Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 PM EST Mon Feb 18 2019

Valid 00Z Tue Feb 19 2019 - 00Z Fri Feb 22 2019


...Southern Rockies...
Day 1...

Shortwave energy diving south along the CA coast will continue to
amplify an upper trough axis over the Desert southwest this
afternoon before ejecting east over the southern AZ border
tonight. An influx of low level moisture from the Gulf of Mexico
and elevated moisture from the Pacific as well as a period of
enhanced divergence aloft support heavy snowfall tonight over much
of NM and the southern half of CO. WPC Day 1 probabilities are
moderately high for foot or more for areas that include portions
of the San Juan and Sangre de Cristo mountains with much of the
higher terrain of the four corners states with probabilities for
four of more inches.


...Central Plains to the Northern Great Lakes...
Days 2/3...

The trough over AZ tonight will shift northeast Tuesday, crossing
the central Plains Tuesday night with surface low development over
KS and reaching the Great Lakes late Wednesday. The associated mid
to upper level forcing is expected to support a broad area of
light to moderate snows spreading north and east from the central
High Plains to the lower half Missouri and upper Mississippi
valley late Tuesday into Wednesday. Heavier snows develop north of
the developing surface low over IA and eastern NE and southeast SD
and into MN/WI on Tuesday night and Wednesday where Day 2 WPC
probabilities for six inches are moderate.  Bands of snow cross
the northern Great Lakes late Wednesday, but with energy shifting
east, rates are expected to decrease over northern MN/WI/the UP
where a few inches are likely.


...Southern and Central Appalachians, Mid-Atlantic, and
Northeast...
Days 2/3

Sprawling high pressure currently centered over the northern
Plains shifts east to NYC through Tuesday. A strong CAD signature
develops east of the Appalachians south of the high center late
Tuesday. This sets the stage for a significant snow icing event
for eastern portions of the southern and central Appalachians and
the adjacent Piedmont. Strong low level theta-e advection and
favorable mid to upper level dynamics ahead of the trough to the
west support a blossoming area of precipitation Tuesday night.
Front end snow develops generally north from the mountains of
western NC and reaches into PA Tuesday night and greater NYC
through Wednesday. A period of moderate to heavy snow supported by
strong upper divergence along with low to mid-level frontogenesis
centered over the central Mid-Atlantic region Wednesday morning
allows moderate Day 2 WPC probabilities of six inches from the
Blue Ridge to the Allegheny Mountains of WV and into south-central
PA.

Advancement of the warm nose will play a key role in amount of
snow vs amount of ice and makes for a tricky forecast. Mostly ice
is forecast for western NC and southwestern VA where the warm snow
quickly overspreads the trapped surface cold air. Day 2 WPC
probabilities for a quarter inch or more of ice are moderate
across the mountains of western NC then up the eastern half of the
Appalachians into PA. This is after several inches of snow. The
complex terrain will likely keep some areas in snow longer before
being in freezing rain longer.

A key note is that Day 2 QPF is one to two inches across the
central Mid-Atlantic Appalachians. Should snow hang on longer, or
freezing rain come on quicker the totals could greatly shift
toward more snow or ice.

By Wednesday night, with the high sliding offshore and warm air
surging north, less frontogenesis should reduce the snow rates
which explains a gap in moderate probabilities for even one inch
of snow over lower upstate NY and coastal southern New England.
Moderate probabilities for a tenth inch of ice area across this
area, from PA to interior New England for Day 3. The surface low
from the Great Lakes tracks east across Maine on Thursday where
moderate probabilities for four inches a spread for Day 3.


...Pacific Northwest to southern California and the Four Corners...
Days 2/3...

The next two waves rounding the persistent ridge into AK shifts
south from BC late Tuesday into Wednesday, eventually closing over
the Great Basin/central CA Thursday. Heavy mountain snows with
some snow down around 1000ft MSL return to Pac NW into the Great
Basin and northern Rockies on Day 2.  Of particular note are rare
extensive snows reaching all the way to higher terrain of
northwestern Mexico and in particularly across terrain above
1000ft MSL over the southern Great Basin and UT/AZ Wednesday night
through Thursday. Moderate Day 3 probabilities for eight inches
are across southern NV/UT and northern AZ along with mountains
south from LA and east of San Diego.

Jackson