Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Sat Oct 27 2018

Valid 00Z Sun Oct 28 2018 - 00Z Wed Oct 31 2018

...Day 1 ...

...New England and Upstate New York...

The nor'easter moving north across New England will result in
widespread precipitation advancing northward across Upstate New
York and interior New England this evening but winding down as the
system departs Sunday.   With a cold surface high initially in
place across the northeast U.S., temperatures will be marginally
cold enough to support a brief period of snow for the Adirondacks,
Green, White Mountains, and ranges of western Maine.  Snowfall
rates should be high enough to support some 2 to 4 inch amounts,
with isolated totals in excess of 4 inches.

Strong warm air advection in the 850-700 mb layer is expected to
result in a transition to mixed precipitation and then light
freezing rain for interior locations in northern NH and Maine,
where spots of one to two tenths of an inch is likely.  WPC
probabilities indicate a low to moderate probability of 0.25 inch
of ice accretion for day 1 over northwest Maine. This threat ends
Sunday as the storm departs. 

...Wa Cascades...
 A strong shortwave tracking towards the Pacific Northwest on
Sunday morning will lead to height falls and strong onshore flow,
along with a strong upper level jet.  Moisture surging northward
ahead of the impulse will be lifted due to positive vorticity
advection and also divergence in the left exit region of the upper
level jet.  Steep lapse rates follow with the passage of the cold
front, with snow levels falling.  Several inches of snow are
possible in favored windward portions of the northern Wa Cascades
Sunday.

...Days 2/3...

...Wa/Or Cascades and Northern Rockies...

On day 2 (Sunday night-Mon), as the upper jet moves from Wa/Or
across ID and southern MT, the moisture plume progresses across
the northern Rockies.  Upper divergence in the left jet exit
region combines with enhanced upslope flow in windward terrain to
produce snow from the Wa/Or Cascades inland to the Absaroka
Mountains and Bitterroots, and then ranges of western MT.

WPC day 2 probabilities are moderate for 4 inches of snow/low for
8 inches for the higher mountains of the northern Rockies.

On day 3 (Mon night-Tue), the upper trough departs the Rockies for
the Plains.  The next 300 mb jet streak moves onshore into the
Pacific northwest.  Moist confluent flow under the jet produces
bands of enhanced moisture and lift from the northern Wa Cascades
and then into the ranges of ID and northwest MT Tue. Several more
inches of snow are possible, especially where longer duration snow
showers persist in the northern Wa Cascades to the Bitterroots of
ID and ranges of northwest MT.

The probability for significant icing (0.25 inches or more) is
less than 10 percent days 2 and 3.

Petersen