Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Wed Nov 27 2019

Valid 00Z Thu Nov 28 2019 - 00Z Sun Dec 01 2019

...Western U.S...
Days 1-3...

Broad but intense upper low will continue to plague the west,
while spokes of energy rotate cyclonically around it. The primary
upper low will drift down into the Great Basin into Friday, before
splitting, with the primary energy moving into the Northern
Plains, while subsequent re-deepening occurs in response to new
energy dropping out of the Pacific off the Oregon coast. While the
former will eventually lead to strong cyclogenesis in the Plains,
the primary low and the wave re-deepening off the Pacific Coast
will continue to spread moisture into the Mountain West. Deep SW
flow on the southern edge of these upper features will advect
significant moisture across the West, with height falls, PVA, and
jet level diffluence all working together to produce deep layer
ascent. Additionally, strengthening S/SW flow at 700mb, especially
D1 into D2, will reach into the terrain orthogonally in the
Sierra, Coastal SoCal ranges, Wasatch, Uintas, and San Juans. This
will drive orographic enhancement and heavy snowfall, with snow
levels falling below 1000 ft to the north, and as low as 4000 ft
in AZ/NM. The heaviest snowfall is expected on D1 and D2 in the
terrain where WPC probabilities in the aforementioned ranges are
high for 12 inches, and 2-day totals could reach 4 feet or more in
the highest terrain. With lowering snow levels, several inches of
snow is even possible into the Valley floors. By D3 /Friday night
and Saturday/ most of the energy will lift into the Plains
bringing a respite from the snow except in the northern Wasatch
and points northeast where WPC probabilities remain high for 6
inches. However, renewed moisture and forcing will produce heavy
snow again in the ranges of NW CA, where WPC probabilities are
moderate for 6 inches.

...Maine...
Day 1...

An upper low across the Great Lakes will move quickly eastward,
leading to pressure falls and surface low development near the
Gulf of Maine. The trend in the guidance has continued to be
subtly southward with the axis of heaviest precipitation, but
broad mid-level divergence and robust WAA will produce a swath of
heavy snow/mix across northern and central Maine. Modest
frontogenesis and a stripe of 700mb deformation may combine to
produce a burst of heavier snowfall as well, and WPC probabilities
are high for 6 inches in the terrain of N/Central Maine, with 2-4"
likely in the lower elevations away from the coast.


...New Mexico/Texas...
Day 1...

A subtropical shortwave and associated Pacific moisture will eject
northeast from near Baja, Mexico as it weakens and becomes
embedded in the SW flow. Ascent through height falls and modest
PVA will produce a region of precipitation lifting north from New
Mexico into Oklahoma. Thermal profiles are favorable for
precipitation to begin as snowfall, but as WAA increases the warm
nose will lift northward atop sub-freezing surface air to produce
freezing rain. Some light snow accumulations are likely in the
High Plains and Texas Panhandle, with much higher amounts likely
in the terrain of New Mexico including the Sangre De Cristos. As
p-type transitions to freezing rain, there is potential for modest
accretions across eastern NM and into the TX panhandle, with heavy
accretions possible in the Sacramento and Guadalupe Mountains
where WPC probabilities are 20-40% for 0.25".


...Central/Northern Plains...
Day 3...

...Significant Winter Storm possible beginning Friday night...

Closed low across the Great Basin will fill as it ejects
northeast, but will re-energize and deepen into a negatively
tilted closed low over the western High Plains. This combined with
an increasing jet streak to the south will drive cyclogenesis in
the lee of the Rockies which will then lift northeast into the
Northern Plains. This system is likely to rapidly deepen, with
pronounced ascent spreading from the Central Plains into the
Northern Plains on D3. While there remains considerable spread in
the guidance, notably the NW displacement and warmth of the
GFS/GEFS, this seems to be an outlier and the forecast features a
blend of the ECMWF/ECENS/NAM which suggest a widespread light
icing event becoming a widespread heavy snow event. The light
icing is likely Friday into Friday night as modest isentropic
ascent ahead of the developing cyclone spreads precipitation
northward. WPC probabilities are low for 0.1" outside of the
Buffalo Ridge on D3. As the storm develops more intensely into
Saturday, ample moisture will likely be thrown back on a LLJ and
warm conveyor into the cold air across the Northern Plains. The
mid-level low and surface low may stall in response to blossoming
ridging to the north, so a prolonged snow event is likely, and WPC
probabilities are already high for more than 8 inches of
accumulation from eastern WY and MT into much of SD and ND.


Weiss