Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Tue Nov 26 2019

Valid 00Z Wed Nov 27 2019 - 00Z Sat Nov 30 2019

...Central Plains through the Great Lakes...
Day 1...
An intensifying surface low moving out of Colorado will move
quickly northeast to be over Lake Huron by the end of Day 1
/Wednesday night/. The guidance this afternoon has shifted subtly
southeast with the low track, but the overall evolution has good
model consensus. A strong upper jet amplifying to 170+kts across
Texas will shift northeast combined with height falls ahead of a
closing upper trough will help strengthen the surface low as it
moves northeast. On the NW side of this system, intense ascent is
likely, both thanks to the synoptic lift and intensifying
mesoscale dynamics. This includes bands of frontogenesis,
overlapping strong deformation which will pivot across MN/WI. The
strongest lift is likely across SE MN and into WI where strong
forcing should produce snowfall rates of 1-2"+/hr at times as
noted by HREF probabilities and theta-e lapse rates slightly less
than 0. Although the system should remain progressive, the intense
snow rates over several hours will produce significant snow
accumulations. WPC probabilities are high for 6 inches from far NW
Iowa through much of SE Minnesota, northern WI, and into the U.P.
of Michigan. Probabilities for 12 inches are above 50% along the
southern shore of Lake Superior and into the U.P. where WAA
ascent, pivoting deformation, and post-low lake effect enhancement
will all combine, and a few locations on the U.P. may approach 24
inches before the storm exits Wednesday night.


...Maine...
Day 2...
The upper low moving through Michigan will shift eastward and
spawn secondary cyclogenesis in southeast Canada. This feature
will then move quickly to the east into the Gulf of Maine early
Thursday morning. Modest WAA ahead of the low will produce ascent
that is marginally cold enough for snowfall. While some mixed
precipitation is possible, most of New England should experience
rain. The exception will be across parts of northern ME, where a
brief but moderate period of snowfall is likely. WPC probabilities
are high for 4 inches.


...The West...
Days 1-3...
An extremely powerful surface low and associated anomalous upper
trough will move onshore the coast of Oregon tonight before slowly
filling as it drifts southeastward into the Great Basin. As this
system fills, it will drive a surface cold front down along the
coast of California through Wednesday, with secondary low
development likely along this front. At the same time, the
extremely cold upper low will shed spokes of energy cyclonically
onshore, which combined with robust Pacific jet energy will
produce periods of intense lift and heavy snow through the periods.

Tuesday night into Wednesday, the heaviest snow is likely from the
Cascades of Oregon southward into the Siskiyous, Shasta/Trinities,
and much of the Sierra of California. In these ranges, the
combination of intense ascent aided by upslope enhancement as
700mb flow becomes orthogonal to the terrain should produce
widespread 12 inches of snow, with more than 2 feet possible,
especially in the Sierra. Snowfall rates may exceed 3"/hr at
times. A brief lull in ascent will occur on day 2 as a secondary
spoke of energy approaches the coast. Significant moisture and
residual ascent lifting northeast will still produce heavy snow in
the highest terrain of the Great Basin including the Wasatch and
Uintas, as well as the San Juans. Heavy snow will redevelop late
on D2 in the Transverse Ranges of southern California as well. WPC
probabilities across these ranges are moderate for 12 inches. On
D3, Thursday night into Friday, another impulse will move onshore
behind the cold front, with increased moist advection developing
on 700mb southerly flow. Flow will be nearly perpendicular to the
Transverse Ranges and the Mogollon Rim, as well as the San Juans
and southern Wasatch. WPC probabilities are high for 12 inches,
and may exceed 2 feet in the higher terrain. Additionally, by day
3 snow levels fall as low as 3000-4000 ft in southern CA into AZ,
and as low as 1000 ft in the northern Great Basin. This should
allow for light accumulations of snow all the way into the valley
floors of Nevada and Utah.


...New Mexico and Texas...
Day 2...
Weakening mid-level impulse lifting northeast from the Baja region
of Mexico will ascent atop a surface high pressure extending down
the Great Plains. Ascent associate with this feature will cause
precipitation to overspread the region. Initially, surface
temperatures will likely be below 0C, and as WAA commences, a
period of freezing rain is possible. The signal has improved for
accumulations of fzra in eastern NM and the Panhandle of TX, and
WPC probabilities are as high as 10% for 0.25" early on Thursday
before the WAA changes all the precipitation over to rain.

Weiss