Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Tue Mar 10 2020

Valid 00Z Wed Mar 11 2020 - 00Z Sat Mar 14 2020



...Western U.S....
Days 1-3...

Two significant systems, a closed low near the Southern California
coast, and a strong shortwave digging into the Pacific Northwest,
will bring significant snowfall to the mountains areas of the West
during the next 3 days.

In the Southwest and towards the Four Corners, the slow moving
closed low off CA will spin two pieces of energy around its
center, and move very little through Thursday before lifting
onshore Friday. This will maintain a persistent moisture feed and
mid-level divergence from Southern CA northeast into Utah and
Colorado. Snow levels will climb on this tropically sourced plume
of moisture and 700mb WAA, so the heavy snow will be confined
above 8000ft. Two distinct surges of moisture are likely, on D1
and again on D3, and WPC probabilities each day are high for 6
inches, first in the Sierra, and then into the San Juans and
Sangre De Cristos D3. Total snowfall over the 3 days may exceed 12
inches in the higher terrain, with 2 feet possible in the San
Juans and highest peaks of the San Bernadinos.

In the PacNW, a strong impulse will dig rapidly in confluent flow
from the Gulf of Alaska to approach the Washington State coast
Friday. Robust moist advection and increasing jet level diffluence
will produce widespread precipitation from Washington into Oregon
and through Montana. Snow levels will gradually drop, and heavy
snow is likely to overspread the Olympics, Washington Cascades,
and into the Northern Rockies. WPC probabilities on D3 are high
for 6 inches in these ranges, with more than 12 inches most likely
in the Olympics.


...Northern New England...
Day 3...

Potent shortwave and accompanying vorticity lobe will move
eastward in fast northern stream flow across the Great Lakes and
into New England Friday. Below this, a surface low will race
eastward across southeast Canada, with secondary wave development
possible along a warm front across New England. This will spread
precipitation across New England on WAA southeast of the primary
low in Canada. The thermal profiles will generally be too warm for
snow, with the exception being far northern Maine and the White
Mountains of NH where some light snow accumulation is likely.
Additionally, as the associated cold front digs eastward late in
the period, cold air will rapidly drop into the region as the
precipitation begins to exit. This could lead to some p-type
transition to snow on the back side, but cold air chasing the
moisture should not lead to heavy snowfall, and WPC probabilities
for 4 inches on Friday are less than 20% and confined to far
northern Maine and the terrain of NH.


Weiss