Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Tue Jan 14 2020

Valid 12Z Tue Jan 14 2020 - 12Z Fri Jan 17 2020

...The West...

Day 1...
A prolonged active winter pattern in the northwest CONUS continues
through tonight as onshore flow revolves around a deep cold-core
low centered over the central BC/Alberta border. This low then
retrogrades off BC as it merges with the next low through
Wednesday night. The trough from this combined low sweeps down the
CA coast through Thursday, pushing inland Thursday night.
Northwesterly onshore flow keeps moisture and snow (from
particularly low snow levels) through tonight from the coast to
the Northern Rockies. Day 1 WPC probs are high for 6 or more
inches for the WA/OR/CA Cascades, and the northern Rockies of
ID/MT/WY/and the northern Wasatch of UT. There is a risk for a
plume of moisture to cross western WA and provide more
accumulating snow for the Seattle metro area tonight with Day 1
probs for 4 inches in the 20 percent range.

Day 2...
Flow becomes southwesterly along the Pac NW ahead of the next low
that swings up off the coast toward Vancouver Island as it begins
to merge with the western Canadian low. The trough from this low
shifts into the OR coast Wednesday night. This directs a deep
plume of Pacific moisture into far northern CA where snow levels
of 3000ft trigger high Day 2 probs for a foot or more for the
Shasta/Trinity Alps/Klamath Mtns of northern CA into southwestern
OR.

Day 3...
This plume of Pacific moisture shifts south in advance of the
trough down the CA coast to southern CA through Thursday. Day 3
probs are high for 12 or more inches from the CA Cascades down the
Sierra Nevada with snow levels generally 3000 to 4000ft over the
state. Snow levels look to be around 4000ft for southern CA when
the plume trough shifts inland.


...Upper Midwest and Great Lakes...

Days 2/3...
A shortwave trough currently shifting through OR will move quickly
eastward across the Northern Plains tonight. Moisture confluence
and merging subtropical/northern jet streams will produce strong
surface low development in the upper MS Valley Wednesday. This low
will then race east-northeast across New England through Thursday.
Deep layer ascent through jet level dynamics and height falls in
conjunction with the strengthening surface low will spread
increasing moisture into the region. Deep high pressure ridging
north of the track over northern Ontario/Quebec will provide
enhanced convergence on the north side of the low. On the south
side, warm air advection ahead of the low will push a warm nose as
far north as Michigan. This will cause a p-type transition from
snow, to freezing rain, and eventually to rain. Due to the fast
progression of this system, WPC probabilities are only moderately
high on Day 2 for 4 or more inches of snow from northern MN/WI and
across the U.P. MI. Further south, there is likely to be a stripe
of light freezing rain accretion in IA, IL, and southern WI.
However, WPC probabilities for 0.1" remain less than 10 percent.

Lake effect snow is expected behind the low Wednesday night and
Thursday, with the favored NW snow belts receiving the heaviest
snow downwind of Lake Superior, and the effective fetch areas off
Lake Michigan and Lake Erie. WPC probabilities are moderate for 4
inches of LES.


...New England...

Day 3...
The surface low moving from Upstate New York Thursday morning will
deepen across New England in response to increase ascent through a
negatively tilting shortwave and merging jet streaks. With the low
moving rapidly, and across central New England, significant
snowfall will be limited to how quickly the low will deepen. As
the low exits into the Gulf of Maine, with a TROWAL lifting
cyclonically around it into Maine causing enhancement Thursday.
Moderate probabilities for six inches are across eastern Maine on
Day 3.


...Southern Plains...

Day 3...
A southern stream jet streak in advance of the trough pushing
across the West Coast Thursday night sets up over the southern
Rockies. A surface trough develops over TX into OK with abundant
Gulf moisture spreading up the Great Plains. Cold air spilling
down the Plains Wednesday night will set preconditions for ice
over OK/KS where there are low probabilities for a tenth inch of
ice Thursday night.


Jackson