Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
434 PM EST Sat Feb 09 2019

Valid 00Z Sun Feb 10 2019 - 00Z Wed Feb 13 2019


...Western U.S....

An anomalously deep upper low currently centered along the Oregon
coast is expected to turn east, gradually weakening and
transitioning to an open wave as it moves east of the Cascades on
Sunday.  However, shortwave energy digging south of the parent low
will continue to produce below normal heights, with the GFS
showing 500mb heights 2 to 3 standard deviations below normal
moving across Northern California into the Great Basin Saturday
night and Sunday.  This will support snow levels near the coast,
with some significant totals in the coastal ranges of Northwest
California.  As the upper low moves inland, the previously noted
energy moving through the base of the trough is expected to
accentuate lift along the trailing baroclinic zone, bolstering
amounts along the western slopes of the Sierra, where WPC Day 1
probabilities (ending 00Z Monday) show a High Risk for
accumulations of a foot or more, while accumulations exceeding two
feet can be expected in some areas.  Closer to the track of the
upper low, significant accumulations are possible across the
mountains of central to northern Nevada, as well as the northern
Rockies, especially along the central Idaho to western Wyoming
ranges.

Shortwave energy will continue to dig southeast, amplifying the
upper trough as it moves farther east across the Rockies Sunday
night into Monday.  This is expected to produce some locally heavy
snows from western Wyoming to southern Utah and western Colorado,
with WPC Day 2 probabilities (ending 00Z Tuesday) indicating the
potential for local amounts of 8-inches or more across this
region.  Meanwhile, models show a well-defined shortwave diving
south through British Columbia with an upper low developing and
then weakening as the wave approaches the Pacific Northwest late
Sunday.  This is expected to produce some generally light to
moderate precipitation as it turns east ahead of the next system
approaching from the east Monday morning.  With ample cold air
remaining in place, these pair of systems are expected to
contribute to another round of lowland snows across western
Washington. Heavier accumulations are forecast for the Cascades,
where WPC Day 2 probabilities indicate a moderate risk for
accumulations of a foot or more along the southern Washington to
Oregon Cascades.

As energy continues to dig south through the eastern Pacific,
models show an elongated positively-tilted trough developing from
western Canada into the eastern Pacific, with snows continuing
across the Northwest into Day 3.  Low elevation snows will extend
farther south through western Oregon into Northwest California,
while additional heavier snows will become likely for portions of
the southern Cascades and the higher elevations of the coastal
ranges of Northwest California.

...Midwest...

Low-amplitude mid to upper level energy lifting out ahead of the
trough in the West will support some light to moderate snows from
the northern Plains and mid Missouri valley to the Great Lakes on
Sunday.  Expect mainly light accumulations, with the WPC
probabilities indicating only a Slight Risk for amounts of
4-inches or more from Iowa and southern Minnesota to Wisconsin. 
Areas farther to the south are expected to see a wintry mix, with
accumulating ice possible from the lower Missouri valley and Ozark
region eastward into the lower Ohio valley Saturday night into
early Sunday.

As the trough in the West moves east into the Plains, a greater
potential for heavy amounts will begin to develop across the the
Upper Midwest by late Monday and continue into Tuesday.  While
there is good deal of spread with respect to the finer details,
the general consensus of the models show shortwave energy in the
base of the trough assuming a negative tilt, lifting northeast
from the central Plains, with a closed low developing as the
system moves over the Great Lakes late Tuesday.  This will support
the potential for moderate to heavy snows developing across the
region, with WPC probabilities indicating the greater threat for
heavy accumulations centered from southern Minnesota to the U.P.
of Michigan.

...Appalachians/Mid-Atlantic/Northeast...

Warm advection precipitation over the Ohio and Tennessee valleys
is forecast to extend east across the central Appalachians into
the Mid Atlantic region Sunday night into Monday.  As high
pressure shifts east into eastern Canada, this will help set the
stage for a wintry mix with light snow and ice accumulation
expected from the central Appalachians to the Mid Atlantic region.
 As the high to the north continues to strengthen, a weak surface
low developing along the Mid Atlantic coast will enhance the wedge
of cold air wedge forming east of the mountains.  This will raise
the potential for additional mixed precipitation lifting north
across the northern Mid Atlantic into the Northeast ahead of the
low moving in the Great Lakes on Tuesday.  Additional light snow
and ice accumulations can be expected across portion of the
northern Mid Atlantic into the coastal Northeast.  Farther to the
north and within the deeper cold air, some significant snow
accumulations are possible across portions of Upstate New York and
interior New England on Tuesday. 

Pereira