Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sun Jan 13 2019

Valid 12Z Sun Jan 13 2019 - 12Z Wed Jan 16 2019


...Central and southern Appalachians/Mid-Atlantic...
Day 1...

A mid-level trough axis is over the lower OH Valley with surface
low pressure beginning to form along the SC coast. Energy will
continue to translate to the coast through today as the surface
low moves along the NC coast which will enhance precipitation
south of the cold conveyor belt. The focus shift to the coast will
also bring an end to the low level southerly overrunning flow that
has brought ice to western NC/southwestern VA. Ice today will be
limited to the transition zone between rain and snow on the
northwest side of the low. The WPC preference was for a solution
similar to the 00Z ARW2, 3km NAM, GFS and ECMWF. These models
feature a stripe of highest snow south of DC over southern MD west
into VA. Day 1 WPC probabilities are low to moderate for 6 inches
(after 12Z) in this area. Rates are not expected to exceed
moderate except for some locales just on the snow side of the mix
line with long duration snows not amounting to more than a few
inches today in most places. The low shifts northeast from Cape
Hatteras this evening with ongoing wrap around snow over the
Mid-Atlantic coast through tonight.


...Southern Rockies...
Day 1...

A southern stream low undercuts an upper ridge centered over the
northern Intermountain West and shifts east from the CA/AZ border
this morning to the central Plains tonight. This is expected to
support snow, with locally heavier accumulations along the
southern Rockies of northern NM and southern CO today. WPC
probabilities are moderate for 8 inches at the higher elevations
of northern NM mountains.


...California and Southwest...
Days 2/3...

An upper low will close and drop southeast across toward the CA
coast today. This low is expected to slow as it approaches a
downstream ridge over the western CONUS. With the ridge remaining
in place, guidance shows the low weakening with energy lifting
back to the north across Northern California late Monday into
Tuesday. This will drag a cold front inland and support periods of
heavy snow along the Sierra Nevada Monday night through Tuesday
night. WPC probabilities are moderate for 12 inches in the High
Sierra. The western ridge drifts east with moisture and high
elevation snow reaching the Four Corners Tuesday night.

Jackson