Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EST Wed Jan 15 2020

Valid 00Z Thu Jan 16 2020 - 00Z Sun Jan 19 2020

...The West...
Days 1-3...
Deep low pressure currently well off the OR/WA border will shift
northeast before merging with another low from western Canada
tonight off the BC coast. Shortwave energy will dig around this
merging low and dive south down the West Coast tonight through
Thursday, pushing inland over southern CA Thursday night. An
enhanced plume of Pacific moisture will stream inland ahead of
this trough with snow levels generally 1000 ft in WA, 2000 ft in
OR and far northern CA, 3000ft in northern/central CA, and 4000ft
in southern CA. Day 1 probabilities for 8 or more inches are high
for the Olympics and highest WA and northern OR Cascades, and
stretch southward into the Sierra and other northern CA ranges.
The heaviest snow on D1 is likely in CA, where 3 ft of snow is
possible in the northern Sierra, as well as the Siskiyous and
Shasta/Trinities.

The trough will be quite progressive as it ejects inland, moving
from the southern CA coast Thursday evening to across the central
Rockies by Friday evening. Therefore, snows across the Great Basin
are expected to be fairly light, on the order of 4 inches or less.
However, high probabilities for 8 inches continue in the Sierra,
as well as spreading eastward into the highest terrain of the San
Juans, Wasatch, and Colorado Rockies as moisture streams northward
ahead of the primary mid-level trough.

By Saturday, yet another shortwave will approach the PacNW,
accompanied by a warm front and significant moist advection. Snow
levels will be high along the coast, 3000-4000 ft, but 1000 ft or
less in the Cascades and points east. WPC probabilities are high
for 8 inches in the Olympics and the WA Cascades.


...Great Lakes...
Days 1...

Lake effect snow is expected behind the a low moving into New
England Thursday. The the favored NW snow belts should receive 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.


...Northeast...
Days 1-2...

A surface low moving across central New England will race into the
Gulf of Maine and then deepen rapidly Thursday night. Modest
ascent early will intensify through Thursday in response to the
deepening system and jet level diffluence/mid level height falls.
Additionally, guidance continues to indicate that a TROWAL may try
to wrap around the deepening low back into Maine, in conjunction
with an inverted trough to enhance snowfall potential locally. 
Much of central/southern New England will be too warm for snow,
but from the Adirondacks, through northern VT/NH and
central/eastern Maine, a swath of heavy snow is likely. WPC
probabilities are high for 6 inches in the northern Adirondacks,
and the Green and White Mountains. There is a secondary area of
higher probability across portions of eastern ME as well where the
deepening low may cause local enhancement of snowfall.

Day 3...

A warm front lifting northward ahead of a rapidly moving but
strong low pressure in the Great Lakes will spread precipitation
into the Mid-Atlantic and New England on Saturday. Initially, the
atmosphere is cold enough to support snowfall as the column is
dominated by cold high pressure. However, robust WAA with the warm
front will quickly transition precipitation from snow to mix,
especially in the Mid-Atlantic. Antecedent dry low-levels may
allow for significant wet bulb cooling affects and there may be a
stripe of moderate sleet/freezing rain, focused in the higher
elevations. WPC probabilities for 0.1" of freezing rain are
moderate in the terrain of WV/VA/PA. More snow is expected further
NE into Pennsylvania, and WPC probabilities are modest for 4
inches towards the Poconos.


...Great Plains through the Great Lakes...
Days 1-3...

Significant storm system will begin to develop late tonight in
response to a deepening mid-level trough ejecting across the
Mountain West. This feature will elongate and begin to tilt
negatively into the Missouri Valley Saturday, forcing downstream
jet streak intensification which will place the strengthening LFQ
and its associated diffluence across the Plains and into the Great
Lakes. Beneath these features, a surface low pressure its likely
to develop in the lee of the Rockies, and then move progressively
northeast while intensifying modestly by the time it reaches the
eastern Great Lakes late on D3.

Moisture will begin to develop in the Panhandles of TX/OK late D1
/Thursday/ as isentropic lift and WAA spread atop a cold Canadian
high pressure. The WAA and isentropic ascent will gradually
intensify into D2, spreading moisture across much of the
Southern/Central Plains and into the upper Midwest. The column
will initially be cold enough for snow north of OK, but the strong
WAA (850mb southerly winds 30-40 kts) will quickly overwhelm the
cold column driving a warm nose and deep warm layer northward.
While the magnitude of the warm nose may not be extremely strong,
the depth of the warm layer will be too strong for much sleet, and
expect a large area of moderate to potentially heavy freezing rain
from the Panhandle of TX through OK, KS, MO, and into IA/IL/IN.
The heaviest freezing rain is likely to accrete across MO/KS/OK
where WPC probabilities for 0.25" are generally 30-40%. Heavy rain
rates and marginal surface temps will preclude greater accretions.
Eventually, the warm nose should completely overwhelm the column
and turn precipitation to rain across the Southern Plains.

Further north, significant snowfall is likely. Continued robust
moist advection on record 850mb winds of 50-60 kts will drive
PWATs to 2-3 standard deviations above the climo mean. This
intense ascent combined with height falls and the aforementioned
increasing LFQ diffluence will drive increasing coverage and
intensity of snowfall from the eastern Great Plains eastward
through MN/IA/WI and into the Great Lakes region. WPC
probabilities for 8 inches are highest from near Minneapolis
through Wisconsin, and both Peninsulas of Michigan. Progression
movement of the surface low and lack of significant
TROWAL/deformation to toss moisture back into a comma head will
limit more than isolated amounts over 12 inches.


Weiss