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

Valid 12Z Tue Dec 11 2018 - 12Z Fri Dec 14 2018

...Mountain West...
Days 1-3...

Persistent plume of moisture will advect into Pacific Northwest as
an Atmospheric River (AR) moves onshore. The moisture plume,
characterized by forecast PWAT anomalies of 2-2.5 standard
deviations above climo as well as high probabilities for IVT of
500 kg/ms into the WA/OR coast, will move onshore today, and then
be reinforced on Thursday. The first surge occurs beneath an upper
trough and associated surface cold front which will bring copious
moisture into the region. Before the cold front and accompanying
height falls move onshore, warm air will raise snow levels to
around 6000 feet, but these will crash down to 2000-3000 feet
tonight. 700mb flow in excess of 50 kts oriented perpendicular to
the Cascades will combine with robust jet diffluence and modest
height falls to produce heavy snow, first across the Cascades, and
then southeast through the rest of the terrain of the Mountain
West as moisture spills eastward. The heaviest snow will be in the
Washington Cascades, where WPC probabilities show a high risk for
12 inches, and up to 3 feet of snow is possible in the highest
terrain. 1-2 feet of snow is likely across the Cascade Crest and
Blue Mountains of Oregon, the Bitterroots of Idaho, and the
Northern Rockies in Montana. Lesser amounts are likely in the high
terrain of Wyoming, Utah, and Colorado, where WPC probabilities
are moderate for 4, and low for 8, inches of accumulation. Lighter
snows are also likely in the lower terrain, with a few inches
possible as low as 2500 feet through day 2.

On day 3, a renewed surge of moisture will advect into Washington
State once again. The upper low responsible for this second surge
will not move onshore until Day 4, beyond this period, but warm
moist advection will begin in earnest on day 3. Heavy snow is
likely to continue across the Cascades and Olympics of Washington,
as well as the far Northern Rockies, but higher snow levels will
confine the heaviest snow to above 6000 feet. WPC probabilities
are high for 4 inches of accumulation only in the Washington
Cascades on Thursday.

Persistent subfreezing air pooled east of the Cascades in WA/OR
will continue to result in pockets of freezing rain until the
arrival of the AR this afternoon. Moderate probabilities for a
tenth inch of ice are present in the lee of the Cascades.


...Southern Plains...
Day 3...

Anomalously strong upper low will cutoff and deepen into the
Arklatex region on day 3, with surface cyclogenesis likely to its
east. There is good model consensus that this cutoff will exist
and be strong, but the placement and timing envelope is large.
Despite that, nearly all guidance suggests at least some light
snow across the Southern Plains Thursday into Friday, with
accumulating snow possible anywhere from the Big Bend of Texas
through parts of Missouri. While temperatures will be marginal, an
axis of mid-level deformation collocated with increasing 850-600mb
frontogenesis north and west of the upper low has the potential to
dynamically cool the column enough for snow to accumulate. A large
area of 1" of snow has been added to the deterministic wwd
forecasts, but WPC probabilities remain low, only around 10
percent, for 4 inches of snow anywhere in this region. While snow
is likely, the combination of the wide model spread and marginal
thermal profiles prevents probabilities from being any higher for
day 3.


Weiss