Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 PM EST Tue Dec 11 2018

Valid 00Z Wed Dec 12 2018 - 00Z Sat Dec 15 2018

...Pacific Northwest/Northern Rockies...

There remains a good model signal for heavy mountain snows for
portions of the Cascades to the northern Rockies in association
with a well-defined shortwave trough forecast to dig
east-southeast across the region Tuesday night into Wednesday. 
Heavy accumulations can be expected across parts of the northern
and central Cascades, as well as the ranges of northeast Oregon
and into the northern Rockies of northern Idaho and northwestern
Montana to northwestern Wyoming, especially for areas above 4000
ft.  WPC probabilities indicate the potential for localized
amounts of a foot or more across these areas during the Day 1
period (ending 00Z Thursday).

Following a brief lull, precipitation is expected increase once
again across Washington into the northern Rockies as onshore flow
increases ahead of a system moving into British Columbia late
Wednesday into early Thursday.  Additional heavy accumulations are
possible, especially across the northern Cascades where WPC
probabilities indicate a Moderate Risk for additional amounts of a
foot or more during the Day 2 period (ending 00Z Friday). 

Snow levels are expected to rise late Thursday as a shortwave
ridge builds ahead of the next approaching system.  The daytime
guidance continued to disagree on the timing of this third system
and therefore confidence in the details is limited, but
probabilities indicated at least a low-end chance for additional
heavy accumulations across the Olympics and northern Cascades on
Day 3.

...Southern Plains...

The leading shortwave trough moving into the Northwest Tuesday
evening is forecast to amplify further as it moves across the
western U.S., with a closed low developing as it moves east of the
Rockies into the southern Plains on Thursday.  Confidence in the
details remains limited at best, however some of the 12z guidance,
particularly the NAM and ECMWF continue to signal the potential
for significant snow accumulations along the tail of a developing
deformation band centered over North Texas.  Both the NAM and
ECMWF show that low to mid level frontogenesis will be sufficient
to produce heavier precipitation and cool the column to support a
change from rain to snow and possibly locally significant snow
amounts, with WPC Day 3 probabilities now showing a Moderate Risk
for amounts of 4 inches or more across North Texas centered west
of the DFW metro into Northwest Texas.

The probability of significant icing (amounts of 0.25 inch or
more) is less than 10 percent. 

Pereira