Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 AM EST Sun Dec 08 2019

Valid 12Z Sun Dec 08 2019 - 12Z Wed Dec 11 2019


...West...
Days 1-2...

A positively tilted longwave trough will shift into the Great
Basin this morning and then split into northern stream and
southern stream pieces of energy. The northern stream will eject
quickly into the Great Plains, while a southern piece will lag
back across California Monday before finally shifting across the
Southwest into Texas on Tuesday. Pronged moisture fetch within a
Pacific Jet streaking eastward from California will bring
anomalous moisture across much of the West, especially south of
45N and early on D1, as noted by PWAT anomalies of 1-1.5 standard
deviations above the climo mean. This moisture will be wrung out
as snowfall due to large scale ascent within the diffluent portion
of this jet streak, as well as height falls associated with the
aforementioned trough. Widespread heavy snowfall is likely through
tonight from the ranges of NW WY southward through the Wasatch of
Utah, the Colorado Rockies and San Juans, and southward into New
Mexico and the highest terrain of AZ. Additional snowfall is also
likely in the Sierra, but the better forcing will gradually shift
eastward. WPC probabilities are high for 12 inches in the Wasatch,
Tetons, and ranges of CO and NM.

By day 2, the remaining snow in the West will be focused in the
vicinity of the southern stream impulse lagging in the positively
tilted trough. This will shift east across AZ/NM and into TX
Tuesday, with mid-level divergence and jet-streak diffluence
producing ascent behind a cold front digging through TX. While
most of the snowfall associated with this feature should be
confined to the terrain of New Mexico, there is potential for some
low-end snow amounts into the High Plains of NM and eastward into
north Texas. WPC probabilities for 4 inches are moderate in the
Sacramento Mountains, with low probabilities for 1 inch extending
eastward to 100W latitude from there.


...Northern Plains into the Great Lakes...
Days 1-3...

Northern stream shortwave racing eastward from the longwave trough
in the west will push from MT through the Dakotas and rotate
around a developing neutral trough into the Great Lakes on day 2.
Beneath this impulse, a weak clipper type low will develop along
an arctic front, further enhancing lift along a narrow but robust
band of mid-level frontogenesis. The rapid progression of this
wave will limit the accumulation potential, but extremely cold air
should increase SLRs considerably to "fluff" the snow amounts.
Despite QPF of around 0.25"-0.40", the brief but intense forcing
within the DGZ and the high SLR could produce heavy snow rates,
potentially reaching 1"/hr as noted by the WPC prototype snowbank
tool and HREF probabilities exceeding 50%. WPC probabilities for
6" are 40-80% across the Dakotas, central MN, and into WI. The
highest amounts are likely in WI where some lake enhanced snowfall
south of Lake Superior is likely.

Behind this system, intense CAA on W/NW winds will drive
increasing lake effect snow in the favored regions downwind of all
the lakes. The heaviest LES through D3 is likely across the U.P.
and NW L.P. of MI, where WPC probabilities for 4 inches are
modest. More significant LES may develop just beyond this current
forecast period.


...Central Appalachians through New England...
Day 3...

A cold front will drop southeast through the region Tuesday
afternoon, while a southern stream shortwave lags back across
Texas. As this shortwave begins to lift northeast, forcing through
height falls and mid-level divergence will lift atop the front
producing anafrontal moisture. As the cold air continues to filter
southeastward, the column will cool, and forcing may become
intense through robust and steeply sloped 850-600mb frontogenesis
overlapped with RRQ diffluence of an intensifying 170+kt upper jet
streak. The combination of intense ascent and a cooling column
will cause precipitation to changeover from rain to snow. The
guidance still differs considerably in timing and location of
precipitation, but confidence is increasing in at least light snow
accumulations along a swath from central TN northeast into
southern ME. WPC probabilities for 2" are above 10% only in the
terrain of the central Appalachians and into the
Catskills/Berkshires, but with such intense forcing it is possible
the column will cool more rapidly, and light snow accumulations
may be possible early Wednesday even into the I-95 corridor.

For Days 1-3, the probability of significant icing (0.25-inch) is
less than 10 percent.

Weiss