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

Valid 00Z Sun Jan 17 2021 - 00Z Wed Jan 20 2021

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

An area of low pressure is forecast to move north across Maine
tonight and into adjacent New Brunswick Sunday.  Ahead of this
low, warm and moist advection is forecast to lead to mid level
temperatures briefly rising above freezing. This will lead to a
challenging p-type forecast as the snow in northeast Maine mixes
with and changes to sleet and freezing rain tonight. The mixed
precipitation aspect of the event ends Sunday as the low moves
away.

Behind the low, a deep closed mid-level low and strong cold
advection behind a cold front moving across the nearly ice-free
Great Lakes will produce periods of lake effect snow downwind of
Lakes Erie and Ontario. Nearly unidirectional flow and enhanced
instability due to the steep lase rates should produce heavy snow
rates into the Tug Hill Plateau into Sunday night.
Snow coverage and intensity taper on Monday as drier air aloft
moves across Lake Ontario and then into the Tug Hill and western
Adirondacks. 
As an 850-700 mb approaches from Michigan on Tuesday, a renewed
surge of enhanced Lake Effect develops off Lake Erie and then
moves into eastern Lake Ontario and the Tug Hill towards the end
of the day. WPC probabilities D1 are high for 8 inches east of
Lake Ontario, and high for 6 inches east of Erie on D3. Days 1-3
snow amounts could exceed 12" east of Lake 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 central Appalachians this evening. A brief
uptick of instability rising to near 200 J/kg and enhanced
mid-level RH supports convective potential until instability wanes
tonight.  Post-frontal west-northwest winds will lead to upslope
snow across WV which is forecast to accumulate to 4-8" days 1-2. 
A secondary impulse moving over the region Sunday into Monday will
renew the CAA and upslope potential.


...Pacific Northwest to Northern and Central Rockies...
Days 1-2...

Confluent upper level flow drives a 700 mb shortwave onshore and
advects moisture inland.  This leads to a period of snow showers
starting the in the WA Cascades and then moving downstream into
the northern Rockies Sunday.  Several inches are possible in the
mountains. Locally heavy snow is expected over a few of the ranges
of central MT, the Bighorns, and Laramie/Snowy Mountains of
WY/adjacent northern CO Sunday night into Monday as moist
confluent flow with 700 mb vertical velocity maxima stream across
the region. 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
in windward areas, snowfall during the 2 days may exceed 1 ft, and
this is most likely in the Little Belt Mountains, Big Horns, and
Black Hills. The event winds down late Monday as the 700 mb ridge
bridges into the northwest and northern Rockies, drying out the
column.

...Southern Rockies/Southwest...

Day 3...

A low-mid level circulation forms over southern CA on Monday night
to Tuesday morning, drifting south into northwest Mexico. The
circulation taps Pacific Ocean moisture and wraps it around the
low, advecting it inland. Upper level divergence in the vicinity
of a jet streak extends from southern CA across AZ and into
southern CO and northern NM.
Lit near the jet along with moisture advecting inland leads to
snow developing as heights/temps aloft fall.  A max is expected
int the terrain of southeast UT across the San Juan Mountains of
southern CO/northern NM.

A cold front will dive across the Plains and bank against the east
side of the Rockies including the Sangre De Cristos. Post-frontal
enhanced upslope flow should lead to heavy snow
in the Sangre De Cristos Monday night into early Tue. WPC
probabilities for D3 are moderate for 8 inches in the Sangre De
Cristos and San Juans, with a foot of snow possible in windward
terrain.

As height/temperatures aloft fall, snow may develops in the ranges
of northern AZ and southern CA, with several inches in the higher
elevations.

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

Petersen