Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Tue Nov 27 2018

Valid 00Z Wed Nov 28 2018 - 00Z Sat Dec 01 2018


...Great Lakes/Northeast...
Days 1-2...

A low pressure system will become vertically stacked over Maine
into Wednesday as a reinforcing trough rounds the low and a new
surface low forms off Nova Scotia into Wednesday night.
Precipitation rates will slowly wane through Wednesday morning as
energy translates east off the coast. However, the upper low will
remain over New England into Thursday, so some wrap around snow
can be expected over New England through this time. The highest
amounts will continue to be over mountains of interior New England
and the Adirondacks.

Meanwhile, strong northwesterly flow/cold air advection will
support lake effect snow showers off Lakes Erie and Ontario,
producing locally heavy accumulations into Wednesday. WPC
probabilities support high probabilities for an additional six
inches in Day 1. By early Thursday, showers are expected to
diminish as a shortwave ridge moves across the lakes.


...Northern Plains...

Southerly flow over the Dakotas ahead of a low over the Canadian
Prairies will result in a warm nose and a wintry mix tonight into
Wednesday. QPF of 0.25 to 0.3 over ND looks to be shared between a
tenth inch of freezing rain and a couple inches of snow with
higher amounts of each are possible along the wintry mix line
where the warm nose and QPF max meet. Moderate to high
probabilities of two inches of snow are north of moderate
probabilities for a tenth inch of ice.


...Western U.S....

A trough pushing into the Pac NW flattens out this evening ahead
of a reinforcing trough that shifts east into CA Wednesday night
into Thursday. Snow levels will remain relatively high ahead of
the low and then drop Day 2 into Day 3 as the low comes ashore. A
focus of heavy snow accumulation is becoming more clear for the
Sierra Nevada in the atmospheric river coming ashore over northern
to central CA tonight into Wednesday (snow level around 7000ft)
then ahead of the low Wednesday night through Thursday when the
snow elevation will be 5000 to 6000ft. Several feet of snow are
possible in the central Sierra through the forecast period.
Blowover will continue east in a narrow swath over NV/UT/CO where
heavy snow is expected for high terrain. The deep trough crossing
the desert SW Thursday night will also allow snows in the higher
terrain of AZ.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Jackson