Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Fri Dec 13 2019

Valid 12Z Fri Dec 13 2019 - 12Z Mon Dec 16 2019

...Central Appalachians/Piedmont...
Days 1,2...

A cold air damming wedge/surface ridge extends southwest from a
1040mb high centered over southern Nova Scotia down the eastern
side of the Appalachians. Precipitation associated with an
upstream upper trough over the southern Great Plains is spreading
through southwestern VA early this morning as a wintry mix of
sleet and freezing rain (rain on ridge tops). This will continue
through the morning with ice mainly in the immediate lee of the
Blue Ridge and in sheltered valleys to the crest of the
Appalachians into PA. The greatest risk for a tenth inch of ice
starting at 7am is in the northern Potomac Highlands/far western
MD. However, guidance nearly always erodes cold air too soon, so
pockets of freezing rain could persist until the developing
surface low from the approaching wave lifts over the region
tonight.

Cold air drawn into the back side of this low, combined with
upslope flow makes for higher elevation snow Saturday afternoon
and evening. Day 2 WPC probabilities are moderate for four inches
over the Allegheny Plateau over WV.


...Western U.S....
Days 1-3...

A mid-level trough will cross the OR/CA border tonight. This is on
the back side of a zonal jet that has been bringing ample Pacific
moisture and high elevation heavy snow over the past couple days.
Snow levels drop behind a baroclinic zone settling south into the
Sierra, Great Basin and central Rockies. Low-to-mid level
frontogenesis will help support a swath of locally heavy mountain
snows extending from the Sierra to the Colorado Rockies Days 1 and
2. Precipitation rates generally decrease today into tonight as
the main Pacific moisture plume is shunted south under the trough,
though probabilities for 8 inches are moderate to high for the
highest elevations of the CA Cascades to central CO Rockies Days 1
and 2.


...Upper Great Lakes...
Days 1-2...
A weak shortwave trough pushes east over northern MN today with
generally light snow totals. Southeasterly flow ahead of the
trough will be allow lake enhanced snow for the North Shore of MN
where there are moderate Day 1 probabilities for four inches.
Northwesterly flow on the back side will allow lake effect snow
for northern WI/western UP where there are moderate Day 2
probabilities for four inches.


...Central Plains and Mid Mississippi Valley...
Day 3...

The well defined mid-level shortwave dropping southeast into
California tonight crosses the CO Rockies late Sunday. A swath of
moderate snow on the north side of the surface low crossing Texas
late Sunday should affect eastern CO, KS into MO through Sunday
night. Day 3 WPC probabilities are generally 20 to 30 percent for
4 inches over central KS though timing differences among global
guidance is lowering the consensus at this time.

Jackson