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

Valid 12Z Mon Nov 26 2018 - 12Z Thu Nov 29 2018


...Great Lakes/Northeast...

Snows will continue across Lower Michigan this morning,
diminishing from west to east as the strong area of low pressure
that brought blizzard conditions to portions of the Plains and
Midwest lifts into eastern Ontario this afternoon/evening.  As the
trailing cold front nears the northern Mid-Atlantic coast, a
secondary low, supported by trailing energy moving through the
base of the upper trough, is forecast to develop near the
triple-point.  This low is forecast to deepen and track north
along the Northeast coast as the energy begins to assume a
negative and pivot north Monday night.  Strong mid-upper level
forcing along with low to mid level frontogenesis is expected to
support increasing precipitation coverage across the Northeast
Monday evening through the overnight hours.  While the overnight
models continue to show disagreement on some of the rain vs. snow
areas, this is expected to be a purely rain event for southern New
York and much of southern New England.  Even areas further to the
north along the northern New England coast are expected to see a
rain/snow mix, transitioning to rain as the strong intrusion of
marine air warms the surface and low levels.  Dynamic cooling is
expected to support a rain-snow transition further inland, while
cold air in place should support a mostly snow event for the
Adirondacks and northern New England mountains - where WPC
probabilities remain high for storm totals exceeding a foot. 
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, producing locally
heavy accumulations through Wednesday before a shortwave ridge
moves across the lakes on Thursday.

...Western U.S....

An approaching shortwave trough 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 a
Moderate Risk for ice accumulations of 0.25 inch or more Monday
morning.

For the remainder of the West, snow levels are expected to remain
fairly high as an upper ridge gives way to the previously noted
trough, establishing a period of zonal flow with Pacific air
flooding the West.  This will hamper the potential for widespread
heavy snow accumulations, however embedded shortwaves may help
support some locally heavy totals across the higher terrain from
the Cascades to the northern Rockies on Days 1 and 2 (Monday to
Wednesday morning).  By early Thursday, an amplifying shortwave
and strong upper jet moving into California is expected to bring
heavier precipitation back into the state, with heavy snow
accumulations possible along the Sierra.

Pereira