Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 PM EST Sat Dec 14 2019

Valid 00Z Sun Dec 15 2019 - 00Z Wed Dec 18 2019


Day 1...

...California to the Central Rockies...
A well-defined mid-level shortwave will dive southeast across
Northern California into the Great Basin before turning east
toward the Four Corners region on Sunday. Some additional light
snow accumulations can be expected across the northern coastal
ranges of California into the Sierra Saturday evening before snows
diminish overnight.  Light accumulations are also expected across
the northwestern to central Nevada mountains late Saturday into
early Sunday as well.  Then by early Sunday, increasing
southwesterly winds ahead of the digging wave will increase
moisture along a low-to-mid level baroclinic zone centered along
the Utah-Arizona and Colorado-New Mexico borders -- supporting
light to moderate snows across the higher terrain.  Snows are
expected to continue into late Sunday across southern Colorado and
far northern New Mexico before the shortwave moves east of the
region Sunday night.  Areas expected to be impacted by the
heaviest totals include the San Juan and Sangre de Cristo
mountains.  WPC guidance continues to indicate high probabilities
for additional accumulations of the 12-inches or more across the
higher terrain.

...Central Plains to the Mid Mississippi Valley...
The first of a multi-wave event to impact the region will develop
as weak mid-level shortwave, originating over the northern Rockies
on Saturday drops southeast into the High Plains early Sunday. 
This will support light to moderate snows moving southeast across
Nebraska.  Meanwhile warm air advection ahead of a low level wave
that is associated with mid-level energy moving east of the
central Rockies will support precipitation spreading east from
eastern Kansas into Missouri.  Snowfall amounts through late
Sunday are expected to be generally light, with WPC guidance only
showing low probabilities for accumulations exceeding 4-inches for
portions of Nebraska and northeastern Kansas.  A wintry mix
developing farther to the south is likely to result in measurable
ice accumulations from southeastern Kansas to southern Missouri on
Sunday.

...Lower Great Lakes...
Models continue to show a surface low strengthening rapidly as it
tracks through eastern New York into southern Quebec Saturday
night.  Strong cold air advection and westerly winds behind the
departing system will produce lake effect snow showers, with
locally heavy accumulations in the lee of the Lower Great Lakes. 


Day 2...

...Central Plains to the Northeast...
A low level wave is forecast to track northeast from the Ozark
region Sunday night and through the Ohio valley on Sunday. 
Southwesterly flow ahead of the wave into a downstream low-to-mid
level baroclinic zone will support precipitation shifting east
from the central Plains and mid Mississippi valley late Sunday
across the Ohio valley overnight and into the northern Mid
Atlantic region on Monday.  Thermal profiles show a wintry mix,
with a widespread swath of measurable snow and ice expected across
the region.  As this leading area of precipitation moves east
across the northern Mid Atlantic, redeveloping precipitation is
forecast back across portions of the central Plains into the mid
Mississippi valley Monday morning.  Models show the previously
noted shortwave that is forecast to impact the central Rockies on
Day 1, amplifying as it moves east into the central Plains on
Monday.  Strong low-to-mid level frontogenesis will support
additional snows developing and moving east across Kansas into
Missouri Monday morning.  During this time-period, latest WPC
guidance shows high probabilities for additional accumulations of
4-inches or more centered across northern Missouri.  By Monday
evening, expect snows to have shifted into Missouri-Illinois on
the northwest side of a developing area of low pressure lifting
northeast through the Tennessee into the Ohio valley. 

Day 3...

...Mid Mississippi Valley to the Northeast...
The mid level shortwave trough shortwave trough over the mid
Mississippi valley late Monday is forecast to lift northeast ahead
of a northern stream trough amplifying over the upper Midwest. 
Snow and ice will lift north well out ahead of the associated low
tracking through the Ohio valley Monday night. Significant ice
accumulations are possible across portions of the Northeast Monday
night into Tuesday -- particularly from eastern Pennsylvania to
western Connecticut and Massachusetts, where WPC guidance shows
high probabilities for ice accumulations of 0.10 to 0.25 inch. 
Guidance currently indicates that best chance for significant snow
farther to the north across the southern Adirondacks, southern
Vermont and New Hampshire, where precipitation is more likely to
remain all snow as the system tracks east from Long Island and
southern New England on Tuesday. 

Pereira