Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EST Wed Feb 20 2019

Valid 12Z Wed Feb 20 2019 - 12Z Sat Feb 23 2019


...Southwest to the Central and Southern Rockies...

There remains a strong model signal for a major winter storm
impacting a large portion of the region, with heavy mountain snows
falling across southern Utah, Arizona, western Colorado and
western New Mexico.

By late Wednesday, widespread snows, bolstered by large scale
ascent and deep southwesterly flow ahead of an amplifying trough,
are expected to develop from the coastal ranges of Southern
California into the mountains of the southern Great Basin and
Southwest.  The heaviest amounts through early Thursday are
expected to fall across southwest Utah and northwest Arizona,
where WPC Day 1 probabilities indicate the potential for
accumulations of a foot or more over portions of the higher
terrain. 

Snows are expected to spread farther south and east through
Thursday.  Models show an upper low closing off over
California-Nevada on Thursday before transitioning back to an open
wave by early Friday.  However, energy digging south into the base
of the trough will continue to amplify the flow, with the GFS
showing 500mb heights dropping to more than three standard
deviations below normal across Southern California and the lower
Colorado Basin Thursday into early Friday.  In addition to the
favorable dynamics aloft and upslope flow, increasing low to mid
level frontogenesis is expected to promote heavy precipitation
rates and snowfall accumulations from northwest Arizona along the
Mogollon Rim into eastern Arizona.  Heavy amounts are also
expected to develop farther northeast into northwest New Mexico
and southwest Colorado across the San Juans.  WPC Day 2
probabilities (ending 12Z Friday) show a High Risk for
accumulations of a foot or more along the Mogollon Rim into the
White Mountains of Arizona, as well as over the San Juan Mountains
in southwest Colorado.  By early Friday, localized areas of two
feet or more are possible along portions of the Mogollon Rim.

Snows will shift farther east with the trough late Friday, with
heavy accumulations moving into the southeast Arizona and
southwest New Mexico mountains, where WPC Day 3 probabilities
indicate a Slight Risk or greater for accumulations of 8-inches or
more for areas above 4000 ft.

Snows are expected to end from west to east as the upper trough
moves east of the southern Rockies late Friday.

...Pacific Northwest and Northern California to the Great Basin...

The previously noted shortwave trough diving south into the
western U.S. is expected to support widespread mountain snows from
the southern Cascades and Sierra into the Great Basin on
Wednesday.  While widespread heavy amounts are not expected,
locally heavy totals are possible, particularly along the southern
Cascades on Wednesday. 

A dry period is expected to follow across much of the region on
Thursday and continuing into early Friday.  By late Friday,
precipitation is forecast to return to the Northwest with the
approach of another shortwave trough, with some locally heavy
accumulations possible over the northern Cascades by early
Saturday.

...Upper Midwest...

An upper level shortwave over the central Plains will continue to
assume a negative tilt and lift northeast ahead of an upper low
closing off over the northern High Plains.  This system is
expected to track steadily to the northeast, supporting widespread
light to moderate snows as it moves across the upper Mississippi
valley and northern Great Lakes on Wednesday.  Heaviest snow
accumulations are expected to center from eastern Minnesota to the
U.P. of Michigan, where WPC Day 1 probabilities (ending 12Z
Thursday) continue to show a High Risk for accumulations of
4-inches or greater.

...Central Appalachians and Mid Atlantic to the Northeast...

Low to mid level theta-e advection and frontogenesis will continue
produce an area of precipitation lifting north across the region,
while a pronounced cold air wedge supports snow changing to mixed
precipitation.  Before changing over to a wintry mix later in the
day, models continue to show a low amplitude mid level shortwave
and favorable upper jet forcing supporting an area of enhanced
precipitation with moderate to heavy snows lifting north from near
the Maryland-Pennsylvania border into south-central Pennsylvania
Wednesday morning.  WPC Day 1 probabilities show a Moderate Risk
for 4-inches and a Slight Risk for 8-inches from western and
north-central Maryland into central Pennsylvania.  A widespread
Moderate Risk for ice accumulations of 0.25-inch or more continues
to cover a large portion of eastern West Virginia, western
Virginia, western Maryland and western Pennsylvania on Day 1.  As
warm air surges north and the heavier precipitation slides
offshore, lighter snow and ice accumulations are expected farther
to the north.  However, some areas of northern New England may see
a brief period of moderate to heavy snow as the shortwave trough
over the Great Lakes advances east Thursday morning.

Pereira