Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Sat Nov 28 2020

Valid 00Z Sun Nov 29 2020 - 00Z Wed Dec 02 2020

...Pacific Northwest...
Days 2-3...

A potent but fast moving shortwave accompanied by the LFQ of a
strengthening Pacific jet streak will translate across the Pacific
and onshore WA Monday morning. This feature will be accompanied by
a brief surge in moisture, upon which the synoptic ascent will
spread precipitation as far east as the Northern Rockies into
Tuesday morning. Beyond that time, forcing weakens and moves SE
away from the area, bringing an end to the precipitation. Snowfall
during this event will be confined to the higher terrain of the
Olympics, WA and OR Cascades, and less impressively into the
Northern Rockies. WPC probabilities are more robust in the WA
Cascades which will be orthogonal to the confluent mid-level flow,
and are high for 8 inches, with locally more than 12 inches
likely. Elsewhere, WPC probabilities indicate a moderate chance
for 4 inches in the Olympics, northern OR Cascades, and into the
Northern Rockies.


...Ohio Valley/Great Lakes/Central Appalachians...
Days 2-3...

A southern stream closed low over OK Sunday morning will advect
quickly eastward while filling, at the same time a northern stream
impulse digs quickly down from Saskatchewan and towards the Great
Lakes. These two features are likely to phase into a sharply
amplified and anomalously deep mid-level low on Monday, with the
NAEFS ensemble tables suggesting departures from climo 500mb
heights approaching -4 standard deviations. This feature is then
likely to close off while beginning to tilt negatively to the
northeast by Tuesday, while another robust impulse dives out of
Alberta late in the forecast period. The interaction of all these
features produces a challenging and potentially significant winter
event early next week.

As the aforementioned phasing occurs to amplify the longwave
trough across the east, intensifying jet streaks will likely
couple to drive a strengthening surface low pressure moving from
the Gulf Coast into New York State. While there remains some
uncertainty into the exact placement of this feature, the current
forecast relied heavily on the EC/ECENS/GEFS/CMC. This suggests
that as precipitation expands northeastward on WAA and synoptic
ascent, it will be nearly all rain initially into Monday morning
across the Ohio Valley. However, as the low begins to deepen and
lift further northeast, colder air will begin to advect
southeastward causing a p-type transition from rain to snow, and
from NW to SE. While the low-level thermal structure looks
marginal for much of the event, a intense deformation axis is
likely to develop as the upper low swings well SW of the surface
feature, and a modest TROWAL throws rich theta-e air westward
behind the system. Dynamic cooling within this intense lift
coincident with deepening of the DGZ suggests snowfall within this
deformation/comma head could become moderate to heavy at times,
with a prolonged temporal duration late Monday through much of
Tuesday as it pivots only slowly to the east. Additional
enhancement is likely on northerly flow off Lake Michigan and
Erie, as well as Lake Superior into the U.P. despite being
somewhat displaced from the main ascent maxima. This setup also
supports heavy upslope snow on prolonged NW flow which will
gradually become more entrenched into the lowering DGZ across the
Appalachians.

WPC probabilities feature several maxima for 4" where locally
enhanced snowfall is likely. These areas include south of Lake
Superior, Lake Michigan, and Lake Erie, as well as within the
primary deformation axis progged to be in SE MI into central OH,
and again in the upslope favored regions of WV. Locally more than
6" is possible in these areas, with the WSE plumes showing a
subtle upward trend in the dProg/dT means. Additionally, light
snows accumulating to several inches are possible in the Southern
Appalachians of NC/TN, and much of the OH Valley.


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


Weiss