Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Mon Dec 24 2018

Valid 00Z Tue Dec 25 2018 - 00Z Fri Dec 28 2018


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

Two distinct shortwaves will advect into the western CONUS this
period bringing rounds of heavy snow, especially into the
mountainous regions. The first will be a potent shortwave digging
southward through California tonight into Tuesday before ejecting
into the Four Corners region. Into day 2, this feature will tilt
negatively and close off near the Panhandles of TX/OK before
lifting into the Plains. Significant moist advection and upper
diffluence associated with Pacific Jet energy will produce heavy
snowfall across the Sierras of California through Tuesday, with an
additional snowfall maximum likely in the high terrain of the San
Juans of Colorado on Wednesday. In these ranges, WPC probabilities
are high for 8 inches or more of snowfall, with over 12 inches
likely. Across the remainder of the West, from Nevada
southeastward into the mountains of AZ/NM, moderate snow
accumulations are also likely. In NV/UT, snow levels will fall to
2000 feet suggesting a few inches of accumulation are possible
down into the Valleys, while further southward snow levels remain
above 5000 feet, limiting snow accumulations to the mountains. WPC
probabilities are moderate for 4 inches of snow.

On day 3, Thursday, another impulse will dig into the Pacific
Northwest accompanied by a 130kt Pacific Jet. Snow levels will
rise ahead of this feature, but significant snowfall is likely
above 4000 feet in the Cascades of Washington and Oregon, as well
as the Olympics. WPC probabilities are high for 4 inches of snow,
with moisture spilling further eastward producing additional light
accumulations towards Idaho.


...Central Plains through the Western Great Lakes...
Days 2-3...

The first shortwave mentioned above will close off across the
TX/OK panhandles and then eject northeast towards the Western
Great Lakes by Thursday night. This will spawn surface
cyclogenesis over TX which will then lift towards Wisconsin on
Thursday. Warm moist advection will accompany this system, and
PWAT anomalies are forecast to reach +4 standard deviations above
the mean in the Eastern Plains and MS VLY. Some of this moisture
will wrap cyclonically around the low within a mid-level TROWAL
and into the cold air, producing heavy snowfall NW of the surface
feature. The combination of lift due to mid and upper diffluence,
intensifying 700mb deformation, and robust isentropic ascent will
produce widespread snow, with cold air wrapping into the low as
925-700mb frontogenesis maximizes. Strong lift within the
saturated DGZ suggests heavy snow, amplified by 700-500mb lapse
rates of theta-e less than 0, and WPC probabilities show a high
risk for 4 inches of snow in a large swath from central NE
northeast into the U.P. of Michigan.

The ECMWF and its ensembles have been consistent, and match well
the FV3 and recent CMC runs, while the NAM appears too aggressive
lifting the surface low into the cold air, and the GFS is too
suppressed and weak with the upper features. The preferred blend
is then heavily weighted on the ECMWF/ECens mean, and many of the
members depict over 12 inches of snow in a narrow band from NE
into MN. Uncertainty still exists as to the exact placement of the
surface low, as well as any banded structures which are likely,
but it is possible over 1 foot of snow will occur in several
locations where the best combination of deformation and upright
instability coincide.

Additionally, there may be an area of freezing rain at onset which
may produce one-tenth of an inch of accretion across western KS
into southern NE. The orientation of the surface high is such that
the surface cold air should erode in time to prevent more
significant accretion, but WPC probabilities do show a few areas
of 10-20 percent for 0.25 inches of freezing rain.  

Weiss