Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EST Tue Mar 05 2019

Valid 00Z Wed Mar 06 2019 - 00Z Sat Mar 09 2019

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

A closed low initially near British Columbia this evening will
eventually begin to propagate eastward as the system opens up.  In
addition, pieces of energy rotating around it and onshore into
California Wednesday 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 130kt upper jet. PWAT values over +3 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 in addition to
orographic effects. The highest accumulations days 1 and 2 will be
across the Sierra Nevada, where 2-day snowfall may eclipse 5 feet
in the highest crests, but widespread 2-4 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.


...Great Lakes...
Days 1-2...

Longwave trough centered near James Bay south of the Hudson Bay
will spin lobes of vorticity across the Great Lakes as shortwaves
rotate through its base. The most significant of these will move
overhead tonight into Wednesday, although some places may see
accumulating snowfall into Thursday.  Unidirectional W/NW flow
over the non-frozen portions of the Lakes (mainly Michigan and
Ontario) will produce light accumulations tonight.  WPC
probabilities are low for 4 inches. On Wednesday, some enhancement
is possible both due to height falls and PVA, but also as the
environment 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 8 inches or more south of
the Tug Hill Plateau.


...Central Plains into Ohio Valley...
Days 2-3...

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 Days 2 and
3. 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 3.

There remains some uncertainty among the models into how
significant QPF will be, but have increased snow amounts along a
stripe from SD/NE into IA and far western IN where better theta-e
advection and upper forcing coincide, and the 700mb wave may
briefly close. WPC probabilities are moderate for 6 inches of
accumulation in western SD/NE on Day 2. Points further east
currently feature lower WPC probabilities, however, better
frontogenesis creates the potential for some banded snowfall which
could locally enhance snowfall even as theta-e advection and
synoptic ascent weaken.

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

Bann