Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Thu Feb 28 2019

Valid 12Z Thu Feb 28 2019 - 12Z Sun Mar 03 2019

...The West...

A continued active pattern across the West as multiple upper lows
interact off the coast before lifting inland and shearing off to
the east in a fast northern stream.

The first of these will drift south along the PacNW coast today
into Friday while a Pacific Jet south of this feature transports
moisture onshore. The best combination of height falls, upper
diffluence, and upslope enhancement on westerly 700mb flow will
occur across the central and northern Rockies on Day 1, and the
heaviest snowfall is forecast in this region. WPC probabilities
are high for 8 inches in the terrain of Colorado, Utah, NW
Wyoming, and western MT. The relatively zonal flow keeps snow
levels above 5000 ft south of 40 N latitude, falling to sea level
across MT. Light snow accumulations are also possible in the
northern Sierra and Oregon Cascades on Day 1.

The first low will open and move onshore during Day 2, with a
secondary low and more impressive moisture plume moving into
California late on Day 2 and into Day 3. The flow aloft remains
zonal/weakly cyclonic so snow levels will only gradually drop, but
longer moisture transport and better jet diffluence will spread
heavy snow from the Sierra through NV/UT/CO and even into the
highest terrain of AZ and NM. PWATs climbing to +3 standard
deviations above the climo mean will support heavy snow
accumulations of 1-2 feet or more in the Sierra, Wasatch, and
Rockies/San Juans of Colorado. Lighter accumulations are likely in
the lower elevations and points further south, with moderate
probabilities for 4 inches outside of these ranges.


...Northern Plains into the Northeast...
Days 2-3...

A wave of low pressure developing off the VA coast will lift
northeast along the offshore baroclinic gradient Saturday and
deepen in response to upper diffluence within the RRQ of a
departing jet streak. Guidance has trended northward with the path
of the surface low tonight, with the Euro/CMC now bringing several
inches of snowfall to the Southern New England Coast/Cape Cod/Long
Island on the northern periphery of this system. These are still
on the NW edge of the guidance envelope, but with a trend
northward of the GEFS mean and SREF as well, chances have
increased for some light snowfall from Long Island to Cape Cod,
including New York City and the entire Southern New England coast.
WPC probabilities have increased to around 30 percent for 2 inches
of snowfall centered around 12Z Saturday.

At the same time the surface low is skirting New England, a piece
of mid-level energy ejected from the Northwest will race eastward
embedded in the fast northern stream flow. This shortwave will be
accompanied by a jet max and at least modest pacific moisture to
produce a swath of snowfall from the Dakotas into Northern New
England Friday and Saturday. The rapid progression of the system
and limited total moisture should keep accumulations low, with WPC
probabilities for 4 inches less than 30 percent most of the area,
slightly higher in the Adirondacks on day 3.


...Mid-Atlantic...
Day 1...

A weak wave of low pressure will lift northeast from TN to off the
VA coast late tonight into Friday morning. WAA on southerly winds
ahead of this system will spread precipitation northward from WV
to the NJ coast. This WAA will be accompanied by a brief period of
diffluence within a weakly coupled jet structure and short but
robust mid-level frontogenesis to produce a burst of snowfall late
on Day 1. The NAM remains a strong outlier in terms of its column
saturation and duration of intense UVV, but a wetter and stronger
trend has been noted in guidance overnight. This has prompted a
subtle increase and northern shift in snowfall, and WPC
probabilities are up to 40-50% for 4 inches in a small region of
the terrain of WV/MD and southern PA. More widespread 1-2" of
snowfall is likely north and south of the 4 inch area, including
the Washington DC metro area. South and west of this region, some
light freezing rain is possible as the precipitation overruns the
cold surface air, but accretion probabilities for 0.1 inches are
less than 30 percent, highest in the terrain of WV.


...Central Plains...
Day 3...

Mid-level low will eject eastward from California Saturday while
filling, but will be accompanied by a 120+kt Pacific Jet and
persistent PVA despite weakening height falls. At the same time, a
cold front will be dropping through the Plains producing a
tightening baroclinic gradient. This baroclinic gradient will be
acted upon by the aforementioned upper forcing and an increasingly
coupled jet structure to produce surface cyclogenesis which will
begin late Saturday night over Texas. Mid-level confluence will
drive Pacific Moisture into the region, which will be wrung out as
precipitation in response to increasing synoptic lift, aided by
isentropic ascent on low-level warm advection and intensifying
mid-level frontogenesis. This all suggests a band of heavy snow
will spread west to east from eastern WY/CO through NE and into
KS/MO. Guidance still differs on the exact position of this
heaviest band of snowfall, as well as its intensity, but WPC
probabilities have increased for a wide swath of 4+ inches of snow
across the area. South of this area of snow, the isentropic lift
atop the cold surface high will likely produce freezing rain
across Oklahoma and into AR/MO. At this time accretions are
forecast to be light with around one-tenth of an inch possible.


Weiss