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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 2127Z Nov 15, 2018)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EST Thu Nov 15 2018

Valid 00Z Fri Nov 16 2018 - 00Z Mon Nov 19 2018


...An Early-Season Winter Storm to bring ice and Heavy Snow from
the Midwest/Ohio Valley to the Appalachians and Interior New
Northeast through Friday...

...Northern Appalachians and Northeast...
Days 1-2...

A broad upper level low pressure moving out of the mid-Mississippi
Valley will be filling as it continues to translate
east/northeastward across the central Mid-Atlantic coast by later
tonight before it changes course and heads northeast along the New
England Coast on Friday. Widespread precipitation from the
central/northern Appalachians into New England will continue to be
fueled by a warm conveyor belt with extends well south into a
moisture-rich atmosphere.  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 the overnight hours tonight. A wintry mix line
is expected near the I-95 corridor and inland with a swath of
mainly snow along the northern and western side of the
precipitation shield...with drier air eventually spreading
eastward and northward behind the system.

Uncertainty lingers as to the exact placement of the various
precipitation types even though the 12Z suite of model guidance
got into somewhat better agreement with respect to the extent and
strength of the warm nose.  The root of the uncertainty appears to
be rooted in the magnitude of vertical velocities within the deep
near-isothermal layer.

Farther north, eastern New England likely see a swath of freezing
rain along and just inland from the coast as a low pressure system
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 later
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.

...North and Central Rockies and Great Plains...
Days 1-3...

A shortwave trough digging into the Northern Rockies, supported by
modest Pacific jet energy, will spread moisture out over the 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 through 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 a roughly equal blend of the 15/12Z
ECMWF/GFS.

Bann