Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 AM EST Sun Dec 01 2019

Valid 12Z Sun Dec 01 2019 - 12Z Wed Dec 04 2019

...California to Great Basin and Rocky Mountains...
Days 1-3...

...More heavy snow for the California Sierra Nevada Range...

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 in the GFS both Sunday and Monday.
The models and means show a split jet structure with difluent flow
aloft resulting in well defined and persistent upper divergence
maxima coming onshore and continuing inland into the CA Sierra
Nevada mountains.

With a secondary max in the northern branch crossing eastern OR
into central ID, light to moderate snow are expected in those
areas on day 1. 

On Monday, Day 2, the upper jet starts to gradually ween and move
southeast with time, so the strength of the upper divergence
maxima weakens over time.
This results in lower amounts than day 1 in the CA Sierra and
signal the major event will wind down on Monday.

Further inland with the upper trough crossing the Pacific
northwest towards the northern Rockies, the jet maxima departs ID
and comes into Wyoming, where upper divergence maxima supports a
period of snow, with several inches in the Tetons and Yellowstone
National park.
This ends as the upper trough passes the latter half of day 2.

On Day 3, the continuation of the Pacific jet coming onshore into
southern CA and into the southern CA Sierra Nevada leads to
persistent snow, with accumulations up to a foot possible.

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 Columbia River Valley in northwest OR and
southwest WA.

...Upper Great Lakes...

Day 1...

The ongoing major winter storm will gradually move eastward, with
coupled upper divergence/lower convergence and lake enhancements
supporting heavy snow on the north side of this low across
northern WI through northern lower MI and the southern UP of MI.
 Sunday evening. The heaviest snowfall is likely where some lake
enhancement is expected on the W side of Lake Michigan.  A second
maxima is downstream of Lake Huron in northeast lower MI, where
the longest duration of deformation occurs. The event tapers later
tonight as the deformation bands moves east out of MI as the upper
low departs.

Further south, a transition of p-types is likely, with a band of
mixed sleet and freezing rain across central MI that gradually
changes to snow as falling heights lower temperatures aloft.
Surface temps are expected to be only at or slightly below
freezing, so ice accretion efficiency may be low. Loss of the mid
level inversion should end the freezing rain threat tonight.


...Lower Lakes/Mid Atlantic/Northeast...

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

Days 1-3..

The models indicate strong low-mid level warm advection and
moisture advection underneath a well defined upper divergence
maxima progressing across western NY and PA downstream across the
rest of NY?PA today and southern and central New England
thereafter.
Where it is cold enough for mostly snow, a band of heavier amounts
is expected from Rochester and Syracuse NY down the Mohawk Valley
to the Capital District and then the Berkshires and Worcester
Hills.

Further south a mixture of sleet/freezing rain and rain will occur
across southern NY and northern PA into southern New England. As
this band of WAA associated with a warm front lifts northeast from
PA into Southern New England, moderate to heavy precipitation is
likely.
Local enhancements to freezing rain are likely in the lee of the
Allegheny Mountains of PA until the warming ends the event. 
Amounts of 0.25" of accretion are expected. Further northeast,
more sleet is likely across northern PA to northern NJ and
southeast NY and interior CT. Most areas of northeast PA and NJ
are likely to start as a quick hit of snow with 1-2" possible.

Tonight, 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 east northeast into Monday.
Confluent flow north of the developing cyclone produces mid level
frontogenesis and a low level jet maxima of 40-50 kt easterly flow
across central New England.

This should result in heavy snow in the Catskills, Berkshires,
southern Green Mountains, and Monadnocks.  Precip should change
back to snow across the Delaware River Valley, lower Hudson
Valley, and Litchfield hills of northwest CT.
  
From the Catskills, into the Berkshires, the Worcester Hills, and
Monadnocks of NH,
event totals snow could be around 18 inches with locally higher
amounts.

Closer to the MA 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.

On Tuesday, the models show the system turning north and cross the
Gulf of Maine, with QPF and snow differences resulting from the
cyclone timing and intensity differences.  The cluster of models
closer to the coast like the NAM, ECMWF, and UKMET are preferred.
But each has a different axis and orientation of QPF and resultant
snow, so uncertainty is greater than normal and confidence
lacking. Late Tuesday, the upper low moves into the Canadian
maritime and the lift decreases and the event ends.

The probability of 0.25 inches of freezing rain is less than 10
percent days 2 and 3.

Petersen