Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 PM EST Sat Nov 30 2019

Valid 00Z Sun Dec 01 2019 - 00Z Wed Dec 04 2019

...California to Great Basin and Rocky Mountains...
Days 1-3...
...More heavy snow for the ranges of central to northern
California...

An upper low centered west of Oregon will retrograde SW to a
position off the northern California coast Sunday. A prolonged and
robust Pacific jet will advect copious moisture into California,
noted by PWAT anomalies of more than 2 standard deviations above
the mean.
The period of surging 700 mb moisture advection and ascent is Sat
into early Sunday across the ranges of northern and central CA,
with heavy snow resulting in the Siskiyous and Shastas down to the
northern CA Sierra Nevada mountains.

The lift continues as the moist flow persists through Sunday
across the Sierra Nevada while a break occurs in the ranges of
northwest CA. The persistent Pacific jet will create more heavy
snow, but by day 2 it is primarily in the Sierra, with even less
forecast on D3 as the strongest forcing lifts away to the
northeast. At the same time, some of this moisture streaming
inland combines with more modest ascent across Oregon and into
Idaho/Nevada where light snows are expected day 1 into day 2. WPC
probabilities for snow are highest on D1, and remain high only in
the Sierra D2, falling to low on D3 for heavy snow. Over the 3
days, total accumulations may exceed 5 feet in parts of the
Sierra, with widespread 6-10" in the terrain of the Great Basin
and into Wyoming.

There is also the potential for some light freezing rain in the
vicinity of the Gorge near Portland, OR, as well as in the Great
Basin where surface cold air may become trapped during a period of
overrunning moisture. WPC probabilities are low for 0.1" across
these areas.


...Central/Northern Plains into the Great Lakes...
Day 1...

Ongoing major winter storm will gradually move eastward,
persisting heavy snow on the north side of this low from Minnesota
through the L.P. of MI. Robust moist advection wrapping
cyclonically into the cold air is likely to produce a swath of
heavy snow exceeding 6 inches from central Minnesota, across
northern WI, the U.P. and the northern L.P. through Sunday
evening. The heaviest snowfall is likely where some lake
enhancement is possible on the S/W sides of Lake Superior as well
as where the longest duration of deformation/pivot point occurs
norther WI. Locally, WPC probabilities indicate a moderate risk
for 12 inches.

Further south, a transition of p-types is likely, and significant
freezing rain accretion is possible across portions of the L.P.
This will occur as in-situ cold air gets trapped beneath a
WAA/overrunning precipitation, leading to an extended period of
freezing rain. However, rates at times may be heavy, and surface
temps are expected to be only slightly below freezing, so
accretion efficiency may be low. However, WPC probabilities do
indicate a low chance for 0.25" of freezing rain in a stripe west
to east across the L.P. where the changeover to snow is slower to
occur.


...Mid Atlantic/Northeast...

...Prolonged Snow Event in interior Eastern New York and Interior
New England...

Days 1-3..

Two phased wintry event likely beginning early Sunday. The first
phase will be a WAA type event where a mixture of
snow/sleet/freezing rain and rain will occur. As this band of WAA
associated with a warm front lifts northeast from PA into Southern
New England, heavy precipitation is likely. This will produce a
band of snow to sleet to freezing rain, with the heaviest freezing
rain likely in the terrain of PA. There, a prolonged period of
moderate fzra is likely, and WPC probabilities are moderate to
high for more than 0.25" of accretion. Further northeast, more
sleet is likely, with heavy snow the predominant p-type into
upstate New York where WPC probabilities are moderate to high for
4 inches. Most areas of northeast PA and NJ are likely to start as
a quick hit of snow with 1-2" possible.

By Sunday evening, a surface low will begin to develop off the NJ
coast as energy from the low across the Great Lakes transfers
eastward. This low will move slowly eastward into Monday remaining
offshore, as the upper low drops southeast as well, likely forming
secondary low pressure development again on Monday. This series of
low pressures will spread moisture cyclonically into New England
and upstate New York, while deformation and instability beneath
the upper low will wrap southeast later in the forecast period to
produce additional snowfall. The heaviest snow is likely across
interior New England and in the terrain where the northward
lifting warm nose won't change p-type from snow. In this swath,
from the Catskills, into the Berkshires, the Worcester Hills, and
into the Whites, WPC probabilities are high for 8 inches, and may
exceed 15 inches in isolated locations through the long duration
event. Closer to the coast, amounts will likely be less due to
warm air intrusion turning p-type over to IP/ZR for a time, and
may be even lower as a dry slot wraps in on Monday, potentially
cutting off snowfall for a period of time. Here, WPC probabilities
indicate a moderate risk for 4 inches.

South and east of there, some freezing rain is also likely, and
WPC probabilities are low for 0.1 inches, highest in ne PA.

Weiss