Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Mon Dec 17 2018

Valid 00Z Tue Dec 18 2018 - 00Z Fri Dec 21 2018

...The West...
Days 1-3...

Significant atmospheric river event will bring widespread heavy
snow to the mountains of the West. A strong Pacific Jet, forecast
to be above 160 kts, will come onshore associated with pieces of
shortwave energy and a coincident strong low-level jet to advect
tremendous moisture into the coastal ranges of Washington and
Oregon. Latest GFS probabilities are over 95% for IVT of 500
kg/m/s, with up to 750 kg/m/s likely coming onshore Washington
state late on day 1, pushing PWAT anomalies over 2 standard
deviations above normal. Although snow levels will rise
significantly ahead of the shortwave energy, reaching 6-7 kft,
they will crash back down towards 3000 feet on day 2. This
suggests that the heaviest snow will be confined above 6000 feet,
and most likely occur in the Washington Cascades, with lesser
amounts in the Oregon Cascades. Across the Washington Cascades,
3-day amounts may exceed 5 feet, and WPC probabilities are high
for 12 inches on both day 1 and day 2. Lesser, but still
significant amounts, are likely in the Olympics and Oregon
Cascades where WPC probabilities show a slight risk for 8 inches
of accumulation. By day 3, longwave ridging amplifies across the
west pushing the heaviest snow northward into Canada.

Elsewhere across the West, moisture will spill over into the
ranges of ID/MT/WY, with significant snowfall also expected across
these areas as jet level diffluence combines with 700mb upslope
enhancement through day 2. Although snowfall will not be as
significant as across the Washington Cascades, sufficient moisture
and lower snow levels suggest accumulations of 1-2 feet are
possible in the high terrain of the northern Rockies, the ranges
of Idaho north of the Snake River Valley, and much of northwest
Wyoming. In these areas, WPC probabilities indicate a moderate to
high risk of more than 8 inches of snow.

Further south, a few inches of snowfall is possible across the
highest terrain of Arizona and New Mexico as a southern stream
shortwave amplifies and moves across the Four Corners region on
day 1. The best chance for 4 inches will be in the terrain above
8000 feet in southern Arizona and New Mexico, where better PVA and
jet level diffluence accompany the height falls.


The probability for significant icing (0.25 inches or more) is
less than 10 percent.


Weiss