Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Thu Dec 17 2020

Valid 12Z Thu Dec 17 2020 - 12Z Sun Dec 20 2020


Day 1...
...Northeast...
Moderate to heavy snow is expected to continue across portions of
the Northeast through early Thursday.  Low-to-mid level
frontogenesis along with favorable upper level forcing will
continue to support areas of moderate to heavy snowfall north of a
low moving along the Long Island/southern New England coast today.
 Overnight this system has shown the capacity to produce bands
generating snowfall accumulations well in excess of 2 in/hr across
portions of Pennsylvania and Upstate New York.  The overnight
HREF, along with recent runs of the HRRR, indicate that banded
snowfall, producing snowfall accumulation rates of over an
inch/hr, is likely to move east from Hudson Valley to the New
England coast.  Latest WPC PWPF indicates additional accumulations
of 4-inches or more are likely from the Capital District of New
York to the New England coast, from the Boston Metro to Down East
Maine.  Snows are expected to diminish from west to east Thursday
afternoon and evening as the associated upper trough begins to
shear east of the region. 

...Western U.S....
A well-defined upper level shortwave and associated frontal band
extending from the northern Rockies to California Thursday morning
will swing east this period, reaching the central Rockies by early
Friday.  This system is expected to produce widespread
accumulating snows from the northern Rockies and Sierra, through
the Great Basin and into the central Rockies, with locally heavy
totals across portions of the higher terrain.  Meanwhile, onshore
flow ahead of the next system will be sufficient for periods of
snow, with additional heavy accumulations possible across the
northern Cascades.

Days 2-3...
...Pacific Northwest...
A shortwave trough is expected to bring another round of organized
precipitation, including heavy snows to portions of the northern
Cascades late Friday.  Shortwave ridging is expected to bring a
brief, relatively drier period on Saturday, before strong,
moisture-laden onshore flow begins to settle south into the
region.  While heavy precipitation is expected to develop across
western Washington Saturday night into Sunday, snow levels are
forecast to be relatively high at the onset, with some further
increase expected -- confining the threat for heavy snowfall
accumulations largely to the higher terrain on the northern
Cascades.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira