Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Thu Nov 28 2019

Valid 00Z Fri Nov 29 2019 - 00Z Mon Dec 02 2019

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

Broad but intense upper low will continue to drift across the
Great Basin into D2 before ejecting eastward. As this occurs, a
secondary upper trough will amplify off the Pacific Coast and dig
towards California on D3. On Day 1 /tonight and Friday/,
persistent moist flow on low to mid level S/SW winds will provide
ample moisture for snowfall. At the same time, strong ascent
through upslope enhancement as well as spokes of PVA/jet
diffluence rotating around the trough will produce periods of
heavy snowfall across nearly all of the terrain of the West from
the Southern California transverse ranges northeast through the
Big Horns of Wyoming. Snow levels will be quite low as well, so
light snow accumulations are possible even onto the valley floors
of the Great Basin. The heaviest snow is likely in the highest
elevations of the ranges most orthogonal to the flow: The Mogollon
Rim, The Wasatch, Uintas, and the San Juans. In these ranges, WPC
probabilities are high for 12 inches, and may exceed 2 feet.

By day 2, most of the energy shunts eastward so the mountains will
likely get a brief respite. However, Saturday night the next upper
low and surface reflection approach the west with a renewed surge
of moisture. A strong Pacific Jet and moist confluent low-level
flow will produce snowfall once again, this time focused into the
Sierra and other ranges of Northern California, where WPC
probabilities are high for 12 inches. Lighter accumulations are
likely spreading northeast into the Cascades of Oregon and into
portions of the Great Basin.


...Central/Northern Plains into the Great Lakes...
Days 2-3...

...Significant Winter Storm likely beginning Friday night...

Shortwave pivoting around a large upper low over the Great Basin
will amplify into a closed mid-level circulation as it ejects
northeast into the Central Plains Friday night. This feature will
then slow as it becomes blocked by strengthening high pressure to
the north, before redirecting to the E/SE through Saturday night.
Beneath the mid-level low, surface cyclogenesis is expected in the
lee of the Rockies, with a deepening surface low lifting northeast
into the Plains before racing eastward towards the Great Lakes by
the end of the forecast period. The GFS remains a deep outlier
compared to the preferred NAM/ECMWF, while the CMC is a weaker and
more progressive system. Using the NAM/ECMWF heavily in the
forecast blend produces a period of freezing rain during the first
several hours of the event, before transitioning to heavy snow.
Confidence is still modest into the evolution, but has increased
in a significant winter storm beginning Friday night.

In the leading WAA regime, low-levels will saturate on southerly
flow. As this occurs, mid-levels, including the DGZ, remain dry,
so despite temps below freezing, an extended period of freezing
drizzle or light freezing rain is likely. QPF during the time of
freezing rain should be limited, and although FZRA/FZDZ may be
widespread, WPC probabilities for 0.1 inches are less than 10
percent across most of the region. The exception is in the
vicinity of the Buffalo Ridge where some topographic effects may
allow for heavier accretion, and WPC probabilities for 0.25 inches
are 5-10%.

As the low strengthens and moves eastward, isentropic lift will
cause precipitation to become heavy and widespread from Wyoming
through Wisconsin. Pronounced ascent of the warm conveyor into a
TROWAL should produce heavy snow, and potential CSI banding noted
by upright-to-folded theta-e surfaces could produce rates of
1-2"/hr. There is likely to be a large swath of snow of greater
than 6 inches, and WPC probabilities indicate moderate to high
chances for 12 inches from NE Wyoming into SE Montana, along the
vicinity of the ND/SD line, and into parts of Minnesota and
Wisconsin. The heaviest snow, potentially greater than 2 ft, is
likely in the Black Hills where upslope enhancement will also
occur.

Along the southern edge of the heavy snow, especially in eastern
SD and into MN/WI, there remains uncertainty into how far north
the dry slot will wrap around the low pressure system. If it does
lift well into MN, snow amounts would be less and freezing rain
could increase due to an extended period of freezing drizzle as
noted by soundings. However, if it were to stay further south,
snow amounts could increase even into the Twin Cities of MN. This
will need to be monitored with future updates, and the current
forecast reflects a blend of the guidance as noted above.


...Northeast...
Day 3...

Surface low pressure moving across the Great Lakes will transfer
its energy to new development off the Mid-Atlantic coast. WAA
ahead of this system will produce a swath of precipitation lifting
northeast from the Mid-Atlantic into the Northeast on Saturday
night into Sunday. The leading edge of this precip shield will
encounter air that is cold enough for mixed precipitation, and
some light accumulations of snow and freezing rain are possible.
For snow, WPC probabilities are moderate for 4 inches in the
terrain of upstate NY. For freezing rain, the highest
probabilities for 0.1" are around 50% and focused in central PA.
Much of this will depend on the evolution of the transferring
system, so confidence is lower than normal at this time.

Weiss