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

Valid 00Z Thu Nov 22 2018 - 00Z Sun Nov 25 2018

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

Heavy snowfall is expected across all the high mountain ranges of
the western CONUS, from California to Colorado and points north,
through the entire three day forecast. Up to 3 feet of snow is
possible in the hardest hit and highest terrain.

A deep mid-level trough will move onshore tonight with an
associated surface cold front beneath it. This will be accompanied
by robust Pacific moisture and jet level diffluence as a 140kt jet
maximum lifts into California. Significant moist advection and of
the 700mb trough will enhance deep column RH, and as moisture
spills across the coastal ranges, snowfall will spread from the
Cascades and Sierras, into the ranges as far east as the Colorado
Rockies. Snow levels initially of 6-8 kft will fall through
Thursday to 3-5 kft, especially north of California. The highest
snow totals are likely in the Sierra where synoptic ascent and
forced upslope enhancement will combine for the longest duration
in an increasingly moist column. WPC probabilities are moderate
for 18 inches of accumulation in the high Sierra, with 2 feet
possible. Elsewhere, WPC probabilities are highest for 8 inches in
the Oregon and Washington Cascades, as well as the Uintas and
northern Wasatch of Utah.

For D2/D3, an atmospheric river (AR) event is likely to advect
into northern California and then shift eastward into Colorado
late in the forecast period. GFS probabilities are over 90 percent
for IVT of 250 kg/m/s affecting northern CA on D2, Friday, and
UT/CO on Day 3, Saturday. This AR will be accompanied by intense
synoptic ascent as the left front quadrant of a 130kt upper jet
drops into region, and favorable 850-700mb will upslope to enhance
lift. PWAT anomalies of +2 to +2.5 standard deviations above climo
suggest ample moisture for heavy snow. The entirety of the western
ranges should see heavy snow, and high probabilities exist for
over 12 inches from the Washington Cascades, to the Sawtooth of
ID, the ranges of NW WY, NE UT, and the northern Rockies in
Colorado, and 2 feet of snow is possible above 8000 feet. As snow
levels lower behind a secondary cold front, even the valleys down
to 3000 feet may see accumulating snow, but with much lower
accumulations likely.

Further east towards western SD including the Black Hills, WPC
probabilities show the potential for a few inches of snow beneath
a departing upper jet streak and ahead of the sharpening upper
low.


...Southern Appalachians to southern Mid-Atlantic...
Day 3...

Mid-level trough moving across the CONUS will gradually tilt
negatively as it approaches the Appalachians Friday night. A
strengthening 850-700mb LLJ will advect increasing moisture into
the column, and modest isentropic lift atop a slowly retreating
cold Canadian High will cause precipitation to overspread the
region. Initially, the cold surface high will keep temps below
freezing despite a rapid advection of the warm nose causing 850mb
temps to climb to +5C by Saturday morning. Area forecast soundings
show the surface cold layer eroding quickly as lack of mid-level
confluence allows the wedge high to move away quickly. Some light
freezing rain/sleet is likely across the southern Appalachians,
from the NC/SC border, to as far north as south central PA, but
accretions should be light as the precipitation changes to rain.
WPC probabilities are near 0 for 0.25 inches of freezing rain, but
do reach 20-30 percent for 0.1 inches.


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

Weiss