FOUS11 KWBC 280917
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EST Sat Dec 28 2024

Valid 12Z Sat Dec 28 2024 - 12Z Tue Dec 31 2024


...The West...
Days 1-3...

Ongoing wintry precipitation over the Northwest and northern
Rockies terrain into Monday before tapering off for at least a
day.

Broad ridging over the eastern Pacific and trough over the Gulf of
Alaska drive impressively confluent and zonal flow with embedded
impulses into the Northwest through tonight before a notable trough
pushes ashore Sunday and tracks over the northern Rockies through
Sunday night. At atmospheric river centered along and south of the
jet maximum and its associated high snow levels (6000-8000ft) will
shift north from CA through OR today/tonight while snow levels
remain lower over WA (around 4000ft). Day 1 snow probs are 50-90%
for the CA and OR Cascades with most WA passes impacted. Moisture
already farther inland will continue to produce heavy snow in
terrain over the Rockies from northern CO through northern
UT/western WY, central ID, and western MT where Day 1 snow probs
for >8" are 50-90%.

The base of the trough/vort max digs south to the far northern CA
coast by Sunday morning. This will surge heavy precip up the PacNW
which will be accompanied by rapidly falling snow levels under the
trough, creating a heavy snow situation for much of the Cascades
Sunday. Snow levels drop below 4000ft Sunday afternoon in OR and
3500ft in WA. Day 2 snow probs for >8" are 60-90% along the extent
of the Cascades.

The AR surge inland means increasing snow levels Sunday over the
northern Rockies with values generally 6000ft in central ID before
dropping to around 4000ft Sunday evening before ridging cuts off
precip late Sunday night. Day 2 snow probs over the northern
Rockies are 60-90% for >8" in the Bitterroots, Salmon
River/Sawtooths, and around Yellowstone including the Red Lodge and
Tetons down toward the Wind River.

Moisture will get shunted east of the Northern Rockies late Sunday
night into Monday though high pressure building south from the lee
of the Canadian Rockies will maintain snow in and around Glacier
NP. Day 3 snow probs for >8" are 60-80% there. A much needed break
looks to last 24-36 hours over the rest of the Northwest on Monday.


...Mid-Atlantic and Northeast...
Days 1-2...

Pockets of light icing persist this morning over interior sections
of the northern Mid-Atlantic and interior New England. Freezing
drizzle is a concern tonight for portions of inland Maine. Further
light icing is possible over northern Maine Sunday night ahead of
the next system tracking up the Great Lakes. These icing totals are
expected to stay below a tenth inch.


...North-Central Plains...
Day 3...

The powerful trough that shifts over the Pacific Northwest coast
Sunday will dive ESE on a 150kt jet and reach southern KS by
Monday, pushing into MO that evening. This will produce an
efficient overlap of height falls and diffluence atop a warm front
to drive sfc cyclogenesis over northern OK into northern AR. Warm
and moist advection ahead will lift into a modest TROWAL, providing
support for ascent and a somewhat narrow swath of snow from
northeast WY and along the SD/Neb border before tracking into less
favorable thermals over IA Monday night. Day 3 snow probs for >4"
are around 50% for the Black Hills and around 20% for much of SD.
There is latitudinal differences in guidance with the 00Z ECMWF/GFS
farther south/more into Neb than the farther north (and heavier)
Canadian.

Jackson
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