Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 AM EST Wed Nov 21 2018

Valid 12Z Wed Nov 21 2018 - 12Z Sat Nov 24 2018

...Northeast...
Day 1...

A potent closed low will track across Quebec today as the surface
low tracks just north of the Adirondacks and across Maine. An
arctic cold front will push across the area behind this low, and
snow showers are expected to accompany this frontal passage.
Moisture is relatively limited and and the system is progressive,
so snow accumulations will likely be light. WPC probabilities for
four inches are limited to the Tug Hill Plateau which will have
lake enhancement, the Adirondacks, and the Green and White
mountains where upslope processes aid totals. However, of note is
the upper trough axis trailing the surface low that crosses the
northeast this afternoon. Instability under that trough with peak
heating should allow for snow squall development over upstate NY
and into interior New England into this evening.


...The West...
Days 1-3...

Heavy snow is expected across all high mountain ranges of the
western CONUS during at least a portion of this three day
forecast. A few feet of snow is likely for the High Sierra and
northern Wasatch.

A very meridional trough (it spans the entire west coast) will
move ashore tonight with an associated surface cold front. This
will be accompanied by deep Pacific moisture and jet level
diffluence producing snow in the highest coastal ranges as early
as this afternoon. Significant moist advection ahead of the 700mb
trough will produce increasing column RH and as moisture spills
atop the coastal ranges, snowfall will spread from the Cascades
and Sierras, eastward across the Intermountain West to the Rockies
for Day 2. Snow levels will initially be 6-8 kft along the West
Coast states, but will fall to 3-4 kft in the Pac NW and 4-5 kft
for CA by Thursday morning. 24hr WPC probabilities for Day 1 range
from moderate for 18 inches in the high Sierra to high for 8
inches in the highest Cascades.

An atmospheric river (AR) event is expected to arrive into far
northern CA during the day Thursday. This will be accompanied by a
strong Pacific jet maximum and anomalously high PWAT and with that
warm clouds. Snow levels actually rise over CA/OR through Thursday
night in association with the AR. North of the baroclinic gradient
in northern OR/WA and east to the Sawtooth and Salmon River
Mountains of ID and mountains of western WY will experience heavy
snowfall with 12 inches or more possible in the highest terrain
from Day 2 through Day 3. As the trough that brought the AR to the
coast moves inland on Day 3, snow levels lower to 2-4 kft in ID/MT
which will allow the northern high plains and valleys to receive
snows Friday as moisture spreads well inland from the coast.


...Southern Appalachians to southern Mid-Atlantic...

A departing Canadian High, pushing east from the northern
Mid-Atlantic will leave a cold air damming wedge southwest to the
central and southern Appalachians Friday. The trough that reaches
the west coast tonight on a 150kt jet streak reaches the
southeastern CONUS Friday night. South flow will overrun this cold
air wedge and should result in some freezing rain that begins
Friday night. There are already low WPC probabilities for a tenth
inch of ice for Day 3 in western NC, VA, and into the Potomac
Highlands of the central Appalachians.


Otherwise, the probability of significant ice (0.25-inch or more)
is less than 10 percent.

Jackson