Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 PM EST Wed Nov 14 2018
Valid 00Z Thu Nov 15 2018 - 00Z Sun Nov 18 2018
...An early-season winter storm to bring ice and snow from the
Midwest/Ohio Valley to the Appalachians and Interior Northeast
...Mid Mississippi Valley into the lower Ohio Valley...
A significant early-season winter storm will develop through the
evening into the overnight hours as an upper low continues to
deepen as it tracks from the lower Mississippi into the lower Ohio
valley. The GFS continues to show 500 mb heights 2-3 standard
deviations below the mean as the system lifts across the region
this evening and overnight. Overall, models remain in good
agreement, indicating an intense deformation band of snow which
will likely setup and pivot over far eastern Missouri and southern
Illinois. WPC probabilities for the 24-hr period beginning 00 UTC
Thu indicate that snow accumulations of 4 inches or more are
likely, with accumulations of 8-inches or more possible, over far
eastern Missouri, including the St. Louis Metro, and across
southern Illinois to the Indiana border.
...Appalachians and Northeast...
As the previously noted low ejects from the Mississippi Valley
toward the Northeast, warm advection ahead of this feature is
forecast to produce widespread precipitation from the Southern
Appalachians through Maine. At the surface, a cold high pressure
centered over New England will only slowly retreat to the east,
but maintain a wedge down the east side of the Appalachians.
Significant forcing and ample moisture will produce heavy
precipitation along the East Coast, with mixed precipitation
Expect sleet and freezing rain to the be predominate precipitation
type for most of the event along the central Appalachians with the
models continuing show a significant signal for freezing rain
amounts of 0.25 inch or more, especially along the western
Virginia and eastern West Virginia mountains. Some models
continue to show some greater amounts across this area, however
there remains the question of how much sleet will occur - which
would temper those amounts.
Further to the north, although areas as far north as southern New
England may experience freezing rain, a colder overall airmass and
mid-level confluence will prevent the warm nose from getting
nearly as strong as it will further south, affording more sleet
and snow at the onset and less freezing rain. As the upper low
moves moves from the Ohio valley into the Mid-Atlantic, a surface
low will develop along the Mid Atlantic coast and lift northward
toward New Jersey Thursday evening. The surface low is expected to
hug the coast as it tracks along the Northeast coast into Atlantic
Canada on Friday. A deformation band forecast to setup northwest
of the low is expected to enhance snowfall amounts from
north-central PA to northern New England, with WPC probabilties
indicating the likelihood of accumulations of 4-inches or more
across the area.
The major cities from Washington DC through Boston will likely be
too far southeast for significant snow or ice. There is moderate
potential that some snow/sleet/freezing rain will occur across
these cities with the initial warm advection precipitation, but
low-level winds becoming east-southeast will quickly transition
precipitation type to rain.
A shortwave will dig into the Northern Rockies Thursday into
Friday accompanied by modest Pacific jet energy. At the surface a
cold front will dive southwards into the Northern Plains and bank
against the mountains producing a sharpening baroclinic gradient
and upslope low-level flow. A wave of low pressure will develop
along this temperature gradient, while Pacific moisture causes a
steady increase in column relative humidity. Lift associated with
upper diffluence and the developing surface low will produce
widespread elevation snows from the Northern Rockies near Glacier
National Park, southward towards the Laramie mountains of Wyoming.
The highest snowfall is expected near Glacier National Park as
well as the Absaroka and Big Horn ranges, where snowfall totals
may reach 12 inches.