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

Valid 12Z Wed Mar 20 2019 - 12Z Sat Mar 23 2019


...Western U.S....

Models continue to show a negatively-tilted upper trough shifting
east across California on Wednesday into early Thursday.  Mountain
snows are expected from the Sierra to southern Utah.  Some of the
heaviest totals of the Day 1 period are expected to fall across
southern Utah where left-exit region upper jet dynamics is helping
support a small Moderate Risk area for accumulations of 8-inches
or more within the southern mountains.

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, supporting additional mountain snows across the central
Nevada and Utah ranges, while extending farther east into Colorado
and northern New Mexico.  Upper jet support along with orographic
forcing will encourage heavy snows along the southern slopes of
the San Juan Mountains in southwest Colorado, where WPC
probabilities indicate a Moderate Risk for accumulations of a foot
or more on Day 2 (ending 12Z Friday). 

Accumulating snows will begin to shift farther north into the
northern Rockies as the low begins to lift north ahead of an
upstream trough approaching the Northwest on Friday.  While
widespread heavy accumulations are not expected, WPC probabilities
indicate a Slight Risk for localized amounts of 8-inches or more
as far north as southwest Montana on Day 3 (ending 12Z Saturday). 

Meanwhile, mountain snows are expected to return to the southern
Cascades and northern Sierra, with generally light amounts
expected through the end of Day 3.

...Northeast...

Models continue to show a developing surface low, associated with
phasing shortwave troughs over the east, tracking north along the
Mid Atlantic to the southern New England coast Thursday night.  As
the upper trough begins to assume a negative tilt and lift north
ahead of an amplifying upstream trough over the Great Lakes, the
low is expected to deepen quickly as it tracks north along the New
England coast on Friday.  While the models have moved into better
agreement, the timing of the system remains a source of forecast
uncertainty.  While most areas should begin as rain, strong cold
air advection on the backside of the system will support a
changeover to snow, with accumulating snows likely across the
Adirondacks by the end of Day 2 before spreading east into the
northern New England ranges, where strong upslope flow could
bolster totals.  For the two day period ending 12Z Saturday, the
latest WPC probabilities indicate a 40 percent chance or greater
for accumulations of 8-inches or more across the Adirondacks and
northern Greens.

Pereira