Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Mon Dec 09 2019

Valid 00Z Tue Dec 10 2019 - 00Z Fri Dec 13 2019

...Great Lakes...
Days 1-3...

Low pressure lifts northeast from Michigan this evening with lake
enhanced wrap around snow ending tonight for the western Lakes.
Strong cold air advection will continue into Tuesday as a
northwest wind shifts westerly with lake effect snow expanding to
Erie and Ontario. Lake effect snow continues into Wednesday night
until a clipper wave enters from the west Wednesday
night/Thursday. Heaviest snow through the three days looks to be
on the Keweenaw Peninsula where more than a foot is likely.


...Tennessee Valley through New England...
Days 1-2...

A strong cold front will cross The Southeast and Northeast through
Tuesday. A southern stream impulse lagging back into TX will
slowly migrate northeast, producing favorable divergence and
ascent in an anafrontal precip type setup where rain changes to
snow. At the same time, an intense jet streak of 170+ kt will
pivot northward, arcing into Canada, and leaving the favorable
right entrance region for diffluence and ascent from the TN Vly
through New England.

Rain ahead of the cold front overspreads the Southeast tonight. As
the column begins to cool, pockets of snow on the back side of the
precip shield are likely with some risk over the Mid-South late
tonight, spreading east across the TN Valley through Tuesday.
Strong low-mid level frontogenesis will likely lead to heavy
precipitation and dynamic cooling from the TN Valley, up the
Appalachians to New England. This suggests a burst of snowfall is
possible before the precipitation exits to the east, with the
heaviest snow likely in the terrain from WV and into the Poconos,
Catskills, and Berkshires, where WPC probabilities for 6 inches
are generally 20 to 30 percent. Farther south and east, and at
lower elevation, warm antecedent air will take longer to overcome
and cause a transition to snow, but minor accumulations are still
likely into the I-95 corridor and as far south as TN/AR. WPC
probabilities are low for 2 inches west of Nashville, but
moderately high east, including over eastern KY.

A secondary band of precip is progged by CAMs to cross the TN Vly
Tuesday evening. This precip is mainly snow over TN, but mainly
ice on the higher portions of Smokey Mountains. This raises the
risk for a quarter inch of ice on those peaks to around 25 percent
for Day 2.


...Pacific Northwest...
Day 3...

A mid-level trough moving onshore the Pacific Northwest Wednesday
will be accompanied by a 130kt jet streaking across the northern
Pacific Ocean, advecting moisture onshore. 850-500mb flow becomes
unidirectional from the west, and moisture spills into British
Columbia and then southeast into Washington/Idaho/Oregon/Montana.
Prolonged moisture fetch will produce heavy snow in the WA
Cascades where WPC probabilities of 18 inches on Day 3 are
moderate for the highest peaks. Higher elevations of ID/MT/OR to
Yellowstone have moderate Day 3 probs for 12 inches.



Jackson