Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EST Sun Dec 13 2020

Valid 12Z Sun Dec 13 2020 - 12Z Wed Dec 16 2020


Day 1...
...Pacific Northwest/Northern California/Northern Great Basin...
The next in a series of waves reaching the Pacific Northwest
(expect a rather active next 10 days) is pushing ashore over the
WA/OR/northern CA coast early this morning. What was a robust
trough is getting sheared apart as a reinforcing shortwave trough
swings into OR/CA today. This resultant wave follows in a line
storms that are tracking over the southern tier of the CONUS and
looks to develop into a Nor'easter over the Mid-Atlantic/Northeast
through the midweek. Broad and ample Pacific moisture brings a
widespread round of mountain snows to the Cascades and Sierra
Nevada with Day 1 snow probabilities moderate to high for 8 or
more inches along these ranges as well as the higher peaks of
eastern OR and northern NV. There continues to be a signal for
freezing rain and accumulating ice across portions of the Columbia
River Gorge today, as shallow easterly flow helps to maintain low
level cold air.

...Southern Plains to the Mid-South...
A wave over the southern Rockies closes at 500mb this morning with
low level frontogenesis allowing ongoing heavy snow over southwest
KS that will refocus farther south in transient bands over the TX
Panhandle then across northern and central OK this morning as new
surface low pressure develops over east TX. Increasing moisture
generally decreases snow coverage later today as the precip shield
expands, but the AR Ozarks look to have enough elevation for some
locally moderate snow. Day 1 snow probs are low for 4 or more
inches over the TX Panhandle and the northern 2/3 of OK, but are
moderate for the AR Ozarks.

...Northern Maine...
Warm advection precipitation east of the system tracking north of
the St. Lawrence and west of a new coastal low east of Maine along
with high pressure over Labrador will maintain low level cold air
today with further light icing and light snow in Aroostook Co.

Day 2...
...Great Basin to the Northern and Central Rockies...
The wave moving into the OR/CA border today will dig down to AZ on
Monday effectively cutting off moisture over the Intermountain
west with decreasing snow rates across UT to the CO Rockies with
moderate Day 2 probs for 4 or more inches of snow for the Wasatch
and western CO Ranges.

...Central Appalachians into the Northern Mid-Atlantic and
Northeast...
The southern Plains shortwave trough continues a fast pace as it
shifts east-northeast from the TN Valley across VA/NC Monday
following a SWly jet streak lifting from the Midwest to the
Northeast tonight. The central Mid-Atlantic will be in the right
entrance of the jet streak Monday morning aiding lift and banding
structures on the northwest side of the low track. This dynamic
banding, along with terrain in the central Appalachians allows Day
1.5 snow probs to be 20 to 30 percent for 6 or more inches for the
Allegheny Mtns of WV. Day 2 snow probs for 2 or more inches are
low over eastern WV into south-central PA (and then lower values
for the Poconos, Catskills, and Berkshires in MA) where a deep
isothermal layer at 0C makes for a difficult forecast, but where
banding sets up should allow enough cooling for snow to reach the
surface no matter whether it's a ridge or valley.

Day 3...
...Central Plains...
An upper trough digging into AZ/NM Monday will shift east across
the southern Plains Monday night/Tuesday. This will bring another
round of accumulating snows to the region with snows developing
initially along an elongated trough extending from western
Oklahoma into central Nebraska on Tuesday before focusing/closing
farther to the south across central into eastern Oklahoma late in
the day. Day 2.5 snow probs for 4 or more inches are currently low
over western KS and southwest Neb and Day 3 snow probs are low for
four or more inches over northwest OK.

...Pacific Northwest...
A cold front associated with a wave that moves into BC reaches the
WA/OR coast early Tuesday and pushes to the northern Rockies
Tuesday night. Ample Pacific moisture with this front and the next
wave immediately behind it reinforces Pacific moisture which
brings widespread moderate to heavy precip with heavy snows for
the Olympics WA/OR Cascades to northern ID/western MT Rockies. Day
3 snow probs are moderate for 12 or more inches for the northern
WA Cascades.

Jackson