Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Sun Dec 06 2020

Valid 00Z Mon Dec 07 2020 - 00Z Thu Dec 10 2020

...New England...

Day 1...
The Nor'easter currently over New Brunswick will continue to lift
north this evening with ongoing light to possibly moderate wrap
around snow over northern Maine will taper off before midnight
tonight. Day 1 snow probabilities are low for an additional 2
inches this evening.

Day 2...
The next phasing system will develop as it tracks north-northeast
from off Cape Hatteras Monday afternoon to points east of Nova
Scotia Tuesday. However, strong northeasterly flow under the
northern stream upper trough should provide enough lift and
instability to allow some ocean effect snow bands to spread across
southeastern Mass late Monday night into Tuesday. Convective
allowing models are not too strong with these bands as of yet, but
it will be cold enough for accumulating snow so this is worth
monitoring.


...South-central Appalachians and Mid-Atlantic...

Day 1...
Phasing between northern and southern stream shortwave troughs
will occur tonight over the TN Valley with mid-level low pressure
closing off Monday morning over the upper OH Valley that tracks
east as a second, reinforcing, northern stream shortwave digs
across the Carolinas. This begins to develop into a potent system
off the NC coast later Monday, but before then some light to
moderate precip will develop as it spreads from the Appalachians
to the southern Mid-Atlantic Piedmont. A band or two of
accumulating snow is possible with the south-central VA Blue Ridge
to Richmond area the most likely target east of the Appalachians
for an inch or more of accumulation per the 12Z consensus. Upslope
flow allows for low Day 1 snow probabilities for two inches back
along the southern Appalachians.


...Pacific Northwest...
Days 2/3...

A Pacific jet streak looks to mainly remain north of WA Tuesday
with reduced risk of heavy snow in the Cascades and northern
Rockies with only low probabilities for 4 or more inches in the
Day 2.5 time frame. However, some light to locally moderate precip
spreads east of the northern Cascades Tuesday/Tuesday night which
given cold antecedent conditions in the northern Columbia Valley
may need to some thicker ice accretion. As of now the Day 2 ice
probabilities are around 10 percent for a quarter inch or more of
ice in north-central WA in the lee of the Cascades.


Jackson