore flow with
embedded energy aloft will follow, supporting additional periods
of precipitation into the weekend.  The potential for heavy
precipitation is expected to increase as a long fetch of
subtropical moisture begins to extend inland from the Oregon coast
through the Intermountain West on Saturday, before being
reorinented farther to the north ahead of an amplifying low moving
across the Gulf of Alaska and northeastern Pacific on Sunday. 
Saturday is also expected to mark the onset of rising snow levels
that are forecast to continue into Sunday.

WPC probabilties indicate that snow accumulations of a foot or
more are likely (greater than 70 percent) along much of the
Washington and Oregon Cascades, including the passes, before snow
levels start to climb this weekend.  Heavy accumulations of a foot
or more are also likely farther east across the Blue Mountains and
into the northern and central Idaho ranges, and along the
southeastern Idaho and western Wyoming into the northern Utah
ranges.

...Central Plains into the Midwest & Northern New England...
Days 1-2...
A shortwave trough now emerging from the Southwest/southern
Rockies is expected to assume a negative tilt as it moves into
Texas and Oklahoma this evening.  The associated strong forcing
aloft overlapping low-to-mid level frontogenesis will support
precipitation blossoming on the northwest side a surface low
tracking from southwest Oklahoma to Missouri tonight into early
tomorrow.  There remains the potential, but still a great deal of
uncertainty, regarding the development of a narrow swath of
moderate to heavy snowfall developing within this deformation zone
from the Panhandle Region to southern Iowa overnight and early
Friday.  While strong dynamic cooling is likely to support a
changeover to frozen precipitation on the northwest edge of the
precipitation shield, the progressive nature of the system and low
SLRs are expected to limit the potential for widespread heavy
amounts.  However, the HREF guidance continues to show relatively
high probabilities for snowfall rates briefly reaching 1 in/hr. 
Models generally show precipitation waning on the backside of the
low during the day on Friday, but then redeveloping by the late
afternoon and evening as an upstream shortwave lifts from the
central Plains through the mid Mississippi Valley, before reaching
the Great Lakes Friday evening.  With this potential for
redevelopment, the probabilities for accumulations of an inch or
more have increased, with WPC guidance now showing some moderate
probabilities (greater than 40 to 70 percent) from extreme
northeastern Kansas and southeastern Nebraska to northern
Illinois. 

Meanwhile, precipitation is expected to spread out ahead of the
surface low and along a slow-moving boundary extending from the
Great Lakes into the St Lawrence Valley.  Accumulating snow of at
least an inch or two can be expected from southern Michigan on
Friday to northern New England Friday evening.

...Southwest & Southern/Central Rockies...
Days 1-2...
A series of shortwaves moving quickly on the heels of the wave
moving into the southern Plains this evening will move through the
base of a broader scale trough as it moves from the Southwest into
the central and southern Rockies on Friday into Saturday.  While
not expected
to produce widespread heavy accumulations, WPC probabilities
continue to highlight the threat for additional locally heavy
amounts across the higher terrain through late Saturday.  This
includes Arizona's White Mountains and the north-central New
Mexico and the western Colorado ranges.  Probabilities for
additional amounts of 6 inches or more are high (greater than 70
percent) for much of these areas, with moderate (greater than 40
to 70 percent) probabilities for 8 inches or more across some of
the higher peaks.

Pereira

***Key Messages for Pacific Northwest Winter Storm***
--Prolonged Winter Storm in the Northwest
A long-duration winter storm along and west of the Cascades
continues through this weekend. Periodic heavy precipitation with
rain for the valleys and snow at many mountain passes can be
expected..

--Heaviest Snowfall in the Cascades
Through Sunday, there is a high chance (>90%) of at least 24" of
snow for Cascade passes, including I-90. Snow levels remain below
4000 feet through Saturday before rising above pass level Saturday
night. Travel will be difficult and hazardous due to both heavy
and blowing snow.

--Mountain Snow through the Northern Rockies
Periods of heavy snow are expected to bring over 12†snow to
higher terrain in the northern Intermountain West and Northern
Rockies through Sunday.

--Potential for River Flooding this Weekend
Several inches of rain in valleys and the rising snow levels
Saturday night will likely produce minor to moderate river
flooding late this weekend into next week.