Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Thu Mar 28 2019

Valid 12Z Thu Mar 28 2019 - 12Z Sun Mar 31 2019

...Pacific Coast...
Days 1-2...

Mid-level low off the Oregon coast will gradually shear out and
shed its energy into the Coast through Saturday morning. As pieces
of vorticity spokes rotate onto the coast, they will be
accompanied by modest Pacific Jet energy, but significant moist
advection, especially on Day 1 into California where PWAT
anomalies are forecast to climb to +2 standard deviations above
the mean. Increasing diffluence aloft will force ascent across the
West Coast, while 700mb S/SW flow will transport moisture and
enhance lift where this flow becomes orthogonal to significant
mountain ranges. Snow levels are forecast to be modest, 4-6 kft
early, falling to 3-4 kft into day 2, so the heavy snow should be
confined above these levels. The heaviest snow is forecast today
and tonight when the best combination of lift and moisture
coincide, especially across the Trinity/Shasta/Siskiyou ranges of
CA, where WPC probabilities are moderate for 8 inches. As the
upper jet gets forced northward tonight and Friday, higher snow
chances will lift into the OR/WA cascades, aided by weak impulses
embedded within the flow from the shedding trough to the west, and
WPC probabilities late Day 1 into Day 2 suggest as well a moderate
chance for 8 inches in the highest peaks.


...Northern and Central Rockies into the Central Plains...
Days 1-3...

Undulating Pacific Jet and accompanying pieces of vorticity
rotating beneath a ridge across western Canada will bring rounds
of snowfall through the forecast period. This will produce heavy
snowfall, with the axis generally sinking southward into the
weekend. The heaviest snow accumulations are forecast across the
mountains of ID/MT on day 1 where enhanced synoptic ascent is
driven by a weakly coupled jet structure and height falls in the
vicinity of an embedded weak mid-level trough and increased
baroclinic gradient. Here, WPC probabilities feature a high risk
for 8 inches of accumulation, with snow levels falling gradually
through the day to near 3000 feet.

By day 2 /Friday/ the best forcing will trend southeast as ridging
over the Pacific forces amplification of the flow and drives the
best jet energy southward towards WY/CO. On day 2, strong ascent
in an environment with continued high Pacific moisture, and high
WPC probabilities for 8 inches exist across much of the terrain of
WY and Rockies northern CO. At the same time, the rapid eastward
progression of the jet streak will work in tandem with a modest
low-mid level wave of low pressure and an increasing baroclinic
gradient into the Central Plains to produce a stripe of moderate
snow into SD and NE, where WPC probabilities are high for 4 inches
across the western portion of these states.

By day 3, drier air within Canadian high pressure sinking
southward will begin to erode the column moisture at the same time
synoptic forcing weakens. This leaves residual snowfall in the
terrain of CO/NM including the San Juans and Sangre De Cristos
where WPC probabilities are less than 30 percent for 4 inches.


...Great Lakes...
Day 3...

A strong cold front ahead of Canadian high pressure will shift
through the Great Lakes Saturday into Sunday. Moist advection
ahead of this feature is abundant, but will primarily occur while
the column is too warm for snowfall. However, as cold advection
commences in earnest and precipitation lags the front due to
continued SW flow aloft post surface FROPA, the column will
attempt to cool in time to produce a period of snowfall from
eastern WI into MI, OH, and upstate NY. Although some guidance
does produce a stripe of heavy snowfall during this changeover,
cold air chasing moisture is never a good setup for heavy snow,
and the antecedent conditions are less than favorable for
accumulation. WPC probabilities have increased slightly for snow
in this area, but remain less than 20 percent for 4 inches except
for far eastern MI.


The probability for significant (0.25 inches) freezing rain is
less than 10 percent all 3 days.


Weiss