Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 PM EST Thu Nov 22 2018

Valid 00Z Fri Nov 23 2018 - 00Z Mon Nov 26 2018

...Western U.S...
Days 1-2...

Two distinct shortwaves, one on Friday and one more potent on
Saturday, will both advect into the west accompanied by Pacific
jet energy and moisture. The first shortwave will move across
Oregon and race southeastward into the Colorado Rockies,
accompanied by ascent driven by jet level diffluence and upslope
enhancement on increasing westerly 700mb flow. The more vigorous
shortwave will follow quickly in its wake on Saturday, accompanied
by even stronger forcing as well as a low-level cold front which
will allow snow levels to fall from north to south. This stronger
forcing combined with falling snow levels will create significant
and widespread mountain snows from the Washington Cascades,
southeast into the Colorado Rockies, with the heaviest snow likely
in the mountains of Utah. WPC probabilities for 8 inches of snow
are high in the Sawtooth, Cascades, and Absarokas on day 1. By day
2, the highest probabilities for 8 inches shift into the Uintas of
Utah and the Colorado Rockies, as well as continuing in the
mountains of NW WY. Through the 2-day period, many peaks above
8000 feet will see at least 1-2 feet of snow, with locally up to 3
feet possible. At lower elevations down to 3000 feet,
accumulations of much lower magnitude are likely, and WPC
probabilities are low for 4 inches.


...Southern Appalachians to Southern Mid-Atlantic...
Day 2...

Mid-level trough will begin to take on a negative tilt as it moves
across the Mississippi Valley, spawning low pressure along the
Southeast coast. This will persist SW mid-level flow atop a slowly
retreating surface wedge of high pressure. The increasing
isentropic lift in response to this setup causes precipitation to
overspread the region from south to north late Friday night
through Saturday. Initially, surface temperatures will be below
freezing in the terrain of the southern Appalachians, but a warm
nose between 850-700mb will advect northward causing a mix of
wintry precipitation. Although precipitation is expected to
changeover entirely to rain by Saturday morning, a brief period of
freezing rain is likely. WPC probabilities have increased for up
to 0.1 inches of accretion across the mountains of NC/VA/WV, and
are now up to 30-40 percent. Probabilities for 0.25 inches remain
10 percent or less, and any accretion should melt with warming
temperatures and rainfall later on Saturday.


...Central Plains into the Great Lakes...
Day 3...

Potent shortwave ejecting from the west will move eastward from
near CO Saturday night, to NE MO by Sunday evening, while taking
on a negative tilt and possibly closing. Associated with this
feature, a coupled upper jet structure will work in tandem with
the height falls to rapidly strengthen a surface low as it lifts
northeast through the Central Plains. As the low strengthens,
widespread snowfall is likely in a swath from Nebraska
east-northeastward towards Lake Michigan. While guidance differs
on the exact track of this surface low, heavy snowfall is likely
just NW of the track where the best frontogenesis and deformation
coincide to produce ascent within a saturated DGZ, and theta-e
lapse rates fall below 0. This suggests that snowfall rates will
likely exceed 1"/hr at times on Sunday, and dynamic cooling due to
this intense snowfall will overcome any minor warm nose along and
north of the eventual surface track. During the period of heaviest
snowfall, winds will intensify as the gradient pinches between the
deepening low and a surface high pressure centered over central
Canada, and blizzard conditions are possible.

The GFS is a south/fast outlier, even from its ensemble mean, and
as such a NAM/ECM/CMC blend is preferred for this forecast cycle.
With uncertainty existing in the exact track, spread in forecast
accumulations remains high, but enough confidence exists at this
time that WPC probabilities have been raised so that areas
centered near southern Iowa have a high risk for 4 inches of
accumulation. A risk for 8 inches also exists in that same area,
and the 90th percentile now reaches 12 inches, but this is a low
probability and will depend on a long enough period of heavy snow
rates beneath the pivoting deformation band.


The probability of significant ice (0.25-inch or more) outside of
the Appalachians is less than 10 percent.

Weiss