Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Mon Nov 26 2018

Valid 00Z Tue Nov 27 2018 - 00Z Fri Nov 30 2018


...Great Lakes/Northeast...

Low pressure that had been based at the surface over MI this
morning is translating east to the Mid-Atlantic coast this
afternoon as southern stream energy phases in. This low will
deepen and track north along the Northeast coast into Tuesday.
Strong mid-upper level forcing along with low to mid level
frontogenesis is will support increasing precipitation coverage
across the Northeast this evening through the overnight hours.
Dynamic cooling will support a rain-snow transition inland with
all snow across the Adirondacks and interior New England where WPC
probabilities for 8 inches are high. Snow continues through
Tuesday night as the low stalls as yet another lobe takes the
surface development focus off the coast. Synoptic snows are
expected to wane as the low transitions east on Wednesday.
However, strong northwesterly flow/cold air advection is expected
to support lake effect snow showers for mainly Erie/Ontario,
producing locally heavy accumulations through Wednesday before a
shortwave ridge moves across the lakes on Thursday.

The probability of 0.25 inches of ice accumulation Days 1-3 in the
east is less than 10 percent.

...Western U.S....

A long wave trough reaching the Pacific Northwest today will
support warming/moist air aloft, while easterly winds help to
maintain subfreezing surface temperatures - generating a freezing
rain threat for the northern Cascades passes, where WPC
probabilities continue to show moderate probabilities for ice
accumulations of 0.25 inch or more Monday morning. Heavy snow in
Day 1 is sequestered to the highest north Cascades and mainly
Canadian Rockies.

The trough will push ashore across the entire west coast through
Wednesday. Snow levels will slowly lower under the trough with
heights generally 5-7kft for the Pac NW Tuesday/7-8kft in CA and
4-5kft in the Pac NW Wednesday/6-7kft in CA. Low pressure comes
ashore in northern CA Thursday which will lower to around 5kft in
CA and 3-5kft in the Pac NW. Snow over the Sierra rapidly ramps up
Wednesday into Thursday with very heavy snow by Thursday. Leading
Pacific moisture also causes high elevation heavy snow in a narrow
corridor of higher elevations of central NV/UT/CO.

Jackson