Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 PM EST Wed Jan 08 2020

Valid 00Z Thu Jan 09 2020 - 00Z Sun Jan 12 2020


Days 1-3...

...Western U.S....
An active period in the West is expected to continue with a
compact mid-level shortwave diving southeast into western Oregon
and northern California Thursday morning.  This system is expected
to produce locally heavy snows beginning Wednesday night and
continuing into Thursday morning across portions of the southern
Cascades into the northwestern California ranges, as well as the
northern Sierra.  This system is forecast to quickly dive south
into Southern California by late Thursday, producing mainly light
amounts along the southern Sierra into the Transverse Ranges. 
Meanwhile, a more more northerly shortwave moving from western
Canada into the Northwest will support widespread light to
moderate snows, with locally heavy totals possible across parts of
the northern Rockies.

An upper ridge shifting east across across the West will provide a
brief lull Thursday night before a more powerful system moves into
the Northwest on Friday.  This storm is likely to produce heavy
snowfall totals across portions of the Olympics, Cascades, as well
as the eastern Washington, northeastern Oregon and the northern to
central Idaho ranges.  WPC 24-hr probabilities for the period
ending 00Z Sunday indicate that accumulations of a foot or more
are likely for portions of these areas.

...Southern and Central Plains to the Upper Great Lakes...
As the previously noted shortwave moving into the Southwest
amplifies and moves east across the southern Rockies into the High
Plains, a significant multi-faceted system will begin to develop
late from the southern Plains into the Midwest.  Deep southerly
flow ahead of the wave will support an expanding area of
precipitation across the region.  As a northern stream trough
moving across the northern Plains helps to move low level cold air
in place, a wintry mix is forecast to develop to the west of
tightening surface front.  Light snow and ice accumulations are
forecast to initially develop over portions of the central Plains
into lower Missouri and mid Mississippi valley by late Thursday. 

Models show the system continuing to amplify and assume a negative
tilt as it moves toward the Mississippi valley on Saturday.  With
low level cold air remaining in place, this will raise the
potential for a more widespread wintry mix, with significant icing
possible from the lower Missouri valley northeastward toward the
Great Lakes.  Increasing cold air advection on the backside of the
deepening system is expected to support a wintry mix with
accumulating snow and ice as far south as northern Texas. 

Models show the low lifting out to the northeast from the mid
Mississippi valley toward the Great Lakes on Saturday.  Meanwhile,
strong high pressure will slide northeast and remain centered over
Ontario -- supporting a wintry mix including strong freezing rain
potential, with significant icing possible over southern Lower
Michigan.

Pereira