Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Sun Feb 10 2019

Valid 12Z Sun Feb 10 2019 - 12Z Wed Feb 13 2019


...Western U.S....

A particularly active pattern persists downstream of an eastern
Pacific ridge that reaching well into AK for at least the next
week.

An anomalously deep upper low currently centered over the coastal
OR/CA border will open into a positively tilted trough as it
swings east across the Great Basin today and crosses the CO
Rockies Monday. An associated cold front will continue to push
down the Sierra Nevada today with low snow elevations in the
trailing cyclonic flow. Day 1 WPC probabilities are moderate for a
foot or more across the southern Sierra Nevada. Along the
mid-level trough axis expect briefly heavy snow in terrain of
central to northern Nevada, ID/WY/UT where moderate to high
probabilities for six inches are also present in Day 1. The trough
continue to swing east and take on a neutral tilt by the time it
reaches the southern/central Rockies Monday. This will continue to
produce some locally heavy snows at elevation for CO into NM for
Day 2.

The next activity is a low downstream of the deep ridge that will
push into interior WA tonight while an upstream low cuts off the
ridge and phases with the downstream low through Monday. The
resulting highly positively tilted trough will draw ample Pacific
moisture into OR while low heights allow snow elevations to drop
around 1000ft. Extremely heavy snow can be expected for the OR
Cascades (with a few feet possible above 4000ft centered around
Day 2) while the Willamette Valley looks to be warm enough to
limit snow accumulation to near zero.

The next wave of Pacific moisture reaches CA by late Day 3 with
rapidly rising snow elevations for the central CA coast, but
remaining low north from northern CA.


...Midwest/Great Lakes...

Low-amplitude mid-upper level energy lifting out ahead of the
trough in the West will set up a tight baroclinic zone with
freezing rain along the zone and light to moderate snows north
from the central/northern Plains and upper Missouri valley to the
Great Lakes today. Day 1 WPC probabilities are moderate for
4-inches or more southern MN to central WI. The swath of freezing
rain extends northeast from the Ozarks eastward across the OH
Valley. This swath will shift north ahead of a developing cyclone
for Day 2 into Day 3.

The trough from West spills onto the Plains as a CO low Monday
with rapid cyclogenesis as it shifts northeast to the Great Lakes
through Tuesday. A low re-closes over the Great Lakes with a
negatively tilted trough swinging over the OH Valley. This will
support moderate to heavy snows developing across the region, with
WPC probabilities indicating the greater threat for heavy
accumulations centered from southern Minnesota to the U.P. of
Michigan. However, deep southerly flow will spread a warm nose
with sleet mixing in Day 2 from southern IA into southern WI and
MI. Immediately south of the sleet is a swath of freezing rain.
With cold antecedent conditions and a 1038mb surface high centered
over Ontario/Quebec, the baroclinic zone will increase and could
provide the needed reinforcing cold air to allow for more
significant ice accretions. Low probabilities for a quarter inch
of ice for Day 2 are in this swath from southern IA to northern IL
and across southern MI.

Heavy snow across the northern Great Lakes can be expected across
the north side of the low from northern MN across the UP and
northern WI to the northern LP of MI on Day 3. Low probabilities
for a foot or more are over northern MI. Wrap around flow is
expected to allow LES over Lake Michigan Tuesday night.


...Appalachians/Mid-Atlantic/Northeast...

Warm advection precipitation develops over the Ohio and Tennessee
valleys today and shifts east across the central Appalachians into
the Mid-Atlantic region tonight into Monday. Snow and freezing
rain at the onset can be expected over the WV highlands with warm
advection west of the Appalachian crest quickly changing wintry
precip to rain. However, strong surface high pressure centered
over Quebec spreads east and allows cold air damming (enhanced by
ongoing precipitation), making for an extended wintry mix over the
central Mid-Atlantic until the cold front crosses late Tuesday.

Fine details on this wintry mix remain uncertain, but the best
solution for now is snow north from northern MD slowly becoming
sleet as the warm nose pushes into PA Monday with freezing rain
farther south, particularly in sheltered valleys. Areas around
Garrett Co, MD tend to retain cold air in situations like this
with 72hr ice accretion around a half inch possible. Pockets of
freezing rain would persist in the enhancing CAD wedge from the
east side of the Blue Ridge to the Allegheny Front and down the
Shenandoah Valley into southwestern VA.

The developed cyclone center reaches MI on Tuesday with the
negatively tilted trough tapping Gulf and Gulf Stream moisture
with cold air conveyor convergence extending east from the low to
across New England. Deep warm air advection looks to reach the
Mohawk Valley through Day 3 with suppressed snow totals south of
there. Moderate Day 3 probabilities for a foot or more are across
the Adirondacks and Green/White mountains of VT/NH. Meanwhile
moderate probabilities for a quarter inch of ice is across PA into
southern NY and CT/MA for this large scale overrunning.

Jackson