Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EST Wed Mar 04 2020

Valid 12Z Wed Mar 04 2020 - 12Z Sat Mar 07 2020



...Pacific Northwest...
Days 1-3...

Yet another shortwave trough will dig through British Columbia and
into the Northern Plains through Thursday morning embedded within
persistent confluent flow. PWAT anomalies briefly reach 0.5
standard deviations above the climo mean, and 700mb omega
maximizes across the Northern Rockies, but total forcing is quite
progressive. With snow levels generally above 4000 ft, any
significant accumulating will be confined to the higher terrain,
and WPC probabilities for 4 inches are above 60% Wednesday only in
the vicinity of Glacier NP.

Ridging behind the aforementioned trough means a break in precip
over the Pac NW into Thursday with the next low dropping southeast
out of the Gulf of Alaska Thursday. This turns the flow for the WA
coast from NW to SW on Thursday, advecting enhanced Pacific
moisture into the Northwest with snow levels generally 2000 to
3000ft. Persistent onshore flow maintains moderate precip across
western WA late Thursday through Friday, and then extending down
into California Friday into Saturday as the trough elongated
southward towards northern CA. Day 2 snow probabilities for 6 or
more inches are high for the Olympics and Washington Cascades. By
D3, snowfall spreads southward with moderate probabilities for 4
inches extending into the Oregon Cascades and Northern Rockies.


...Northern Great Lakes to the Northeast...
Days 2-3...

A potent shortwave will close off as it digs across the Great
Lakes Thursday and continue southeast to move off the Southern New
England coast Friday. Beneath this feature, a modest wave of low
pressure will progress quickly to the southeast to fill into a
trough as it attempts to interact with a fast moving southern
stream system lifting along the Atlantic Coast. In the Great
Lakes, a brief period of WAA ahead of the low will spread
precipitation across parts of MI, with moderate snow likely only
in the U.P. where thermal profiles are cold enough for frozen
precipitation. As the low digs further SE, a brief period of
strong CAA will encompass the Lakes, with LES likely in the
favored N/NW snow belts south of Lake Superior. Inversion heights
are shallow and the fetch is generally unfavorable for more than
multi bands of short duration, but WPC probabilities across the
U.P. are high for 4 inches, with the best chance for 6 inches in
the eastern portion.

As the low digs further southeast, there is likely to be at least
some interaction with the more significant southern stream system
ejecting northeast away from the Atlantic Coast. Although phasing
of the southern and northern streams is unlikely, a band of
mid-level deformation and low-level trough will likely develop
between the two, causing enhanced lift and a band of moderate
snowfall across parts of upstate New York, with light to moderate
snow also likely across central/northern New England in response
to jet diffluence and a stripe of modest frontogenesis. Forcing
and moisture are expected to be modest, so WPC probabilities for 4
inches are less than 30% across all of this region. There is the
potential for some light snow out near Cape Cod as well, which may
get brushed by moisture of the more significant system as it pulls
away late on D3.

Additionally, as the larger southern stream low pulls away,
upslope flow is likely into the central Appalachians near WV,
which may accumulate to several inches. WPC probabilities are high
for 4 inches here.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss