Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EST Wed Feb 26 2020

Valid 12Z Wed Feb 26 2020 - 12Z Sat Feb 29 2020


Day 1...

...Great Lakes to the Central and Southern Appalachians...
The models indicate that low pressure in the Ohio Valley drifts
east towards the central Appalachians and then intensifies quickly
as the lows moves out of PA into western NY.  Expect light to
moderate snows to develop within the associated deformation axis
and move northeast from IL/IN across southeast lower MI.  Higher
amounts are expected near the thumb of southeast Lower MI, with
possibly lake enhancement near Lake Huron, so 4-8 inches is
expected in this area.
The intensifying low allows post-frontal cold air advection to
change rain to snow starting in northwestern Ohio, then crossing
into western PA and then western NY. The limited duration where it
is cold enough for snow limits snow potential in northeast OH east
into western NY. In upstate NY and VT, the warm/moisture advection
that produces the snow leads to a likely transition from snow to
mixed precip types.  Locally heavy snow is possible in the NY
Adirondacks before the change over occurs.

Further south in the central Appalachians, post-frontal cold
advection produces a change from rain to snow with
GFS soundings of 20-30 kt from the sfc-850 mb producing strong
upslope flow in windward terrain.  Mid level deformation maxima
cross the central Appalachians, providing a synoptic scale source
for lift.  This should result in locally heavy snow in the
mountains of central WV, including the Snowshoe area.  The event
winds down early day 2 as drier air aloft moves across the
mountains.  

Days 2 and 3...
...Northeast/Great Lakes...

A triple point low development leads to rapid low pressure
development as the system crosses New England, including interior
Maine. By the end of Day 2, the low moves out of Maine into New
Brunswick.  A well defined mid level deformation/frontogenesis
maxima crosses Maine, producing heavy snow in the interior where
it remains cold enough for snow. There remains potential for a
foot of snow in interior NH and Maine.
The inland low track supports the precip type transition zone to
be in eastern Maine, so snow accumulations quickly lower as you
head towards coastal NH and Maine.

Over the Great Lakes, the heaviest snows across Upstate NY will
likely be lake effect driven and are expected to occur as the
primary low lifts north of Lake Ontario on Thursday.  Strong
westerly flow and cold air advection on the backside of the low
will set the stage for a prolonged, major lake effect event with
heavy accumulations, on the order of 1-3 feet likely east of Lake
Ontario into the Tug Hill region beginning Thursday and continuing
into Friday.

Secondary maxima are expected in the lee of Lake Erie, with
persistent west-northwest flow leading to lee shore convergence in
southwest NY to northwest PA. A prolonged period of snow is
expected on Day 2, with the possibility of a slot of dry air
crossing the lake leading to a lull period at the end of day 2
into early day 3.  2 day totals of a foot of snow are possible Thu
and Fri in southwest NY.

Day 3
...Pacific Northwest...
The models show cyclogenesis over the northeast Pacific, with the
low possibly approaching Vancouver Island on Day 3 Fri.
The models show up to half inch precipitable water values moving
onshore, with bands of 700 mb convergence crossing WA State.  The
pre-frontal lift should result in snow developing in the higher
elevations of the WA Olympics and Cascades, with several inches
possible in the mountains, with maxima approaching a foot.  The
models vary on the cyclone track and timing, which impact
resultant amounts.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen