Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 PM EST Sat Jan 19 2019

Valid 00Z Sun Jan 20 2019 - 00Z Wed Jan 23 2019


...PA/NY/New England...

Heavy snow is still expected across northwest PA across
western/northern New York and northern New England as the
intensifying low-mid level circulation provides strong
moisture/warm advection across the region.  Amounts of 18-24
inches are possible in favored areas of the NY Adirondacks and
portions of the Green and White mountains.  A general area of 12
to 18 inches is expected elsewhere across much of western to
northern New York to northern New England. 

Strong upper divergence supported by a coupled upper jet, along
with low to mid level frontogenesis is expected to produce a swath
of heavy snow accumulations.
With the latest forecast trends showing the 850 mb low tracking
25-50 nm west of yesterday's forecast, the warm low-mid level air
is forecast to advance further inland in eastern New York and
across MA into southern VT/NH and coastal Maine.
This leads to snow changing to a mixture of sleet and freezing
rain across
north central PA to southeast NY and southern and central New
England.
As a result, snow amounts were reduced from prior forecasts in
northeast PA across southeast NY and southern New England.
Freezing rain accumulations are possible after the period of sleet
across southeast NY and MA into nearby southern VT/NH/southern ME.
Up to a quarter inch of freezing rain is possible in the Catskill
and Berkshire Mountains.

Strong cold air advection in the wake of low is forecast to
support lake effect snow showers, contributing to additional
locally heavy totals in the lee of lakes Ontario and Erie into
early Monday.  Significant additional totals are also possible
further to the south along the upslope regions of the central
Appalachians.


...Ohio Valley Day 1...

Radar trends have shown a circulation developing along the tail
end of the 850-700 mb trough with rain changing to snow as cold
air plunges south across MO/IL. Several inches of shows are
expected as the continuing band of mid level
deformation/convergence continues this evening and tonight across
southern IN/northern KY, then northeast across Ohio. Latest WPC
probabilities show a Moderate Risk of 4 inches of snow along the
Ohio river, and then a high risk into northern Ohio. 


...Western U.S. into the Central to Northern Plains and Upper
Mississippi Valley...

A frontal band pushing into the northern Rockies is expected to
support some locally heavy mountain snows across the northeast
Oregon and central/southern Idaho ranges tonight into Sunday,
continuing into WY as the upper trough moves onshore and inland,
driving the upper jet out of ID and into WY.
The heavy snowfall threat also continues in the ranges of CA as
the upper trough approaches and upper divergence and low level
moisture advection bring a couple rounds of snow expanding south
from the OR Cascades across the Shasta/Siskiyous and then CA
Sierra Nevada.  Event totals of 1-2 feet total are expected in the
CA Sierra Nevada mountains the next 2 days.

As the trough continues to move east, probabilities for locally
heavy amounts increase across the ranges of northern NV, followed
by the Wasatch/Uintas Sunday night- Monday.
There is locally potentials for a foot of snow in each of these
ranges.  On Monday,
the upper trough departs UT and crosses the ranges of CO,
prompting the upper jet to cross into eastern CO, placing the
favored left exit region across central to northern CO and
southeast WY, where several inches of snow are expected in the
front range.

On Tuesday, the snow ends in the front range as spreads across the
central to northern Plains as the mid-upper level trough
progresses east northeast. The short duration of precip in the
central to northern Plains and upper MS Valley leads to
accumulating snow but amounts less than those that feel in the
Rockies. 

The probability of a quarter inch of icing is less than 10 percent
days 2 and 3.

Petersen