Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Wed Mar 20 2019

Valid 00Z Thu Mar 21 2019 - 00Z Sun Mar 24 2019


...Ranges of CA/OR/Great Basin/Rockies...

On Day 1 tonight-Thu, Models continue to show a negatively-tilted
upper trough shifting east across California into Nevada.  An
upper low is expected to close off within the base of the trough
and rotate east into the Great Basin on Thursday into early
Friday. Mountain snows are expected to taper in the Sierra and
move across the ranges of central NV to Utah, and then into
southwest CO in the San Juans.  Some of the heaviest totals of the
Day 1 period are expected to fall across the southern Utah Wasatch
and Co San Juans, where Moderate Risk areas for accumulations of 8
inches are indicated.

On day 2 Thu night-Fri, additional mountain snows are forecast
across the central Nevada and Utah ranges, while extending farther
east into Colorado and northern New Mexico ranges.  Upper jet
support along with orographic forcing will encourage heavy snows
along the windward slopes of the San Juan Mountains in southwest
Colorado, where WPC probabilities indicate a Moderate Risk for
accumulations of 8 inches and low risk of a foot or more on Day 2.
 The snow winds down Fri night in CO with the departure of the 700
mb wave.

On day 3 Fri night-Sat, the next surge of 700 mb warm/moisture
advection and ascent leads to likely accumulating snows in the CA
Sierra Nevada, extending across the Siskyous and north into the
southern Cascades.  As the wave moves further inland, a period of
snow is expected in the ranges of southern ID and then into
northwest WY, as well as the mountains of central NV.
Widespread heavy accumulations are not expected. WPC probabilities
indicate a chance of 8 inches or more in the CA Sierra Nevada and
as far north as southwest Montana and northwest WY on Day 3. 

...Interior New York/New England...

Models continue to show a developing surface low, associated with
phasing shortwave troughs over the east, tracking north along the
Mid Atlantic to the New England coasts Thursday night through
Friday night.  The low is expected to deepen quickly as it tracks
north along the New England coast on Friday. 

Areas near the coast are expected to be rain with a transition to
snow in interior portions of New York and New England.  Snow is
expected to change to rain along the low level cyclone track in
western Maine.  The highest confidence for heavy snow is west of
the low track in the Adirondack Mountains of New York and points
north to the Canadian border. A foot of snow is possible in this
region.

Strong cold air advection on the backside of the system will
support a changeover to snow, spreading east into the northern
Vermont Green Mountains, where strong upslope flow could bolster
totals.  For the two day period covering Fri and Sat, the latest
WPC probabilities indicate accumulations of 8 inches or more are
likely across the Adirondacks and northern Greens.

The probability of significant freezing rain is less than 10
percent.

Petersen