Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Thu Jan 09 2020

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

...Western U.S....
Day 1...
A compact mid-level shortwave diving southeast along the CA coast
around the multi-day mean trough today. Precipitation associated
with this low will weaken today with low probs for six inches for
the high Sierra Nevada (snow levels around 4000ft) near Tahoe with
decreasing snows south through the Transverse Ranges. However,
this trough will amplify as it turns inland near the US/Mexico
border tonight with mountain snows in AZ with the highest peaks
having low to moderate probabilities for six inches tonight.

Meanwhile, a northern stream shortwave moving from western Canada
across the Northwest CONUS will support widespread light to
moderate snows, with locally heavy snow and moderate to high
probabilities for 6 or more inches across parts of the northern
Rockies through tonight.

Days 2/3...

An upper ridge shifting east across the West Coast will provide a
brief lull to the PacNW tonight before a more powerful system
moves into Vancouver Island on Friday. This deep cold core system
is expected to produce heavy snowfall totals across portions of
the Olympics, Cascades, and northern Rockies from ID/MT to WY
through Day 3. Snow levels in western WA will be initially higher
on Friday in the moisture plume ahead of the low...around 3000ft,
but crash under the low/trough Friday night into Saturday dipping
below 1000ft as precip ends Saturday. The next system brings more
precip into the NW coast Saturday night. WPC 24-hr probabilities
for both Days 2 and 3 are moderate to high for a foot or more for
the Olympics, WA/OR Cascades and moderate for the northern Rockies
of eastern WA/OR and across ID into western MT.


...Southern and Central Plains to the Upper Great Lakes...
Days 2/3...
The shortwave moving into the Southwest CONUS today amplifies and
moves east across the southern Rockies through Friday. A
significant multi-faceted system will develop as it crosses the
southern Plains Friday night, turning more northerly as the trough
becomes sharply negatively tilted over the Midwest through
Saturday night. Deep southerly flow laden with Gulf moisture ahead
of the wave will support an expanding area of excessive
precipitation across the region. A leading northern stream trough
crosses Lake Superior today, ushering in cold Canadian air across
the northern Plains and into the Midwest. As the system overruns
this shallow cold layer, a wintry mix is forecast to develop to
the west of tightening surface front. With low level cold air
remaining in place, expect a more widespread wintry mix, with
significant icing possible from the southwest MO northeastward
across the central Great Lakes/the LP of MI. Increasing cold air
advection and development of a comma head on the backside of the
deepening system will support a wintry mix with accumulating snow
and ice as far south as northern Texas. The Day 2 risk or six
inches of snow is 10 to 30 percent from the OK Panhandle across
west-central KS, stretching then to the Upper Midwest for Day 3
with greater totals there where lake enhancement could allow a
foot or more to fall over eastern WI and the northern LP of MI.

00Z guidance is in much better agreement that previous model
suites with the surface low lifting out to the northeast from the
mid Mississippi valley and up the Midwest and Great Lakes through
Saturday night. Meanwhile, strong high pressure will slide
northeast from MN Friday night and remain centered over Ontario
into Sunday, supporting a wintry mix including strong freezing
rain potential, with moderate risks for a quarter inch of ice from
north-central MO to northern IL on Day 2 and moderately high
across southern Lower Michigan on Day 3. There will be a stripe of
sleet in there too, so these fine lines will be borne out with
more detail in coming forecasts.

Jackson