Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sun Mar 03 2019

Valid 12Z Sun Mar 03 2019 - 12Z Wed Mar 06 2019

...Mid-Atlantic and Northeast...
Days 1-2...

A shortwave moving through the TN VLY will work in tandem with an
increasingly coupled 300mb jet structure to produce surface
cyclogenesis across the Southeast today. This low will then travel
northeast to off the VA coast tonight, and then race just outside
of the Benchmark and into Nova Scotia Monday evening.

On the northwest side of this low, snowfall will spread across the
area, with light accumulations from Missouri through Ohio, with
heavier snowfall likely east and northeast from there. Although
the low will move rapidly thanks to a modest and open 700mb wave,
guidance suggests a period of heavy snowfall is likely from WV and
points northeast into Maine. It is in this region where the best
overlap of synoptic ascent through jet level diffluence and
mesoscale forcing due to increasing frontogenesis and mid-level
deformation will occur.

The models have trended subtly weaker and further southeast
overnight, and the highest accumulations are forecast along and
just NW of I-95 from Philadelphia to Boston, and along the coast
of Maine. A 4-8 hour window exists where snowfall may approach or
exceed 1"/hr where the aforementioned fgen and deformation
overlap. Some guidance is indicating this will occur in the
NYC-PHL corridor, where low-level thermal advection occurs in
conjunction with the frontogenesis to produce a burst of snowfall.
However, best forcing is below the DGZ during this time so
snowfall efficiency may be limited and the high snow totals being
predicted may be overdone. The highest snowfall accumulations are
more likely from NYC to Boston, with the highest numbers actually
in Downeast Maine as the low wraps up into Day 2. Snowfall rates
of 1"/hr are more likely in this region, and with snowfall ratios
of 10-12:1 expected, WPC probabilities are above 50% for 6", with
some amounts near 8" possible, and even isolated 10" in far
eastern Maine. By Monday afternoon the low will be wrapping up and
moving away to the northeast, bringing an end to the accumulating
snow everywhere but eastern Maine.

Some mixing of precipitation type to sleet and freezing rain is
likely on the southern periphery of the snow swath, but accretion
is expected to be light and WPC probabilities are less than 20
percent for 0.1 inches.


...California, Great Basin, into Central Rockies..
Days 1-3...

Weakly cyclonic flow aloft across the West will persist today as a
Pacific Jet max and modest shortwave eject eastward from
California into the Central Rockies. The forcing for snowfall will
be nearly zonal from northern California into Colorado, with the
best overlap of LFQ diffluence and low-level confluence between a
Pacific airmass and Canadian airmass behind a cold front will
occur across Colorado through tonight. This ascent in an airmass
of weakly anomalous PWAT will produce heavy snow in a narrow
corridor above 4000 ft, with high probabilities for 8" confined to
the Wasatch, Colorado Rockies, and San Juans, where over 12" is
possible.

After a respite from heavy snow on day 2 in response to ridging
blossoming across the Great Basin, a renewed trough and
accompanying surface low will move towards California on Tuesday.
An associated plume of moist Pacific air of tropical origin will
raise snow levels, but also be ripe for heavy snow as PWATs soar
to 3-4 standard deviations above the climatological mean. Despite
the high snow levels, heavy snowfall is likely above 7000 ft in
the Sierra where WPC probabilities feature a high risk for 12
inches or more of accumulation. As moisture spills eastward,
several inches of snow is also possible in the high terrain of the
Wasatch and ranges of Colorado, as well as into the Cascades of
Oregon.


Weiss