Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EST Sat Dec 26 2020

Valid 12Z Sat Dec 26 2020 - 12Z Tue Dec 29 2020


...Great Lakes..
Day 1...

Bands of locally heavy lake effect snow have developed across Lake
Erie and adjacent onshore areas from Ohio across northwest PA and
southwest NY.
The global models and high res windows show southwest boundary
layer flow today across Lake Erie, setting up a lee shore
convergence area with continuing bands of snow in southwest NY
from the PA border to Buffalo NY. Several additional inches of
snow are expected today.  Drying aloft tonight deepens into the
dendritic crystal growth zone, which should cut back the snow
shower coverage and intensity in southwest NY.

The heaviest lake effect snow is expected in the lee of Lake
Ontario, including the adjacent Tug Hill and western Adirondacks.
The flow backs to the west from the surface to 700 mb, with ascent
in that layer.  The minimal directional shear in the vertical
supports a strong single bands of heavy snow developing and moving
from the eastern shore of Lake Ontario across the Tug Hill and
adjacent western Adirondacks.
Most high res windows show an inch to inch and half liquid
equivalent precipitation on day 1.
With snow to liquid ratios in the high teens to mid 20s to one, a
band of 18-24 inches of snow is expected.

WPC probabilities on D1 are moderate to high for 12 inches SE of
Erie and E of Ontario heavy snowfall likely the first half of D2.
Event total snowfall could reach 2 feet in the Tug Hill Plateau,
and while Buffalo itself may be just north of the heaviest snow,
amounts near 18" are likely east of Erie as well.

Days 2-3...
A wave of low pressure is expected to  move across the Midwest and
Great Lakes Sunday night into Monday.  Snow is expected to center
on northern lower Michigan with a band of mid level deformation
north of the low track.  Most models have a few tenths liquid
equivalent, supporting 2-4 inches across portions of northern
lower MI.
Further north in the UP of MI, A very cold air mass will support
high snow to liquid ratios. Post-low lake effect snow off Lake
Superior could produce accumulations in excess of 4" in a few
locations. Southwest flow coming into the Tug Hill of NY favors a
few more inches of snow on Monday in windward facing areas.
   

...Pacific Northwest/CA/Great Basin/Rockies...
Days 1-3...

A potent 130 kt Pacific jet streak will approach the California
coast this morning, providing robust moisture transport into the
West. The jet extends inland this afternoon across northern NV and
UT. North of the jet, upper divergence maxima accompany moisture
spreading inland across ID and then into western WY, favoring a
period of snow in windward facing terrain.  A closed low off the
WA Coast moves slow north today, decaying as it occludes.  After a
short respite, the next developing upper low offshore induces
another increase in moisture advection and upper divergence coming
into the WA coastal ranges and Olympics/ Cascades tonight,
resulting in another round of mountain snows. WPC probabilities
indicate moderate probabilities for 8 inches in the Cascades,
Olympics, ranges of northwest MT, and high probabilities for 8
inches in the WY Tetons through D1.

Sunday into Monday, an upper low off the CA moves slowly east
southeast towards the coast and then towards the Great Basin.  As
the upper trough approaches, the upper jet maxima develops and
moves onshore, continuing inland into the central to southern
Sierra Nevada mountains.  The jet will drive moisture advection
and embedded upper divergence maxima drive ascent Sunday night
into the central to southern Sierra Nevada mountains.  High
probabilities for 8 inches exist in the central-southern CA Sierra
Nevada range.

As the jet moves further inland on Monday, the snow should develop
downstream in difluent flow aloft north of the jet across the
ranges of central NV to central UT and western CO. Warm advection
on southwest flow comes into the San Juan Mountains of southwest
CO with potential for locally heavy snow in windward terrain
there. A secondary maxima in 8 inch snow probabilities exist in
the San Juan Mountains for the Sun night-Tue morning period.  

...Central Plains...
Height falls across the northern to central high Plains leads to
southwest flow in advance of the upper trough.  Low-mid level warm
advection develops Mon night, with moistening of the column
resulting in light snow developing in the central high Plains Mon
night in northern Nebraska to South Dakota.
This area is expected to grow in coverage/intensity on Tuesday.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen