Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Wed Feb 13 2019

Valid 12Z Wed Feb 13 2019 - 12Z Sat Feb 16 2019

...Northeast and Great Lakes...
Days 1-3...

Occluding low pressure is moving northeast from Lake Huron with
secondary low development occurring over southern New England.
This coastal low will become the primary low today over Maine. A
wintry mix along the eastern Maine coast and heavy snow inland
will quickly shift east this morning. A north-south oriented band
between the lows will produce brief moderate snow as it shifts
east from the Adirondacks this morning to across Maine this
afternoon. Wrap around snow from the occluded low shifting along
the St. Lawrence valley will provide some snow streamers tonight
with the most notable an LES band off Lake Ontario. Day 1 WPC
probabilities are moderate for eight inches over Aroostook County
with high probabilities for four inches over the Tug Hill and east
from northern VT across central Maine. The wintry mix this morning
along the Maine coast should be mainly sleet, though there are low
Day 1 probabilities for a tenth inch of freezing rain in far
eastern Maine.

The next CO low crosses the Plains Wednesday night into Thursday
with another northeast track into the Great Lakes Thursday and
toward Lake Huron Thursday night. A surface ridge over the Gulf
coast keeps Gulf moisture suppressed/out of the system until
Thursday, so little precip is expected until Thursday night over
the Great Lakes where there are moderate probabilities for four
inches over the northern MI on Day 2 and over northern Maine on
Day 3 on a southerly flow as the low lifts north across Quebec.
This southerly flow allows some freezing rain risk over eastern
Maine though the risk is low for less than a tenth inch of ice.


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

A very positively tilted trough off WA shears apart today as yet
another low digs south from the Gulf of AK and reaches a point off
the Pac NW coast Thursday. The remnant energy from the western
side of the shear phases with this new low off the CA/OR border
Thursday with the resultant low shifting inland from the Pac NW
coast Friday night.

South of the existing trough is a broad plume of tropical-sourced
moisture streaming into CA which will get shunted south to the
Baja through Thursday. Snow levels rapidly rise in this warm
plume, remaining above 8000 feet south of 40N latitude in CA later
this morning through tonight. Only the highest Sierra Nevada will
see heavy snow today, though areas above 8000ft could see 2 to 3
feet of snow. Snow levels decrease in CA as the plume/atmospheric
river is shunted south, though precipitation rates also decrease
(from extreme to merely heavy Day 2 and moderate on Day 3. The
phased low looks to focus precip and 2000ft snow levels over far
northern CA on Day 3, though uncertainty remains with the position
and track of this low.

Spillover moisture is significant with WPC probabilities for 8 or
more inches high over higher terrain of UT/ID/WY/CO for both Days
1 and 2 with high probabilities of four inches on Day 3 over this
terrain (and into MT) as the trough digs and prepares for another
low track across the rest of the CONUS.


...Great Plains to Kentucky...
Day 3...

The mid-level remnants of the trough moving into southern CA
Thursday shift across the central Rockies on a potent zonal jet
Friday morning and reach the southern Appalachians by late Friday
night. This mid-level energy undercuts a ridge axis up the central
and northern Great Plains which allows low level convergence of
cold Canadian air and warm Gulf-sourced air over the central
Plains Friday morning. Surface low development occurs Friday night
over the TN Valley (ECMWF solution) or a bit farther south (GFS
solution). Convergence and cold enough air allows a stripe of snow
from SD to MO to the KY/WV/VA border with Day 3 probabilities high
for two inches along this stripe into KY with some moderate
probabilities for four inches, particularly over central MO. South
of the snow is a potential stripe of wintry mix with Day 3
probabilities moderate for a tenth inch of ice over southeastern
MO. The strength of the jet (150kt+) means any slight shift could
cause great displacement of this forecast on future model runs.
Will need to monitor the progression of the fairly southern snow.


Jackson