Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Sun Nov 15 2020

Valid 00Z Mon Nov 16 2020 - 00Z Thu Nov 19 2020


...Western U.S....
Early in the period, favorable mid-to-upper level forcing along
the leading edge of strong Pacific flow will support a
continuation of snow across portions of the northern Rockies, with
additional heavy accumulations possible, especially across the
western Wyoming ranges Sunday evening.  Meanwhile across the
Northwest, onshore flow will continue to support snow showers
across the Olympics and Cascades.  By late Monday, a well-defined
front surging north will help produce some additional snows as it
lifts across the northern Cascades.  However, with snow levels
rapidly rising, any additional heavy accumulations will likely be
confined to the high peaks.  Dry conditions can be expected across
much of the West Monday night into early Tuesday as the front
continues to lift north in Canada ahead of amplifying trough and
rapidly intensifying surface low approaching Vancouver island. 
Wet weather will return behind the trailing cold front as it
pushes across the Northwest, Northern California and northern
Rockies late Tuesday into Wednesday.  As snow levels fall, this
will raise the potential for locally heavy accumulations along the
northern Sierra and Cascades, as well as the central Idaho ranges.

...Northeast and Great Lakes...
A sharp occluded front associated with a negatively-tilted upper
trough is forecast to sweep across the Northeast Sunday night and
Monday morning.  Guidance continues to show a weak triple point
low developing along the front moves across northern New England. 
This will be enough to disrupt the strong warm advection ahead of
the boundary and help maintain enough cold air to support a wintry
mix, with a few inches of snow likely across northern Maine and
the northern White Mountains in New Hampshire. 

Lake effect precipitation is expected to ensue on Monday in the
deep cyclonic, westerly flow, with an embedded shortwave, behind
the front.  This will likely produce at least a few inches in the
favored areas east of lakes Erie and Ontario, including the Tug
Hill and western Adirondacks.  Winds will become more
northwesterly as the shortwave moves east, with snows diminishing
by Wednesday as an upstream ridge moves over the region.

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

Pereira