Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 AM EST Mon Dec 02 2019

Valid 12Z Mon Dec 02 2019 - 12Z Thu Dec 05 2019


...Mid Atlantic/Northeast...

...Days 1-2...

...Heavy Snow in portions of the Northeast...

Moderate snow occurring across much of central and western New
York spreads east into the Catskills and Delaware Valley of
northeast PA to northern NJ and southeast NY, leading to an
extended period of snow.  The Canadian Regional Gem and several
high res windows indicate the possibility of high local maxima in
response to elevated instability, but confidence is lower. 
Greater confidence is on higher amounts in the Catskills of NY
where 700 mb lift is combined with upslope flow to produce heavy
snow amounts.
Additional local maxima are expected in strong level confluent
flow with embedded convergence/vertical velocity maxima in the
Vermont southern Green Mountains, the Berkshires, and Worcester
Hills.

With a new low forming off the NJ/Long Island NY coast, low level
convergence increases nearby.  Initially, most of eastern  PA/NJ,
and the New York metro area are too warm for snow.  Cooling as the
day progresses allows precip to change back to snow with light
accumulations possible. Increasing low level convergence as the
low approaches southeast MA allows a surge of snow in northeast MA
to southern NH by Tue morning.

A mid level dry slot remains through much of southern England
early today. Low level moisture remains. This supports an extended
period of light freezing rain/freezing drizzle as surface wet
bulbs temps struggle to rise above freezing.  Additional ice
accretions are expected to be on the order of 0.01 to 0.10 inches
across the lower Hudson Valley of NY, and into portions of CT/RI.

On Monday, prolonged moist upslope flow in the central
Appalachians supports repeated snow showers, with several inches
likely in windward terrain.  Coverage and amounts taper on Tue as
the upper trough moves east and the layer relative humidity drops
in response.

On Tuesday, the low pressure off the NH coast moves northeast
towards Nova Scotia. Snow is likely to occur in the band of mid
level deformation across much of eastern and northern Maine.
There remains some discrepancy in the track of this low, with the
00z ECMWF and Canadian global trending further offshore, like the
GEFS Mean.  The GFS, NAM, and UKMET are closer in.  The 00z NAM
parent had the highest QPF but the Conus Nest had lower amounts
similar to the ECMWF/UKMET, and GFS, so less weighting was given
to the parent NAM in Maine. Light icing is possible in mid level
warm advection early Tue in coastal portions of downeast Maine.

...Lower Great Lakes on Day 3...

The models indicate an 850-700 mb trough crosses the lower Great
Lakes into New York, with the sfc cold front moving along in
tandem with the mod level trough.  Post-frontal cold advection
develops and leads to cross lake flow, with lee shore convergence
developing downstream from lake Ontario and also to the lee of
Lake Erie in northwest PA and southwest NY. 
The GFS and ECMWF indicate half an inch liquid equivalent,
supporting potential for clusters of 6 inches of snow in the Tug
Hill and other areas near the southwest Ontario shore and also
where Erie lake heat and moisture fluxes maximize instability for
snow showers moving off the lake into northeast PA to southwest
NY. GFS soundings show near moist adiabatic lapse rates and deep
moisture, favoring widespread snow showers late Wed-Thu.

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

An upper low centered west of northern California will retrograde
to the southwest through Tuesday before slowly ejecting northeast
towards California on Wednesday. On day 1, continued moist
advection within a robust Pacific jet streak upper-level
divergence will produce additional snow in the CA Sierra Nevada,
with moisture spreading northeastward to produce snowfall as far
east as the Wind River range of WY. Snow levels will be relatively
high so heavy snow will be confined to the higher peaks.

On Monday night and Tuesday, the low level flow weakens and upper
divergence subsides,
so areas of snow will be scattered and light. On Tuesday night
into Wed., the upper low west of California will begin to lift
northeast bringing renewed moisture and an increase in the upper
jet returning to southern CA into the desert southwest. Difluent
low aloft supports upper divergence maxima and areas of light to
moderate snow in the southern Ca Sierra Nevada, and ranges of
northwest AZ to southern UT and southwest CO.

The probability of 0.25 inches of icing is less than 10 percent
Days 1-3.

Petersen