Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Wed Dec 08 2021

Valid 12Z Wed Dec 08 2021 - 12Z Sat Dec 11 2021

...A Winter Storm Develops Over the Southwest Tonight...Tracks
Northeast From the Central Rockies Friday and Through the Great
Lakes Saturday...

...California...Southwest...Southern Rockies...to the Central
Plains...
Days 1-3...
An amplifying trough shifts east across the northern CA coast late
tonight, digging southeast into interior southern CA through
Thursday before shifting east to the Four Corners Thursday night.
Low pressure currently stalling off Baja CA today, allows Pacific
moisture to stream northeast across much of Mexico and over the
Desert Southwest today through Thursday, moistening the
Inter-mountain west ahead of the approaching trough. Furthermore,
a reinforcing trough digs southeast from WA late Thursday likely
phasing with the trough near the Four Corners by Friday morning
allowing mid-level low development likely just east of the
Colorado Rockies Friday with rapid lee-side surface cyclogenesis
that ejects northeast across the central Plains Friday and crosses
the central Great Lakes Friday night into Saturday morning.

Models remain in overall remarkably good agreement with the
timing, development, and track of the system. A slight southerly
shift in the track over the Great Plains is noted in the 00Z
consensus. The 00Z NAM takes an even more southerly track with the
low on Day 3 in the central Plains, so the WPC forecast generally
strays away from the NAM by Day 2. WPC probabilities indicate that
brief heavy snow is likely to shift south down the Sierra Nevada
late tonight/early Thursday, with precip expanding in coverage and
intensity over the Great Basin, Utah and Colorado terrain
Thursday. Day 1.5/2 snow probabilities are moderately high for 6
or more inches over much of the Sierra Nevada, the many ranges of
central NV, the Wasatch and high terrain of UT and the western CO
and south-central WY ranges. Heavy snow spills onto the central
High Plains Thursday night in strong convergent banding forming
north of the surface low from southeastern WY eastward across
northern Neb with Day 2.5 snow probabilities moderately high there
for 6 or more inches. The heavy snow swath then shifts northeast
Friday with the now occluded low resulting in an expanded comma
head that may bring snow into northern KS (the ECMWF historically
does a good job depicting comma head/wrap around snow on large
Plains lows like this). Day 3 probabilities for 6 or more inches
are moderate from northeast Neb across northern IA, southeastern
MN, and central to northeastern WI.


...Maine...
Day 1...
An upper level trough shifting east from the Ohio Valley today
allows surface low pressure to develop off the Mid-Atlantic coast
this afternoon and rapidly develop as it tracks northeast to a
point off Nova Scotia by late tonight. As the upper trough takes
on a negative tilt over New England this evening, the expanding
outer band northwest of the low center approaches and likely
reaches the Maine coast, particularly Down East Maine late tonight
just ahead of the mid-level trough axis. Heavy snow is likely to
develop in this outer band/TROWAL with Day 1 WPC snow
probabilities for 6 or more inches above 20 percent for the
eastern half of the Maine Coast increasing to 40-60% for the
shoreline on Passamaquoddy Bay.


...Pacific Northwest...
Days 1-3...
Northwesterly onshore low level flow with embedded shortwave
trough activity persists onto the WA/OR coasts today through
Thursday night with flow backing to southwesterly ahead of a very
potent cold front and dominating trough that approaches from the
Gulf of Alaska Friday night/Saturday. Snow levels drop to around
1500ft today under height falls and persist there through Thursday
night before they rise in the increasing moisture Friday/Friday
night. The consistently moderate precip rates make for some
notable mountain snows in the Cascades the next few days. Day 1
snow probabilities are moderately high for 8 or more inches over
the WA Cascades, expanding to the breadth of the WA and OR
Cascades for Day 2. The higher snow levels and backing flow shift
the focus back to WA with Day 3 probs for 8 or more inches in the
WA Cascades and Olympics.


For Days 1-3, the probability of ice accretion over 0.1" is less
than 10 percent.

Jackson