Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Wed Mar 06 2019

Valid 12Z Wed Mar 06 2019 - 12Z Sat Mar 09 2019

...Western United States...
Days 1-3...

A closed low initially near British Columbia will eventually begin
to propagate southeastward as the system opens up.  In addition,
pieces of energy rotating around it and onshore into California
this morning and again Thursday night will enhance coverage and
snowfall rates during those time frame.  Each piece of shortwave
energy will be accompanied by a surface low, as well as anomalous
moisture and heavy snowfall.

California is positioned to bear the brunt of the first system
coming on-shore, with channeled moisture directed from the Eastern
North Pacific into the Sierra Nevada range.  A modest Atmospheric
River will lift northeast through CA and into the Great Basin
north of 40N on increasing SW 850-700mb flow and within the LFQ of
a powerful 120kt upper jet. PWAT values over +2 standard
deviations above the climo mean will create an environment ripe
for heavy snow, with synoptic ascent through the aforementioned
jet level diffluence and approaching height falls combining with
orographic enhanced lift to produce heavy snow. The highest
accumulations days 1 and 2 will be across the Sierra Nevada, where
2-day snowfall may eclipse 3 feet in the highest crests, but
widespread 1-2 feet is likely above 6000 feet. Elsewhere across
the West through Thursday morning, WPC probabilities are high for
12 inches in the mountains of Utah and Colorado, with generally
lesser amounts spread across the remaining western ranges.

After a brief respite Thursday, another shortwave will eject
around the parent trough and move onshore atop an associated
surface low into California. This system is weaker and will dig
further south than the first, but is accompanied by a similarly
strong jet max.  Snow is again expected to overspread the west,
and snow levels will collapse down to the south, becoming
3000-4000 ft along 40N, and only marginally higher to the south.
This will bring the potential for even heavier snow into the
Mountains of Utah and Colorado on Day 3 while drying out areas
near the coast, and WPC probabilities are high for 12 inches in
the highest terrain of the Colorado Rockies on Friday.


...Great Lakes...
Day 1...

Today, enhancement in the lake effect snow is likely due to height
falls and PVA, but the environment also becomes more favorable for
heavy snowfall, especially east of Lake Ontario when a core of
colder temperatures aloft slides in from the north/northwest.
Modest unidirectional flow along the long fetch of the Lake and
robust low-level forcing into a saturated DGZ with an high EL
suggests a window for heavy snow, and WPC probabilities are
moderate for 6 inches or more south of the Tug Hill Plateau.


...Central Plains into Ohio Valley...
Days 1-3...

Shortwave ejecting out of California will traverse near the Four
Corners and through Wyoming before cresting the ridge and shifting
southeast through the Plains and into the Ohio Valley. This will
be accompanied by high column moisture and a weakening Pacific
Jet, through which diffluence embedded in the LFQ will provide
synoptic ascent across the region. A slowly developing surface
wave will be preceded by warm moist advection and a sharpening
700mb temperature gradient /frontogenesis/ to produce mesoscale
lift coincident with the upper diffluence. Guidance is in good
agreement that a progressive band of snowfall will spread from
eastern WY through SD/NE and into IL/IN/OH and WV by day 2.

There remains some uncertainty among the models into how
significant QPF will be, and guidance has backed off slightly
tonight, likely due to a subtly faster wave moving through the
northern stream with less amplification of the 700mb wave. WPC
probabilities for 4 inches are high along the western NE/SD
border, but fall off rapidly to the east except in the terrain of
WV where upslope flow may enhance accumulations, and probabilities
for 4 inches are near 30 percent.

On Day 3, a more significant system is likely to begin to develop
in the Lee of the Rockies and begin to deepen while shifting
northeast into the Central Plains late Friday. There remains
considerably spread in the guidance of the evolution and heaviest
snow from this system, but WPC probabilities are already over 30
percent for 4 inches across NE/SD. A blend with significant weight
on the GEFS and ECENS means was utilized to create these initial
snowfall probabilities.


The probability of freezing rain of 0.25 inches or more is less
than 10 percent all 3 days.


Weiss