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

Valid 00Z Fri Jan 04 2019 - 00Z Mon Jan 07 2019


...Southern Plains into Mid-South...
Day 1...

A closed upper low will push east along the TX/OK border this
evening and reach AR/LA late tonight. Warm air will continue to be
drawn around the system with continued trowal development
overnight. However, 12Z guidance consensus is for this to be just
above freezing with little snow accumulation expected over OK/AR.
Low WPC probabilities for two inches are just NW of the Ozarks.
Light icing is has moderately high probabilities under the trowal
where surface temperatures remain near freezing this afternoon
across northern OK into western MO/AR.


...Northeast...
Days 2-3...

The low currently over the southern Great Plains shifts ENE across
the Mid-Atlantic Friday night with continued trowal development on
the north side of the low across the northeastern CONUS Friday
night into Sunday. WPC probabilities for light icing are
moderately high for higher elevations from the PA/NY border and
across northern New England for Days 2/3.

A northern stream trough behind the departing low off Nova Scotia
on Sunday allows northwest flow and upslope snow to interior
northern New England. Low to moderate WPC probabilities for two
inches of snow exist for the Adirondacks northeast to northern
Maine.


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

An ongoing atmospheric river south of a low off Vancouver Island
will continue to provide high elevation heavy snow to the WA
Cascades and Olympics into Friday. Snow levels and rates decrease
through tonight as the upper trough axis reaches the WA shore.

A stronger low will reach the CA/OR border 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
from the Sierra Nevada to the OR 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 moderate for 12 inches on each of Days 2 and 3.
Elsewhere across the northern California ranges and into the
Oregon Cascades, probabilities are high for 4 inches or more.

The trough axis (as deep as The West Coast) pushes ashore Saturday
night, bringing high elevation across the Intermountain West to
the Rocky Mountains Day 3. Moderate WPC probabilities for six
inches are common for higher elevations from the Mogollon Rim of
AZ/NM to western MT.


Jackson