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

Valid 12Z Tue Mar 05 2019 - 12Z Fri Mar 08 2019

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

A persistent closed low near British Columbia will spin nearly in
place through the forecast period, with pieces of energy rotating
around it and onshore into California Wednesday morning and again
Thursday night. Each of these shortwaves will be accompanied by
Pacific Jet energy and a surface low, as well as anomalous
moisture and heavy snowfall.

The first, and more significant of the two systems, will move
onshore California Wednesday morning, with moisture spreading
inland well ahead of the actual surface low. Channeled moistured
from the Pacific in the form of a modest Atmospheric River will
lift northeast through CA and into much of the west north of 40N
on increasing SW 850-700mb flow and within the LFQ of a powerful
130kt upper jet. Snow levels will rise as the airmass is of
modified tropical origin, climbing to 5000-6000 ft south of 40N,
but remaining near sea level towards the US/Canadian border. PWAT
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. 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 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 despite lower column moisture. Snow is again
expected to overspread the west, and snow levels will collapse
down to the south, becoming 3000-4000 ft along 40N, higher to the
south. WPC probabilities on day 3 are modest for 6 inches, mainly
across the Sierra and high terrain of the Great Basin in Nevada.
The lowering snow levels to sea level will also create the
potential for some light snow in the Columbia River Gorge and into
the city of Portland, OR.


...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, enhancing what is otherwise a
modest environment for Lake Effect snow. Unidirectional W/NW flow
over the non-frozen portions of the Lakes (mainly Michigan and
Ontario) will produce light accumulations today and tonight, and
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. 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 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 IA/IL/IN by day
3.

There remains some uncertainty among the models into how
significant QPF will be, but have increased snow amounts slightly,
especially in SD/NE 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. Points further east currently feature lower WPC
probabilities, however, better frontogenesis creates the potential
for some more banding which could locally enhance snowfall even as
theta-e advection and synoptic ascent weaken. At this time,
probabilities for 4 inches remain less than 50 percent from IA
through IN.


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


Weiss