Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Mon Jan 13 2020

Valid 00Z Tue Jan 14 2020 - 00Z Fri Jan 17 2020

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

Unsettled weather pattern persists into the West as a prolonged
Pacific Jet pivots into the region, with periodic shortwaves and
waves of low pressure embedded within the flow. The first of these
shortwaves will move across Oregon/Idaho on D1, followed by a
second impulse moving onshore Oregon D2, and a third, more potent
impulse lifting onshore Thursday into Thursday night /D3/. Each of
these will be accompanied by height falls, surface fronts,
low-level southerly moist advection, and jet level diffluence. The
most robust combination of forcing and moisture will occur on D3
with the full latitude trough digging onshore, and a surface low
lifting slowly towards British Columbia. This will create the
heaviest snowfall of the period, and WPC probabilities are high
for 12 inches or more in the Oregon Cascades, the northern CA
mountain ranges, and spreading into the ranges of ID/UT/WY. On day
3, snowfall may exceed 3 feet in parts of CA. While snow levels
will likely be rising on D3 as the surface low moves northeast,
the first 2 days will feature extremely low snow levels, dropping
to 0-500 ft to as far south as 40N latitude by Wednesday morning
behind the first two impulses. This will allow snow to accumulate
into the valley floors, and a few inches of snow is possible even
into the Seattle, WA and Portland, OR metro areas. The amounts
should be relatively light outside of the terrain however, but WPC
probabilities on Day 1 are high for 12 inches in the Oregon
Cascades, Biterroots, WY ranges, and northern CA ranges, before a
lull occurs Tuesday night/Wednesday in between systems.


...Upper Midwest and Great Lakes...
Day 1...

A shortwave trough ejecting from the Northwest will move quickly
eastward across the Northern Plains, spawning weak cyclogenesis in
the Dakotas. This low pressure will move northeast on day 1 across
Lake Superior, and a band of moderate snowfall will likely
accompany this feature due to ascent forced by increasing
mid-level divergence as the trough tilts negatively, and modest
LFQ jet level diffluence. There will also be some enhanced
moisture transport ahead of the surface low, which will be wrung
out as snowfall in a stripe across eastern MN/WI, and again into
the Arrowhead of MN where upslope enhancement is possible. The
strongest forcing is expected from far NE IA into W-Central WI,
where the best overlap of DGZ saturation and forcing exists, and
snowfall may reach 1"/hr briefly. WPC probabilities for 4 inches
are modest in this swath, as well as on the north shore of Lake
Superior.

Days 2-3...
Yet another northern stream shortwave will dig out of the
Northwest Wednesday and move across the Northern Plains and Great
Lakes through Thursday. This will be accompanied by mid-level
moisture confluence and merging subtropical/northern jet streams
to produce more significant surface low development in the Central
Plains Wednesday. This low will then race northeast to Upstate New
York 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. Cold high
pressure ridging into the region will initially allow the entire
column to be cold enough for snow. However, as the low
strengthens, WAA ahead of it will enhance ascent, but also 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. At
this time snow amounts appear modest due to the rapid movement of
the system, but WPC probabilities are high for 4 inches of snow
from far eastern MN through norther WI, and into the U.P. and L.P.
of 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" are currently less than 10 percent.

There is also likely to be some LES behind the low on 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 low for 4 inches
however as mid-level ridging quickly returns to shut off the 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 should be confined to Northern New England, and amounts
are likely to be modest. However, there are some model signals
that the low may deepen significant just as it exits into the Gulf
of Maine, with a TROWAL lifting cyclonically around it into Maine
causing some enhancement late on D3. The most widespread WPC
probabilities for 6 inches are across central/eastern Maine, but
some higher snowfall is also likely in the terrain of the
Adirondacks and Green Mountains.


The probability of significant icing (0.25-inch or greater) is
less than 10 percent.

Weiss