Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Thu Nov 29 2018

Valid 00Z Fri Nov 30 2018 - 00Z Mon Dec 03 2018


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

A potent shortwave will move into California this evening before
ejecting southeastward into the Four Corners region Friday night.
This shortwave will be accompanied by a 130 kt upper jet maximum,
transporting significant moistured noted by PWAT anomalies over +2
sigma into California and the mountain west. A secondary impulse
and even stronger jet energy of 150 kt will then drop into
northern California on Day 2 before continuing to trek southeast
towards the Four Corners once again.

This series of impulses and accompanying moist advection will
produce lowering snow levels within a deep western CONUS trough,
and heavy snow is likely across most of the western mountain
ranges. Strong upslope enhancement on westerly 700mb flow will
combine with the synoptic ascent driven by jet level diffluence to
produce heavy snow Friday and Saturday across the Sierras of
California, northward through the Cascades of Oregon. Along these
coastal ranges, WPC probabilities feature a high risk for more
than 8 inches of snow, with 1-2 feet likely above 8000 feet.

As the moisture spills across the coastal ranges and into the
remainder of the Mountain West, snowfall will overspread these
mountains as well, with snow levels lowering to below 3000 feet
behind the secondary impulse into Day 3, especially north of the
40N latitude line. The combination of significant moist advection,
substantial synoptic ascent through jet diffluence, as well as
upslope enhancement within the 700mb trough axis supports heavy
snow across the region, with Day 1 and Day 3 favored for the
heaviest snowfall. WPC probabilities are high for 8 inches across
the Uintas and Wasatch of Utah, as well as the San Juans of
Colorado and southward into northern Arizona on day 1, with the
heaviest snow totals likely confined above 8000 feet. Lighter
snows are likely in the ranges of Idaho, Montana, and Wyoming
where WPC probabilities are moderate to high for 4 inches of
accumulation.

On days 2-3, the heaviest snowfall is likely across the southern
Wasatch of Utah into the ranges of northern Arizona and New
Mexico, as well as southern Colorado, where the best combination
of moisture and lift coincide. Snow levels fall to around 5000
feet and WPC probabilities are high for 4 inches, but low for 8
inches. Further north, moisture is less so snow accumulations
should be limited. However, with snow levels falling to below 3000
feet, light accumulations are possible down the valley floors with
a few inches of snow likely.


...Northern Plains into the Western Great Lakes...
Days 2-3...

...Significant snowstorm becoming more likely Saturday and
Sunday...

A sharply negatively tilted mid-level trough will eject eastward
beginning Friday night and then likely close off over the Central
Plains. This combined with an upper level coupled jet structure
will spawn cyclogenesis, with this surface low deepening rapidly
while lifting northeast into the Great Lakes on Sunday.

Initially, low-level temperatures across Nebraska, Iowa, and into
southern MN/SD will be warm as WAA induces precipitation across
the area. This will likely produce a period of mixed precipitation
as surface temperatures remain sub-freezing in the vicinity of a
large area of high pressure to the north. In this area, WPC
probabilities have increased for freezing rain accretion, and are
now up to 30 percent for one-quarter inch. As forcing intensifies
in the vicinity of the deepening low, the column should
dynamically cool allowing for p-type to changeover to snow across
areas along and north of the surface low track, with rain likely
to the south.

As the surface low deepens, strongly sloped frontogenesis will
increase and lift northward into SD/MN while a deformation axis
hangs back through the panhandle of Nebraska. The guidance has
come into better agreement with a stronger slightly more southern
low, likely in response to mid-level ridging out ahead of this
advecting surface trough. While model spread still exists in
amounts, the envelope has decreased in amounts and there is better
agreement and hence higher confidence in snow accumulations.
Although parameters do not support a long period of snow rates
exceeding 1"/hr, a prolonged period of moderate, to at times
heavy, will accumulate and WPC probabilities are high for 8 inches
of snow, with 12 inches possible. Areas further northeast from the
maximum snow area will see lesser amounts, but a strong TROWAL
collocated with the best frontogenesis axis will produce a stripe
of heavy snow across southern MN into WI, where WPC probabilities
are high for 4 inches of snow.

A secondary axis of mid-level stretching and an elongated inverted
surface trough will also extend northwestward from there through
MT. This will continue ascent in a moist 1000-500mb environment,
and light to moderate snow will accumulate on Saturday into the
northwest High Plains, before moving slowly across the Central
Plains on Sunday. This will enhance snowfall totals across the
same areas, and although new accumulation will likely be light,
storm total may exceed 15 inches in a few locations near the NE/SD
border.

A 700 mb wave moves east from WY/CO onto the central high plains
Fri afternoon, evening, with an embedded closed low developing and
moving east roughly along the KS/NE border by Sat morning. 700 mb
warm/moisture advection and convergence develops within the jet
axis with several inches of snow possible where it it is cold
enough for snow in north central NE to south central SD. Precip
type uncertainty exists further south in Nebraska as warm
advection presents the possibility of mixed precip types or a
change over to rain, limiting snow amounts.


...Upstate New York and Northern New England...
Day 3...

Low pressure occluding and moving through the Great Lakes will
push a warm front and associated WAA into New England during
Sunday. As this occurs, it will force the cold surface high to
retreat slowly to the northeast leaving a column with sub-freezing
surface temperatures despite a warm nose lifting across New
England. With precipitation overspread the region from the SW, a
period of light freezing rain is likely in the terrain and
elevated valleys before warm advection changes the precipitation
to all rain. A few hundredths to a tenth of an inch of accretion
are possible, but WPC probabilities are less than 30 percent for
0.25 inches.

In the highest terrain of NH and Me, where cold temperatures will
hang on longer, some snow is possible at onset before changing to
freezing rain and then rain. Light accumulations of snow are
possible in the White Mountains of NH and points NE through
northern Maine. WPC probabilities are low for 4 inches, but a few
inches of accumulation is likely before the warm nose advects
northward.


Weiss