Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EST Mon Dec 16 2019

Valid 12Z Mon Dec 16 2019 - 12Z Thu Dec 19 2019


Day 1...

...Central Plains through the Ohio Valley and Mid-Atlantic to
southern New England...
The remnants of an initial wave will push off the north-central
Mid-Atlantic coast this morning, bringing with it the round of
snow ongoing over the area. 

The positively tilted long-wave trough pushing across the CO
Rockies now will promote surface cyclogenesis over the lower MS
Valley this morning that will track along the baroclinic boundary
laid by the first wave. Mid-level forcing ahead of the trough axis
above low level convergence will continue to promote snow across
southern KS (south of the previous round). However, the more
northeast trajectory of this convergence band that becomes the
comma head snow with the developing low will cross the previous
snow swath over eastern MO this afternoon and track across the
interior Northeast trough Tuesday night. Day 1 WPC probabilities
for four inches are around 30 percent just south of St. Louis,
decreasing then through central IN.

Warm air advection ahead of the developing surface low that tracks
up the existing stationary front and crosses the southern
Appalachians tonight will promote a wintry mix over the
Mid-Atlantic for an extended amount of time. A cold surface and
warm nose will maximize freezing rain over the northern Potomac
Highlands and Laurels of PA where Day 1 WPC probabilities for a
quarter inch of ice are moderate to high. Low probabilities for a
quarter inch of ice spread east from there to northern NJ. A
lesser area of freezing rain is progged over southern MO this
morning, north of the developing low, though low probabilities for
a quarter inch of clear ice are there for Day 1.

Days 2/3...

...Northeast...
The largest issue for the Northeast remains the exact track of the
low and northward extend of the associated precip shield. 00Z
guidance trended north with good agreement in the snow axis over
upstate NY into New England among the 00Z GFS/ECMWF/UKMET/NAM.
Magnitude differences remain and there has been rather large
differences in placement over the past few runs. That said, a
swath of moderate to locally heavy snow can be expected as the
trough is drawn into a cold core low that crosses north of the
Great Lakes Tuesday night. Day 2 WPC probabilities for four inches
are moderate over mainly higher elevations from the Tug Hill,
Adirondacks, Green and White mountains. However, the progressive
nature of the systems affecting this area results in nearly nil
probabilities for six inches on Day 2.

...Great Lakes...
A strong cold front associated with the cold core low will push
southeast through the Great Lakes Tuesday through Tuesday night.
Behind the front, strong northwest winds across the Upper Lakes
and west-northwest winds across the Lower Lakes will support lake
effect snow showers, with significant accumulations possible over
the eastern U.P. and northern Lower Michigan, as well as western
New York and the Tug Hill region before showers diminish Wednesday
night. Day 2 probabilities for four inches are moderate over the
coastal UP and northern LP of MI with moderate for four off Lakes
Erie and Ontario on Day 3.


...West Coast... 
A deep low shifts across the Gulf of Alaska Tuesday night through
Thursday with a plume of ample Pacific moisture streaming to the
south with the initial wave pushing into the Pacific Northwest
Wednesday night. Day 3 probabilities for 8 inches of snow are
moderate to high for the Olympics and WA Cascades. This plume then
continues to push into the West Coast into or through the weekend
for an extended heavy precipitation event.

Jackson