Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
453 PM EDT Tue May 14 2019

Valid 00Z Wed May 15 2019 - 00Z Sat May 18 2019

...Northern New England...

Some lingering snow showers are likely across the higher
elevations of northern Maine and extending westward to the White
Mountains of northern New Hampshire.  A mid-upper level low
situated over Maine Tuesday night will slowly lift to the
northeast, and with some lingering moisture and lift available,
the potential exists for an additional 1 to 2 inches for these
areas, with the heaviest snowfall likely over New Brunswick,
Canada based on the WPC probabilities.

...Western U.S. Mountains...

Significant snow accumulations are forecast to return to the
Sierra during the Day 2 period on Thursday as a late season winter
storm affects the West Coast region.  A potent shortwave trough
over the eastern Pacific, with anomalously low 500mb heights by
May standards, will support an occluded surface low that will
track towards northern California and result in widespread
precipitation.  Initially high snow levels at the onset of
precipitation are forecast to lower significantly across the
Sierra and then over the Great Basin as the system moves onshore. 
Heavy accumulations are expected for portions of the Sierra, with
WPC guidance showing a Moderate Risk for accumulations of at least
12 inches for terrain over 7000 feet on Thursday.  This especially
holds true where the low level flow is orthogonal to the terrain.

The probability of significant icing is less than 10 percent
nationwide.

Hamrick