Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Thu Nov 15 2018
Valid 12Z Thu Nov 15 2018 - 12Z Sun Nov 18 2018
...An early-season winter storm bringing ice and snow from the
Midwest/Ohio Valley will intensify as it shifts east to the
Appalachians and Interior Northeast today before turning northeast
...Appalachians and Northeast...
Upper level low pressure over the mid-Mississippi Valley
translates east to the central Mid-Atlantic coast through tonight
before shifting northeast along the New England Coast Friday. A
tropically connected warm conveyor belt ahead of the upper low
will lead to 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 cold air damming wedge down the east side of the
Appalachians into tonight. A wintry mix line is expected near the
Fall Line and inland with a swath of mainly snow along the
northern extent of the precipitation shield from the low.
The extent and strength of the warm nose remains uncertain with
00Z guidance increasing confidence of a large isothermal layer
near 0C developing over the interior northern Mid-Atlantic. 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 for higher elevations
from eastern West Virginia to the Pocono mountains of PA.
Farther north, eastern New England likely see a swath of freezing
rain along and just inland from the coast as the low develops and
becomes vertically stacked. A weaker warm nose in New England
should result in 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 enhance along the
Mid-Atlantic coast and lift northward to New Jersey this evening.
The surface low is expected to hug the coast as it tracks along
the Northeast coast into Atlantic Canada on Friday. A TROWAL is
forecast to setup northwest of the low is expected to enhance
snowfall amounts from central PA to northern New England, with
moderate WPC probabilities for 8 inches over this area.
The major cities from Washington DC to New York City are too far
southeast for significant snow or ice. There is an expectation for
some snow/sleet/freezing rain across these cities with the initial
warm advection precipitation, but low-level winds becoming
east-southeast will quickly transition precipitation type to rain.
Boston has a greater chance of snow with moderate WPC
probabilities for 2 inches on coastal eastern MA.
...North and Central Rockies and Great Plains...
A shortwave will dig into the Northern Rockies tonight bringing
modest Pacific jet energy and moisture into the north-central
CONUS. At the surface a cold front will dive southward across the
Northern Plains and bank against the mountains producing a
sharpening baroclinic gradient and upslope low-level flow late
tonight in MT and Friday, then farther south to the central Plains
and CO Saturday. A wave of low pressure will develop along this
temperature gradient over the northern Plains tonight and Friday
while Pacific moisture leads to 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
toward the Laramie mountains of Wyoming. Also, a band of snow can
be expected to extend from MT/ND Friday to MN/IA with a preference
given to the 00Z ECMWF/FV3 which are between the farther south GFS
and farther north NAM.