Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Thu Dec 08 2022

Valid 12Z Thu Dec 08 2022 - 12Z Sun Dec 11 2022

...Central Plains to Upper Midwest..
Days 1-2...

A small but vigorous shortwave ejecting from the Central Rockies
will race eastward today while amplifying. This features is
progged to deepen into a closed low over Nebraska this evening and
then progress towards Lake Michigan by Saturday evening, driving
enhanced ascent through height falls and PVA. Atop this wave, a
subtropical jet streak stretching across the entire CONUS from CA
to NJ will buckle across the Central Plains resulting in a brief
period of enhanced upper diffluence to provide additional lift,
and this will yield a deepening surface low moving from eastern NE
through lower MI. Downstream of this low, moist isentropic upglide
at 290-295K will become impressive, and the accompanying theta-e
ridge will wrap into a modest TROWAL. The associated WAA will help
drive locally intense omega, which may correlate well with a
saturated DGZ to produce periods of heavy snowfall. The guidance
has trended a bit north overnight, but some uncertainty still
remains, so the footprint of heavy snow will likely be a bit more
confined than what the models are depicting. However, within this
best WAA, a laterally translating fgen-driven band of snowfall
with rates of 1"/hr or greater according to the WPC prototype snow
band tool will move eastward, leading to rapid accumulation,
especially from eastern SD through southern MN, northern IA, and
central WI Friday evening through Saturday morning. WPC
probabilities for more than 4 inches reach 10-30% along the IA/MN
border, and locally exceed 30% in WI. Locally up to 6" of snow is
possible in a few areas where the best banding sets up as shown by
high 75th and 90th percentiles in the WSE plumes.

South of the heaviest snow, an axis of moderate to significant
freezing rain is forecast as the WAA aloft causes a p-type
transition from snow to rain while surface temps and wet bulb
temps remain below 0C. There is also the likelihood that a dry
slot wrapping in around the surface low may dry the DGZ to cause a
p-type transition from snow to freezing drizzle, with even some
more moderate freezing rain possible as the low-levels remain
extremely saturated over NE. WPC probabilities for 0.1" of
freezing rain reach 20-30% in NE where accretion efficiency should
be maximized despite lower QPF, with a secondary max possible over
far northeast IA into southern MN and WI. Here, QPF will be
heavier within the more pronounced WAA east of the dry slot, but
rates will also be more intense suggesting some runoff will occur
during the period of freezing rain. Still, WPC probabilities also
indicate a 20-30% chance for more than 0.1" of ice here as well.



...Pacific Northwest and California through the Central Rockies...
Days 1-3...

An active period across the West will begin today as a series of
shortwaves and associated atmospheric rivers (AR) advect into the
region. The first shortwave trough is progged to lift into OR/CA
this evening with downstream divergence, height falls, and leading
WAA driving ascent. At the same time, a modest Pacific jet streak
will surge towards the coast, combining with moist confluent
mid-level flow to push higher PWs onshore to create a weak AR
noted by CW3E probabilities for 250 kg/m/s IVT. The overall
transient feature and modest WAA will keep snow levels around
2000-3000 ft during the heaviest precipitation. While synoptic
ascent will be modest with this first round, upslope flow into the
terrain, especially around the OR Cascades southward through the
northern Sierra, will result in waves of heavy snow rates which
have a higher than 50% chance of exceeding 8 inches. Additional
heavy snow is likely into the WA Cascades, Northern Rockies, and
Blue Mountains of OR. These snow levels suggests impactful snow
will also accumulate at many of the Cascades Passes D1.

A brief break in forcing is likely the first half of D2 as
shortwave ridging briefly builds into the northwest. While snow
may not fully shut off in the terrain, it will be much lighter
during this period. However, a more pronounced shortwave will
likely close off west of OR D3 and then amplify rapidly with
anomalous height falls digging into CA by the end of the forecast
period. Pronounced synoptic ascent will be accompanied by IVT
exceeding 500 kg/m/s shifting onshore, with snow levels climbing
to 4000-5000 ft on the strong WAA. Snow levels will drop the
latter half of D3 as the closed low approaches however. This is
likely to be an impressive snowfall event above these levels, with
orographic enhancement likely driving both intense snow rates and
impressive snowfall accumulations by D3, especially in
Shasta/Trinity/Siskiyou region, the length of the Sierra, and into
the Sawtooth. WPC probabilities D3 for more than 12 inches are
higher than 50% in the Sawtooth, and above 80% in the CA ranges
where locally 2-4 ft of snow is likely.


Weiss