Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Wed Jan 02 2019

Valid 12Z Wed Jan 02 2019 - 12Z Sat Jan 05 2019

...Southwest to Southern Plains Day 1...

An anomalously deep upper low will drift southeast across
southeast AZ and then turn east and move across NM towards the TX
panhandle by Thu morning. The models and ensemble means cluster on
the primary snowfall occurring across the southwest New Mexico
ranges. 
With the low starting to move east, amounts are not forecast to be
as heavy as prior days, with enhanced low level convergence
supporting a moderate risk of four inches in the area on day 1
(Wed). Lighter amounts are possible east of the low into southeast
NM and the TX panhandle, with guidance trending a bit drier in the
northern TX panhandle.

East of the low in Texas and Oklahoma, much of Oklahoma and north
central Texas have dropped below freezing as cold air has been
advected south by high pressure further north. Low-mid level
southwest flow will advect warmer air over the top of the surface
cold air, resulting in profiles that are too warm for snow. 
Consequently a blend of sleet and freezing rain appears likely
from north central Texas to south central OK today, with a
consensus of a tenth to quarter inch accumulation expected.

...Southern Plains Day 2...

As the upper low continues to move east into the Southern Plains
on Thursday, consensus of the 00Z models remains a little
slower/further south than previous runs.
00Z guidance showed the greatest threat for heavier snow in
southwest Oklahoma and Northwest Texas and the southeast Texas
Panhandle.  WPC Day 2 probabilities indicate a Moderate Risk for
4-inches across this area.  An icy mixture of sleet and freezing
rain  is forecast further to the east, with light ice
accumulations from North Central Texas to central Oklahoma, with
most areas expecting measurable to a tenth inch of ice accretion. 
 Probabilities are low for a quarter inch, but the 00z GFS
forecasts indicate that much is possible in south central OK.  

...Ranges of northern WA State and northern Rockies Days 1-3...

Heavy snow is expected across the northern Cascades Wed Night
through Thursday night as a deep low moves across the northeast
Pacific towards the northern tip of Vancouver Island and the
British Columbia coast.  Snow levels will remain high, limiting
the threat for heavy accumulations mainly to the northern Cascades
through Day 2.  The models indicate heavy totals possible.  For
example, the SREFMean 3 day snow snow total indicates 2-3 feet of
snow in the northern WA Cascades just south of the Canadian
border. Lighter maxima
with several inches are likely in the windward terrain in the Wa
Olympics, northern ID to northwest MT, with locally a foot of snow
possible.
 
On Friday the low weakens as it moves onshore and inland, with a
dry slot moving onshore and into the Pacific northwest.  This
should weaken the snow coverage and intensity.

The probability of a quarter inch of freezing rain is less than 10
percent day 3.

Petersen