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

Valid 00Z Mon Dec 02 2019 - 00Z Thu Dec 05 2019

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

An upper low centered west of Oregon 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 combined with warm 700mb
southerly flow and mid-level divergence will produce heavy snow in
the Sierra, with moisture spreading northeastward to produce
snowfall a 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, and WPC probabilities that are high for 8 inches are
confined to the Sierra where isolated totals above 2 feet are
possible.

Weaker forcing Monday night and Tuesday will bring a respite to
the heavy snow across most of the West, with light accumulations
remaining possible in the CO Rockies, Grand Tetons, and the Sierra.

By Day 3, the upper low west of California will begin to lift
northeast bringing renewed moisture and forcing into the region.
Additionally, a northern stream shortwave will approach British
Columbia with moisture and jet level diffluence moving into the
Washington Cascades. In both of these regions the forcing and
moisture appear to be modest, but WPC probabilities indicate that
more than 4 inches of snowfall is likely in the ranges, with up to
12 possible in isolated locations in the Sierra.


...Mid Atlantic/Northeast...
Days 1-3..

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

Two-phase winter event beginning tonight will slowly evolve
through early Wednesday. Tonight into Monday, a mid-level low and
weakening surface low will be dropping out of the Great Lakes,
while secondary development of surface low pressure begins off the
NJ coast. In between, a warm front will lift northward spreading
precipitation through isentropic lift across the Mid-Atlantic and
into southern/central New England. During this WAA regime,
precipitation will begin as a burst of heavy snow, but gradually
transition to sleet, freezing rain, and rain, from CT and points
SW. Across portions of MA/upstate NY/southern VT/southern NH, a
period of heavy snow is likely with rates potentially exceeding
1"/hr.

During Monday, a dry slot is likely to lift northward through much
of southern/central New England as well as NJ/lower New York.
During this time, the DGZ will dry out, but low level moisture
remains. This supports an extended period of light freezing
rain/freezing drizzle as surface wet bulbs temps struggle below
freezing. While accretions are not expected to be high, there are
some moderate WPC probabilities for 0.1" from northeast PA, across
the Hudson Valley of NY, and into portions of CT/RI.

Late on Day 1 and into Day 2, the mid-level low will drop
southeast off the NJ coast, spawning secondary cyclogenesis, or at
least spreading the surface trough back to the west. During this
interaction, the environment becomes extremely favorable for a
robust axis of deformation to stretch across upstate NY and then
dig southeastward into PA/NJ. The exact placement of this feature
is still uncertain, but guidance has trended upward in its QPF
within this deformation band, and snow rates could be extremely
heavy, greater than 2"/hr as it pivots eastward. Additionally, as
the low begins to intensify during Tuesday in the Gulf of Maine, a
secondary axis of deformation and frontogenesis may pivot onshore
the Maine coast and angle southward into eastern New England,
re-saturating the DGZ and producing heavy snow across much of the
area. At this time, confidence is low in amounts and placement of
these bands, but WPC probabilities have increased to account for
these axes of heavy snow.

Across upstate NY, and into the Catskills, Poconos, Berkshires,
and Worcester Hills, WPC probabilities are high for 8 inches, and
may exceed 12 inches in many places. Further south and east
including the I-95 corridor, WPC probabilities have also
increased, and now feature a moderate risk for 2 inches in
Philadelphia and New York. In Boston, probabilities for 4 inches
are high. In the urban corridor, there is potential for very heavy
rates and higher snow totals depending on exactly how these
deformation bands pivot, but the highest threat for much great
accumulations will be in Boston.

Additionally on day 1 and into day 2, prolonged and enhanced
upslope flow displaced from the low looks favorable for heavy
snowfall. WPC probabilities are high for 6 inches in the favored
terrain of WV, with low probabilities for 6 inches extending down
the central and southern Appalachians, but confined to the highest
terrain.

By day 3, heavy snow is likely to persist across much of Maine,
gradually lifting northeastward as the strong low pressure slowly
moves away. There remains some discrepancy in the track of this
low, and how much warm air may wrap onshore, but at this time WPC
probabilities are high for 6 inches in parts of ME on both D2 and
D3, and local totals may exceed 12 inches.


Weiss