Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Fri Nov 22 2019

Valid 00Z Sat Nov 23 2019 - 00Z Tue Nov 26 2019


...Northeast...
Days 2-3...

A northern stream shortwave moving across southern Canada will
attempt to capture a southern stream impulse lifting out of the
Missouri Valley. Guidance has backed off on this capture today,
and the resultant surface low is progged to track just offshore
Cape Cod and east of Nova Scotia by Monday morning. The weaker
interaction and further east track will keep the heaviest
precipitation near the coast, where p-type will remain as rain.
Well north into central and northern New England, the column
should remain cold enough for snowfall, but WPC probabilities for
4 inches are confined to parts of N/E ME, and even there are less
than 30 percent. However, a few inches of snowfall is likely
across much of central and northern New England. In the transition
zone between rain and snow, a stripe of freezing rain is likely
where WAA causes a warm nose to overrun surface cold air during
precipitation. The high pressure to the north erodes quickly, so
do not anticipate a long duration wedging event. However, dry/cool
isallobaric wind from the north into the deepening low may persist
sub-0C wet bulb temperatures long enough for freezing rain to
accrete several hundredths of an inch, and WPC probabilities for
0.1" are moderate in the Catskills and Berkshires, with low
probabilities of 10% or less for 0.25".


...Pacific Northwest and Northern Rockies...
Days 2-3...

A robust Pacific Jet with core of 150+kts will begin to impinge on
the PacNW early Sunday, before angling strongly SE towards OR by
Monday. This will accompany a modest atmospheric river event,
within which a shear vorticity impulse will rotate onshore and
dive towards the Four Corners Sunday evening. This feature will
advect moisture onto the coast which will spread inland towards
the Rockies, while jet level diffluence and height falls/PVA
provide ascent. This will produce widespread snowfall, first in
the Northern Rockies of MT on D2, before spreading inland into
UT/CO, while also increasing significantly along the Cascades and
Olympics on D3. WPC probabilities on D2, through Saturday evening,
are moderate for 6 inches near Glacier NP. On Day 3, through
Sunday evening, WPC probabilities for 6 inches are high in the
Cascades of Washington and Oregon, as well as the ranges of ID and
WY. Localized amounts in the Cascades could exceed 12 inches.


Weiss