Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Fri Nov 30 2018

Valid 00Z Sat Dec 01 2018 - 00Z Tue Dec 04 2018


...Northern to central Plains into the Western Great Lakes...
Days 1-2...

...Major snowstorm expected Saturday across north central Nebraska
and adjacent southern South Dakota...

Heavy snow is expected to occur over much of north central
Nebraska to adjacent southern South Dakota north of the slow
moving cyclone on Saturday. The latest forecast guidance still
supports snow totals over a foot across portions of north central
Nebraska and extreme south central SD, generally north of the 700
mb closed low that will track slowly east/northeast across
Nebraska toward the Iowa/Missouri border by Sunday morning.

The 12z model cycle did trend warmer in the low levels with a
prominent warm nose extending into parts of central/eastern
Nebraska which will likely result in a sharp gradient between rain
and snow. As the system deepens and slowly moves east, strong
dynamic cooling will change rain over to moderate to heavy snow
across portions of northeast Nebraska and adjacent southeastern
South Dakota, with potential for 1-2"/hr rates. A prolonged period
of moderate to heavy snow is expected over northeast Nebraska and
WPC probabilities are high for 8 inches of snow and moderate for
12 inches.

Frontogenesis will increase and lift northward into SD/MN where
several inches of snow is also expected across portions of
southern Minnesota. At the onset, soundings indicate that
precipitation will likely begin as a wintry mix. There is
potential for icing across portions of southwest Minnesota,
extreme northwest Iowa, and far southeastern South Dakota. A small
area of significant icing is possible across southwest Minnesota.

Across Montana and Wyoming, a secondary axis of mid-level
stretching in combination with a lingering inverted surface trough
will continue forcing for ascent in a relatively moist
environment. Lift within the DGZ will lead to light to moderate
snow on Saturday before moving into the Central Plains on Sunday.
WPC probabilities are moderate for 8 inches of snow across
portions of northeast Montana, and additional light accumulations
are likely also from Nebraska northeast towards the U.P. of
Michigan on Sunday.

...Western U.S....
Days 1 and 2...

Another upper level closed low dropping across the western US will
bring snow with heavier amounts in the Oregon cascades down into
California Sierra Nevada range, with secondary maxima inland in
the UT Wasatch/Uintas and ranges of western CO through Sunday.

On day 1, several inches of snow are expected across the Oregon
Cascades, spreading into the mountainous region of northern
California. Significant moisture impinging on the terrain will
result in heavy snow, aided by 700 mb convergence. Lighter amounts
are expected in the WA Cascades as the strength of the jet and
moisture transport is weaker further north. The combination of
significant moist advection, synoptic ascent through jet
diffluence, as well as upslope enhancement supports several inches
of snow across the Uintas and Wasatch of Utah, Tetons of western
WY, as well as the San Juans of Colorado and southward into
northern Arizona on day 1. The highest probabilities for 8 and 12
inches are in the Oregon Cascades.

On day 2, the Pacific wave moves inland across NV and UT. The
heaviest snowfall is likely across portions of the southern
Wasatch of Utah and into the ranges of southern Colorado and
northern New Mexico, where the best combination of moisture and
lift coincide.

...Upstate New York and Northern New England...
Days 2 and 3...

Broad isentropic lift and warm advection associated with a warm
front lifting through will lead to precipitation breaking out
across portions of New York and New England. With a retreating
surface high pressure, shallow cold air in place will lead to the
potential for light icing until warmer air arrives and
precipitation changes over to all rain. Probabilities of
significant icing (> 0.25") are greatest (20-30 percent) across
interior New York into portions of VT, NH and upstate Maine.

In the highest terrain of upstate Maine, where cold temperatures
will remain in place the longest, several inches of snow will be
possible. WPC probabilities are moderate for 4 inches in northern
Maine before the change over to mixed precip types.

Taylor