FOUS11 KWBC 132106
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EST Fri Dec 13 2024

Valid 00Z Sat Dec 14 2024 - 00Z Tue Dec 17 2024


...Lake Ontario...
Day 1...

Lingering single band in westerly flow over Lake Ontario will
persist into this evening for the Tug Hill area before diminishing
around midnight. A few more inches are likely after 00Z.


...Western U.S...

A powerful shortwave trough rounding a low centered west of WA
will shift over the CA coast early Saturday before tracking over
the northern Rockies through Sunday. The next plume of moisture
arrives Sunday night into the PacNW Coast ahead of a weakening low
off Vancouver Island. A less potent onshore flow then expands over
the Northwest through Monday.

California...
Day 1...

A moderately strong atmospheric river will continue to stream in
over far northern California tonight before a potent and digging
vort lobe crosses the SF Bay area around 15Z Saturday. This sharp
trough will provide an intense focus for lift and orthogonal flow
to the Sierra Nevada on Saturday with a quick end Saturday evening
behind the trough axis. Snow levels linger around 5000ft over the
CA Cascades and Sierra Nevada through tonight before rising perhaps
500ft just ahead of the wave Saturday. Heavy snow through synoptic
and upslope flow will extend along the length of the Sierra with
high rates persisting longest over the northern half. Per 12Z HREF,
hourly rates exceeding 1"/hr resume around 08Z for the northern
Sierra where they persist until about 22Z with the southern Sierra
seeing heavy rates from about 18Z to 00Z. Max rates exceed 2"/hr
for a few hours as the max precip works its way south down the High
Sierra 14Z to 22Z. The additional snowfall maxima for Day 1 alone
is around 36" in the highest western slopes of the northern and
central Sierra.


Pacific Northwest through Northern Rockies...
Days 1-2...

Farther north, the closed mid-level low pushes ENE over Vancouver
Island late tonight. Persistent moist advection into the Pacific
Northwest occurs through Saturday night as an additional
reinforcing shortwave trough swings through on Sunday driving
widespread heavy precipitation into the Northern Rockies through
Sunday. Snow levels will fluctuate through the period, but
generally remain around 4000-5000 ft. However, strong ascent and
convection could result in locally lower snow levels during heavier
activity. This configuration of back-to-back waves consolidating
over the NW focuses the inland heavy precip on central ID terrain.

Day 1.5 WPC probabilities for >8" are moderately high (50-80%)
along the WA/OR Cascades and the Olympics and high (>80%) over the ,
Salmon River/Sawtooths in ID along with the Wallowa Mtns in
northeast OR. This then expands to western WY down to the Wasatch
where Day 3 snow probs for >8" are 50-80%, highest in the Tetons.
Expect considerable impacts in the passes here.

...Pacific Northwest...
Day 3...

The next wave has an initial burst along the Cascades early Monday
with snow levels around 2500ft in WA and 3500ft in OR. However, the
weakening low and diminishing moisture influx means more moderate
rates through the rest of Monday as snow levels slowly rise. Day 3
snow probs for >8" are 30-60% for the Cascades above those snow
levels.



...Central Plains into the Upper Midwest...
Days 1-2...

A trough currently ejecting from the Central Rockies will amplify
into a deep low over KS tonight before shifting over northern IN
through Sunday. Height falls downstream of this developing low
will combine with weak but coupled jet streaks aloft and a residual
low-level baroclinic gradient to drive surface cyclogenesis over KS
by this evening. As this low progresses ENE, increased Gulf
moisture will be drawn north and around the low and over the I-80
corridor around IA which has been under and Arctic airmass the past
couple days, resulting in a wintry mix. Despite the fairly strong
warm air advection, the plain rain line looks to only modestly work
northward through Saturday night as precip continues over the
Upper Midwest. It will take some time for the DGZ to saturate as
the antecedent air is quite dry, so this setup is better for
freezing drizzle turning into freezing rain/sleet, and snow on the
northern end. Day 1 ice probs for >0.25" are 20-40% over both
western and eastern IA with coverage of >0.1" ice extending from
eastern Neb to just beyond the northern and eastern IA borders.


...Central Appalachians...
Days 2/3...

The same high pressure retreating eastward from the Upper Midwest
will drift over New England Saturday through Sunday, allowing a
strong Cold Air Damming (CAD) wedge to setup east of the
Appalachians over the Mid-Atlantic. Moisture from low pressure
moving into the Great Lakes shifts eastward over the central Apps
Saturday night. Air will initially be cold enough for snow,
especially in the higher elevations, before the robust WAA brings a
wintry mix that lingers over terrain and preferred higher central
App valleys into or through Sunday night. WPC probabilities for
>0.1" of freezing rain are highest 12Z Sun-12Z Mon (Day 2.5) with
70-80% over the Laurels of PA down into the MD Panhandle with >20%
probs extending from north-central PA through southern WV.


...Northern Plains...
Days 2/3...

The potent trough axis crossing SF Bay early Saturday lifts ENE
over the northern Rockies early Sunday with further development
over ND on Sunday. Overrunning flow ahead of the wave brings a
light freezing rain threat to northeastern MT Saturday night into
Sunday morning with the Day 2 ice probs for >0.1" 20-60%. Snow
bands then develop north of the low with Day 3 snow probs for >4"
around 20% for the north-central border of ND.


Jackson


$$

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