Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EST Sat Jan 16 2021

Valid 12Z Sat Jan 16 2021 - 12Z Tue Jan 19 2021

...Central Appalachians, Great Lakes, and Northeast...
Days 1-3...

Complex mid-level and surface pattern will produce waves of
precipitation and snowfall including synoptic snow, upslope snow,
and lake effect snow, through early next week.

Today, an area of low pressure developing south of New England
will deepen and lift north into Maine tonight. Ahead of this low,
impressive warm and moist advection, aided by height falls and PVA
associated with a spoke of vorticity/shortwave lifting northward,
will cause precipitation to expand and shift northward, spreading
rain and snow into the Northeast. The antecedent column is cold
only across Maine, with marginal low-level thermals in place
elsewhere. This will lead to a challenging p-type forecast as the
heavy snow will be dependent both upon elevation and dynamic
cooling. For the former, snow levels are forecast to rise as high
as 1500ft, so elevations above that in the Adirondacks, Greens,
Whites, and much of northern/central ME will see mostly snow and
heavy accumulations. WPC probabilities are high for 6" in these
areas, and may eclipse 1 ft in the highest terrain due to snowfall
rates  in excess of 1"/hr at times. At the lower elevations,
precipitation will likely change to rain for much of the event,
however, periods of heavy snow are possible where ascent is most
robust causing the column to dynamically cool. Strong WAA and weak
elevated instability will combine to produce a few inches of snow
in the Berkshires and Catskills, with light accums even possible
in the Worcester Hills. However, due to varying precipitation rate
and p-type, confidence is a bit lower below 1500ft, and note the
WSE ensemble mean came down quite a bit overnight in these lower
elevation regions. Some lingering heavy snow is expected into
northern Maine to start D2, but as the system pulls away the heavy
accumulation across New England will wane.

Behind the low, a deep closed mid-level low and strong CAA behind
a cold front will lead to upslope snow across WV which may
accumulate more than 6". A secondary impulse moving over the
region Sunday into Monday will renew the CAA and upslope
potential, with additional light accumulations possible both days.
Additionally, the CAA moving across the nearly ice-free Great
Lakes will produce periods of LES, some of which is likely to be
heavy, especially downwind of Lake Erie and Ontario. Nearly
unidirectional flow and enhanced instability both due to the CAA
atop the lakes but also due to the upper low overhead should
produce heavy snow rates into the Tug Hill Plateau D1 and D2, with
a renewed surge enhanced LES off Lake Erie D3. WPC probabilities
D1 and D2 are high for 6 inches east of Ontario, and high for 6
inches east of Erie on D3. Total LES could exceed 12" east of Erie
and 18" in the Tug Hill.

Additionally, although accumulations are expected to be light, the
upper low behind the cold front could produce an environment
favorable for heavy snow showers or even snow squalls across a
large portion of the Appalachians and into the Mid-Atlantic this
afternoon and evening. A brief uptick of instability rising to
near 200 J/kg combined with modest 925mb fgen and enhanced
mid-level RH supports some convective potential. While the snow
squall parameter does increase, 0-2km theta-e lapse rates are
minimal, so its more likely that snow showers will occur this
aftn, but cannot rule out a squall. Accumulations will be minimal,
but brief visibility restrictions and hazardous travel are
possible should any snow squalls develop.


...Pacific Northwest to Northern Rockies...
Days 2-3...

Broad mid-level ridge will give way to an approaching shortwave
trough from the Pacific, accompanied by the RRQ of an equator-ward
diving jet streak on Sunday. This will be followed quickly by a
second shortwave and another jet streak to produce periods of
precipitation from WA state through WY into early next week. While
the duration and intensity of ascent appear to be modest at any
given time, persistent moist advection from the Pacific will be
tapped by this forcing to produce heavy snow in the terrain,
generally above 5000-6000 ft. Where upslope enhancement can occur
from the W/NW mid-level flow, snowfall during the 2 days may
exceed 1 ft, and this is most likely in the Absarokas, Big Horns,
and Black Hills. Otherwise, WPC probabilities are modest for 6"
across much of the remaining terrain.


...Southern Rockies...
Day 3...

A trough and accompanying Pacific jet streak will dive down the
west coast Monday. Two shortwaves embedded within this amplifying
trough will merge into a closed low near the coast of Southern
California, driving mid-level WAA and moist advection towards the
Four Corners in conjunction with enhanced mid-level divergence. At
the same time, a cold front will dive across the Plains and bank
against the east side of the Rockies including the Sangre De
Cristos. While this front likely won't make much further progress
SW, it will lead to enhanced upslope flow, and heavy snow is
likely in the Sangre De Cristos, San Juans, and even into the
Front Range Monday and Monday night. While heavy snow is likely in
these ranges, especially east/northeast slopes, on the downwind
side there may be some shadowing, as there appears to be a gap
between the best low-level ascent and upper forcing. This leads to
WPC probabilities for heavy snow to be confined to the terrain and
not bleed into the lower elevations of NM. WPC probabilities for
D3 are high for 6 inches in the Sangre De Cristos and San Juans,
with locally more than 10" possible.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss