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

Valid 00Z Fri Dec 09 2022 - 00Z Mon Dec 12 2022

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

An vigorous mid-level shortwave low over the central High Plains
this afternoon will continue shifting east to IA tonight before
weakening over IL/WI and MI on Friday. Atop this low, a
subtropical jet streak stretching across the entire CONUS from
southern CA to NJ will ripple around the low like a stone in a
stream through tonight 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 IA tonight.
Downstream of this low, warm/moist advection increases with
associated lift forming a wintry mix zone ahead of/under the low
with snow to the north. Enough moisture/frontogenesis wraps around
the 700mb low continuing to make a TROWAL through a saturated DGZ
to produce bands of heavy snowfall. The laterally translating
fgen-driven band of snowfall with rates of up to 1"/hr according
to the WPC prototype snow band tool/12Z HREF will move eastward,
leading to rapid accumulation, especially from eastern SD through
southern MN, northern IA, and central WI Friday evening through
Saturday morning. Day 1 WPC probabilities for more than 4 inches
are 10-30% from southeast SD, along the 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.

In the wintry mix zone, an axis of locally moderate freezing rain
will shift from eastern Neb to northwest IA/southeast SD/southern
MN/ this evening with a fine line of freezing to plain rain
(currently near Omaha). The 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 after 00Z are limited to
northeast IA into southwest WI at 20-40%. However, the risk of
continued glaze over northeastern Neb/northern IA into this
evening is notable.



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

The first of a pair of waves is pushing inland over the Northwest
and into northern CA through tonight. A modest atmospheric river
associated with this (about 250 kg/m/s IVT) brings in moisture,
but its zonal, easterly direction, and a progressive speed reduces
its inland impacts with precip ending by the western slopes of the
northern Rockies Friday. Day 1 WPC snow probs for 8 or more inches
are moderately high for the higher entire length of the Cascades
and the far northern Sierra Nevada as well as the Wallowa area of
the Blue Mtns in OR and higher peaks in ID.

A much stronger wave is currently shifting southeast from the
Aleutians and will amplify off the BC/Pac NW coast Friday before
shifting to the coast through Saturday. A reinforcing wave digs
offshore Saturday, closing off the trough into an expansive low
centered near the OR/CA coast which then shifts inland across the
state of CA Sunday. Pronounced synoptic ascent will be accompanied
by moderately-strong IVT exceeding 500 kg/m/s shifting onshore
Friday night/Saturday, with snow levels climbing to 4000-6000 ft
on the strong WAA. Snow levels will then drop Saturday night to
around 3000ft under the low. An impressive snowfall event is
expected above these levels, with orographic enhancement likely
driving both intense snow rates and deep snow accumulations. Day 2
snow probs for 18 or more inches are moderately high for the
Shasta/Trinity/Siskiyou region, expanding down the length of the
Sierra Nevada into Day 3. Local 2-4 ft of snow is likely for these
CA ranges through Day 3.

Farther north over OR and WA, onshore flow downstream from the
near shore trough brings nearly continuous precip tonight into
Sunday. Snow levels remain generally around 1000ft tonight then
rise to around 3000ft Friday through Saturday. Day 2 snow probs
for 6 or more inches are high for the Olympics and WA/OR Cascades
as well as the arc of ranges from eastern OR (Blue Mtns) to
central ID (Sawtooth Mtns).

The trough around the low shifts inland enough Sunday to bring the
focus for heavier precip into NV/the Great Basin and to southern
CA with moderate Day 3 snow probs for 6 or more inches over the
5000 ft (NV) to 7000ft (SoCal) snow levels during the heaviest
precip.


Jackson