Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Mon Nov 16 2020

Valid 12Z Mon Nov 16 2020 - 12Z Thu Nov 19 2020


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

Zonal onshore Pacific flow currently into OR and associated with
moderate precip north of a warm front associated with a low well
offshore will back and shift the front north across WA today.
However, with snow levels rapidly rising, additional heavy
accumulations are expected to be confined to the highest peaks of
the northern Cascades where Day 1 probabilities for 8 or more
inches of snow are moderate. Dry conditions expand across much of
the West through tonight before the cold front associated with the
offshore low approaches the WA/OR/northern CA coasts Tuesday
before surging inland and reaching the northern Rockies Tuesday
night. Broad onshore flow behind the front promotes increasing
snow coverage as snow levels fall. Day 2.5 snow probabilities are
moderate for 8 or more inches for all of the higher Olympics,
Cascades, Klamath, northern Sierra Nevada, and Salmon
River/Sawtooth ranges of ID. This extends to the Teton Range of WY
on Day 3.


...Great Lakes...
Days 1-2...

Broad cyclonic flow around a low now centered over northern Quebec
and high pressure over the Great Plains will allow slowly veering
flow across the Great Lakes through Tuesday night. Dual snow lake
effect precip bands have developed from both lakes Erie and
Ontario in marginal conditions for snow that will improve today as
cold air advection continues. Locally heavy snow is expected in
these bands, including the Tug Hill and western New
York/Chautauqua region where 6 or more inches are locally likely,
though Day 1/1.5 snow probabilities are currently low for 6 or
more inches in these areas. Winds will become more northwesterly
behind a reinforcing cold front tonight, with snows diminishing
Tuesday night as an upstream ridge moves over the region.


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

Jackson