Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Mon Mar 09 2020

Valid 00Z Tue Mar 10 2020 - 00Z Fri Mar 13 2020



...California into the Four Corners...
Days 1-3...

Large closed low west of Central California will drift slowly
southeastward to move onshore Southern California Wednesday. As
this feature moves onshore and begins to fill, a secondary
shortwave will rotate cyclonically to its west and deepen into an
even more pronounced closed mid-level low, with this feature again
rotating southeast to come onshore the Baja area of Mexico by the
end of the forecast period. Persistent moist advection through the
column will spread widespread precipitation across California and
into most of the Southwest and southern Great Basin as a prolonged
subtropical jet and mid-level confluent flow drive PWs to as much
as +3 standard deviations above the climo mean. While moisture
will be pronounced and long-lived, snow levels will rise steadily
through the week as the tropically sourced air lifts northward.
Snow levels are forecast to climb from around 6000 ft to 8000 ft
or higher, confining the heaviest snow to the highest terrain of
the Sierra and the transverse ranges of Southern California. WPC
probabilities for 8 inches are high only in these ranges D1 and
D2, and snowfall totals may reach 2 feet. Further east, WPC
probabilities each day are moderate for 4 inches, with event total
snowfall possibly exceeding 12 inches at the top of Mt Charleston.


...Northwest...
Days 2-3...

Shortwave energy digging through the mean CONUS trough will eject
rapidly southeast from British Columbia Wednesday night into
Thursday and then push into the Northern Plains on D3. Confluent
mid-level moist flow combined with modest height falls and subtle
jet level diffluence will produce modest ascent in an increasingly
saturated 700-600mb layer. This shortwave will also drive a
weakening cold front through the region, with some enhanced
low-level convergence further driving lift. The system moves
quickly and moisture is somewhat limited, so heavy snow should be
confined to the terrain of the Washington Cascades and Northern
Rockies on D2, before spreading southward into the Big Horns and
other ranges of NW WY on D3. The heaviest snow is likely D2 when
WPC probabilities for 6 inches are high, with modest probabilities
for 4 inches into WY on D3.


...Northern Maine...
Day 1...

A wave of low pressure moving out of the Great Lakes will extend a
stationary boundary/warm front into Maine. As this wave of low
pressure moves eastward, WAA will cause precipitation to
overspread New England, initially in the form of snow only across
parts of northern Maine. Although this WAA will eventually develop
a warm nose within the atmospheric column to cause a changeover of
p-type from snow, to sleet, to freezing rain, a burst of moderate
to heavy snow is likely tonight as the WAA causes an
intensification of the fgen. The duration of this heavy snow
should be short however, and WPC probabilities for 4 inches are
less than 50% and confined to the northern tip of Maine. South of
this area and across much of the Caribou CWA, a period of sleet
and freezing rain is also likely before precipitation changes over
to all rain Tuesday evening. WPC probabilities for 0.1" of
accretion have increased to 30-40%, but are less than 10% for
0.25".


Weiss