Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 PM EST Thu Feb 06 2020

Valid 00Z Fri Feb 07 2020 - 00Z Mon Feb 10 2020

...Northeast...
Days 1/early Day 2...

The models show a strong front with a developing wave along the
front tonight becoming a closed low sfc-700 mb Fri morning.  The
rapid intensification of the low is expected to produce rapid
post-frontal cooling in OH and then across PA/NY, with rain
changing to snow. The well defined mid level
deformation/frontogenesis zone results in snow coverage/intensity
picking up in western NY tomorrow morning and continuing into
central and then northeast NY as the day progresses.  The models
have a large spread in the amounts of QPF when it is cold enough
for snow across western and central NY.

Heavy snow is expected in northern NY across northern VT and NH up
into Western Maine with Day 1 probabilities for a foot or more
moderate from near Lake Ontario, increasing to high for the
northern Adirondacks, northern Green Mtns, and
northern Maine.

Low-mid level warm advection across eastern NY and New England
occurs over surface sub freezing air, resulting in significant
freezing rain across northeast NY (including the southern
Adirondacks and upper Hudson Valley), Green Mtns of VT, the
Monadnocks of NH...and eastern Maine on Day 1. Amounts of a
quarter to third of an inch are expected.  Lighter freezing rain
amounts of a tenth or two are expected in the precip type
transition zone in southwest to central NY and in the Berkshires
of MA and northern Worcester Hills of MA.

The departure of the low Friday night up into the Canadian
Maritimes ends the synoptic portion of the event, with precip in
Maine winding down during the evening.

...Northwest...

Days 1-2...
Confluent flow arcing from the Pacific Northwest and northern
Rockies continues low level frontogenesis with strong moisture
influx through Friday. Lift is aided by a strong jet stream across
western MT/ID down across WY into CO with embedded 300 mb
divergence maxima tonight.  The heaviest snow are expected from
the Bitterroots and Clearwater Mountains of ID down into the
Salmon River Mountains and Blue Mountains of OR.

The jet max starts moving east on Fri as the next wave approaches
from the northeast Pacific.  
The jet lifting allows snow coverage and intensity to decline
across WY and CO, so the heaviest snow is expected across
the WA cascades as the west to east jet level flow Fri night
produces moisture fluxes onshore and inland. As the jet drops
south Sat, the favored left exit region of the favors snow
continuing in the Wa Cascades, and then developing
south into the OR Cascades.  48 hr probabilities for 18 or more
inches are high for the WA Cascades, with 2 day snow totals of 4
feet in favored mountain upslope areas in steep terrain. 

Snow coverage and intensity wanes on Sun as the upper trough moves
east and an upper level ridge approaches WA/OR from the Pacific.

... Northern Great Plains across Upper MS Valley to Western Great
Lakes...

Days 2/3...
The next shortwave emerging from the northern Rockies triggers
development of a closed 850 mb low expected in eastern MT Sat,
continuing across SD Sat and then near the MN/IA border early Sun.
A low-mid level frontogenesis max develops north of this 850 mb
low track.
Snow develops where it remains colder and gradually increases in
the 850-700 mb frontogenesis axis across central SD into southern
MN and central WI. A mesoscale band of 6-10 inches is possible in
these areas. 
The uncertainties involve precip type with a change over to mixed
types and rain along and south of the 850 mb low track, along with
a tight QPF gradient north of the frontogenesis zone.

Good agreement among global guidance Saturday night and Sunday
warrants moderate probabilities from eastern SD across southern MN
and western WI. 

The probability of freezing rain greater than 0.25 inches is less
than 10 percent Days 2-3.

Petersen