Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Mon Mar 02 2020

Valid 12Z Mon Mar 02 2020 - 12Z Thu Mar 05 2020


...Sacramento Mountains of New Mexico...
Day 2...

A closed upper low moving eastward across Baja today will shift
towards the Big Bend of Texas Tuesday night. Ahead of this low,
increased upper divergence and RRQ diffluence behind the poleward
shifting jet streak will drive ascent locally across Arizona and
New Mexico. Moisture in the vicinity of the Mexico/US border is
significant, but snow levels will be generally 6000-8000 ft, so
moderate to heavy snow is expected to be confined to the highest
terrain. The highest probabilities for 4 inches or more of snow
are in the Sacramento Mountains on Tuesday, with low probabilities
for a few inches extending westward into the Mogollon Rim.


...Pacific Northwest and Northern Rockies...
Days 1-3...

A series of mid-level impulses embedded within confluent flow will
lift onto the British Columbia coast through mid-week. These
features will be accompanied by modest height falls and PVA, and
at least modestly diffluent flow as a NW oriented jet streak dives
continuously into Washington State all 3 days. Although forcing is
modest during this time, the prolonged ascent and ample moisture
noted by surges of PW more than 1 standard deviation above the
climo mean will drive rounds of precipitation, with snow likely in
the mountainous regions. Snow levels D1/D2, when moisture is most
significant, are forecast to range between 3000-5000 ft, and WPC
probabilities are high for 6 inches across the Washington Cascades
into the Northern Rockies today, and again on Tuesday. Snowfall
should ease on Wednesday as the best forcing shifts southeastward
and PW anomalies drop. Total snowfall could eclipse 2 feet in
parts of NW MT, and 3 feet in the Washington Cascades.


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

A mid-level northern stream trough swinging through the Great
Lakes Tuesday into Wednesday will work in tandem with a
strengthening jet streak south of New England to induce surface
cyclogenesis on Wednesday. While there remains considerable
latitudinal spread in the placement of this surface low, the trend
has been south tonight, with much of the guidance pushing this low
east off Massachusetts and into the Gulf of Maine. Although the
feature is expected to remain progressive, significant ascent and
robust moist advection driving PWs to +1-1.5 standard deviations
above the climo mean, will support precipitation spreading across
New England. Some of this may be heavy as guidance indicates a
developing TROWAL may shift into Maine, while a region of
mid-level deformation wraps southeast from Canada. While the
column will likely be marginal for snow, especially at onset, as
the low moves eastward cold air is likely to wrap back into
Northern New England, especially Maine, and WPC probabilities for
6 inches have increased to 20-30% across Maine.


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

Weiss