Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Mon Feb 11 2019

Valid 12Z Mon Feb 11 2019 - 12Z Thu Feb 14 2019


...Western U.S....

A mid-level low over WA this morning shears today with northern
energy phasing into a trough shifting south off the BC coast while
the rest shifts east as the northern stream component to the
trough shifting east from the Great Basin. This trough shifts east
across the Rockies today, becoming a CO low across the central
CONUS. Frontal precip pushes into the CO Rockies this morning,
diminishing on the lee side of the Rockies, leaving the high
plains dry.

The phasing trough over the Pac NW becomes very positively tilted
today and remains nearly stationary through Tuesday night. This
sends ample Pacific moisture into the OR and northern CA coast
where snow levels remain fairly low (generally 2000-3000ft along
the coast and less inland) in the trough zone. Moderate to high
Day 1 probabilities for 18 inches or more are over the OR/southern
WA Cascades with the highest areas getting over two feet on Day 1.
A slight shift south is anticipated for Day 2 with the heavy snow
focus over the southern OR Cascades and northern CA Klamath
Mountains and Cascades where moderate Day 2 probabilities for 18
inches exist.

This trough begins to push south Wednesday with increasingly
tropically sourced Pacific moisture directed farther south over
CA. Particularly heavy and persistent precipitation is expected
for the Sierra Nevada with QPF of 4 to 6 inches. Snow elevations
rise dramatically with the arrival of tropical sourced moisture
Wednesday, rising above 7000ft along the Sierra crest. So snow
will be limited in the middle western slopes, but be particularly
high for the higher terrain. Two to four feet of snow are possible
in 24 hours at high Sierra elevations.

Lesser rates, but still heavy snow extends inland over the ID/MT
Rockies Day 1-2 and spreading south into the higher terrain of the
Great Basin to the CO Rockies on Day 3.


...Midwest...

A CO low develops today as an upper trough crosses the Rockies
this morning. Shortwave energy in the base of the trough is
expected to assume a negative tilt by this evening as it
approaches the Mid-MS Valley.  As the associated surface low
amplifies and tracks northeast from the mid-Mississippi valley
toward the Great Lakes tonight, low to mid-level forcing northwest
of the low will allow moderate to locally snowfall across the
upper Mississippi valley into the upper Great Lakes late tonight
into Tuesday. Day 1-2 WPC probabilities have maintained a moderate
risk for 8-inches or more across portions of southeast MN,
northeast IA and much of WI and the UP. 

Farther south and east, a wintry mix will contribute to light snow
and ice accumulations from along the IA/MO border across northern
IL to Lake Erie, with Day 1 WPC probabilities indicating a
Moderate Risk for ice accumulations of 0.25 inch or more across
southern Lower Michigan to the IN/OH borders. Snows will continue
across the upper Great Lakes into Wednesday as the trough closes
off into an upper low Tuesday night, before shifting farther
northeast from the Great Lakes. A brief, but intense period of
Lake effect snow into western MI is expected Tuesday night,
quickly lifting north Wednesday morning.


...Northern Mid-Atlantic and Northeast...

Light precip will persist and expand north along a baroclinic zone
extending up the OH Valley to the northern Mid-Atlantic coast
through tonight ahead of a developing low over the central Plains
that shifts to the Great Lakes tonight. Furthermore, deep high
pressure shifting into Quebec will wedge cold air damming into the
Mid-Atlantic, countering the increasing southerly low level flow.
A zone of wintry mix will persist, likely over northern MD into PA
today, spreading back south tonight and persisting Tuesday until
the cold front crosses Tuesday evening. The trough over the
Midwest closes over the Great Lakes Tuesday night with secondary
surface low development along the northern Mid-Atlantic coast
Tuesday night before shifting north across Maine Wednesday.

Heaviest ice accumulations are expected to center over western
Pennsylvania and western Maryland, with WPC Day 1 and 2
probabilities indicating a Moderate Risk for ice accumulations of
0.25-inch or more both days. Farther to the north light to
moderate snows will lift north across the Northeast, before
changing over to a wintry mix across much of New York (up to the
Mohawk Valley) and southern New England late Tuesday. Tuesday
night into Wednesday, precipitation will continue to lift to the
north as the coastal wave becomes the primary surface feature and
tracks north. While most areas of the northeast will see some
transition to mixed precipitation, some parts of far northern New
York (like the Adirondacks) and interior northern New England may
remain mostly snow.  These areas have moderate probabilities for a
foot or more of snow for both Days 2 (mainly the Adirondacks) and
3 (northern Maine). A large swath of the northeast from all but
far western PA to southern NY to CT/MA has moderate to high
probabilities for a tenth inch of ice on Day 2. Moderate
probabilities for a quarter inch are also from the Poconos to the
Catskills for Day 2.

Jackson