Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Thu Jan 03 2019

Valid 12Z Thu Jan 03 2019 - 12Z Sun Jan 06 2019


...Southern Plains...
Day 1...

A potent closed upper low will move eastward across Northern Texas
through today as a surface low develops near the Gulf of Mexico
and lifts northward ahead of this feature into the TN Valley. On
the northwest side of the mid-level low a deformation axis will
set up and within this zone the column will cool to allow precip
to changeover to snow from rain. A period of heavy snow is
possible, and while guidance still differs into exactly where the
strongest ascent will occur, but models have come into better
agreement with a subtle shift NE with the depiction of heaviest
snow. Despite marginal temperatures, intense snow rates are
possible as a pronounced TROWAL within the cyclonic branch of the
WCB combines with negative theta-e lapse rates, a near-surface
isothermal layer, and enhanced lift ahead of a PV anomaly reaching
below 500mb to produce snowfall rates which may reach 1"/hr. The
period of heaviest snowfall should be of relatively short
duration, but WPC probabilities show a high risk for 4" from
north-central TX into central OK, with low probabilities for 8
inches in a few locations.

Additionally, an icy transition zone along the eastern extent of
the heavier snow and northeastward into central and eastern
Oklahoma is forecast.  Guidance continues to show light to
moderate precipitation with surface temperatures hovering near the
freezing mark, raising the potential for icing for portions of the
area. However, a lack of dry advection to enhance wet bulb effects
combined with moderate rainfall rates should preclude efficient
accretion, so while WPC probabilities are over 50 percent for 0.1
inches, they remain low for 0.25 inches.


...Western U.S....
Days 1-3...

Two systems will bring heavy snowfall to the west during the
forecast period. The first is a weak shortwave which will lift
into British Columbia tonight bringing heavy snowfall tot he
Washington Cascades and spilling over towards the Northern
Rockies. Snow levels will climb ahead of the surface cold front
which is expected to cross the area into Friday, and the most
robust forcing and highest moisture will shift into SW Canada.
While heavy snow is likely in the Washington Cascades where WPC
probabilities are high for 8 inches or more, most of the moisture
through day 1 will fall with snow levels too high for significant
snow elsewhere. The exception will be the far Northern Rockies
which may also receive a few inches of snow above 5000 feet.

A more significant low pressure will approach California late on
Friday and then lift onshore Oregon Saturday. Snow levels rise
ahead of this feature, but will fall behind the associated cold
front which will move into California on Saturday. Significant
moisture within a Pacific Jet maximum will advect onshore, and
intense lift within the left exit region of the jet max combined
with increasing 700mb flow will produce heavy snow in the
mountains from the Sierras of California and points north into the
Oregon Cascades. The heaviest snow is likely in the Sierras where
low-level winds will be orthogonal to the ranges to produce
efficient upslope enhancement, and WPC probabilities are high for
12 inches. Elsewhere across the northern California ranges and
into the Oregon Cascades, probabilities are high for 4 inches or
more.


Weiss