Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EST Thu Feb 14 2019

Valid 00Z Fri Feb 15 2019 - 00Z Mon Feb 18 2019


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

Multiple systems will continue to impact the mountains of the West
over the coming days as the longwave trough repeatedly becomes
reinforced by shortwave digging in from the north and shedding
across the West. This will produce widespread snows with heavy
accumulations in some areas. 

A closed low will drop southward along the Pac NW coast Friday
before lifting onto the California coast by early Saturday. As
this occurs, a secondary shortwave will dig from the north,
following the same path Saturday night. A third and more potent
trough will follow on the heels of this one as well, closing off
and dropping along the California coast on Sunday. These will all
act to persist the large longwave positively tilted trough across
the west, with pieces of energy ejecting around the base providing
forcing for snowfall. Additionally, persistent and strong Pacific
Jet energy will transport significant moisture in the form of
anomalously high PWAT air, especially D1-D2, south of this
deepening trough. Widespread heavy snowfall is likely Friday and
Saturday across most of the mountainous terrain. Initially, due to
the tropical origin of the airmass, snow levels are high,
4000-6000 ft south of 40N, but will crash such that snow levels
north of 40N become 1000 feet or less by Sunday, with 4000 ft snow
levels expected down to the Mexico border.

The slowly falling snow levels combined with periodic snowfall
will produce heavy snow from the Washington Cascades, east to the
northern Rockies, and southward to the Mogollon Rim. The heaviest
snowfall is likely on D1-D2, with the favored upslope terrain of
the Sierra likely to receive 4-6 feet of snow, with 1-3 ft across
the northern mountains of CA, and the high terrain from MT into
CO. Elsewhere, WPC probabilities are high for 4 inches into the
valleys, especially north of 40N, but a few inches are possible at
much lower elevations across the southwest as well, with some snow
possible in the mountains even around San Francisco and southern
CA on day 3.


...Central Plains and Ozarks to the Lower Ohio Valley and Central
Appalachians....
Days 1-3...

Remnant shortwave energy shedding from an upper low moving into
California will race eastward atop a potent Pacific Jet before
being shunted southward Friday evening as it encroaches upon a
high amplitude ridge. As this feature moves eastward, isentropic
lift on southerly flow ahead of it will top the baroclinic
gradient along a surface cold front to produce lift and
precipitation. A stripe of moderate to heavy snowfall is likely on
Day 1 from extreme SE MT through SD, NE, and into MO. While the
temporal extent of high RH is limited, a brief period of strong
850-600mb fgen correlated with theta-e lapse rates near 0 and very
high SLRs (15-20:1) due to a very cold column suggests snowfall
rates may become intense Friday afternoon from SE NE into S
Central MO. This region will also interact with the LFQ of the
potent upper jet, providing synoptic ascent as well. This suggests
a better potential for heavy snow, and WPC probabilities show a
slight risk for 6 inches in this narrow region, with moderate
probabilities for 4 inches further NW.

South of this, there is a good multi-model signal for freezing
rain as the isentropic lift/WAA pushes a warm nose above 0C along
the AR/MO border and points east into KY. The strongest lift is
forecast to remain beneath the DGZ, so the cold surface
temperatures in this environment support freezing rain. Despite
periodically heavy rain rates and temperatures near 32 which would
inhibit efficient accretion rates, a widespread area of 0.1 inches
of accretion is likely, with minor probabilities now existing for
0.25" northeast of the Bootheel of Missouri.

As this system progresses east on Saturday, a weak surface low
will track beneath the jet maximum from TN to the Outer Banks of
NC. This system will be mostly suppressed due to a weak parent
shortwave, but a swath of snow and ice is likely from southern IL
eastward to the coast. Guidance has subtly shifted north and
south, but the general trend and favored solution is for a
slightly more southern track of the low, and hence precipitation.
This will bring light accumulations even into the DC area Saturday
morning. T heaviest amounts are possible across WV and central VA,
but even here WPC probabilities for 4 inches are less than 30
percent.


...Great Lakes and Northeast...
Day 1...

Shortwave energy digging into MN will close off and eject
northeast into Saturday morning. This will support a deepening
area of low pressure moving through the Great Lakes early on Day
1, before occluding to a triple point across New England. As this
lifts northeast, WAA will produce snow tonight across MI and then
into the terrain of New England on Friday. Snowfall will be modest
as temperatures warm to turn the precipitation over to rain, at
the same time a dry slot races in from the southeast. WPC
probabilities are highest for 4 inches across the U.P. of MI, the
mountains of ME, and the Tug Hill/Adirondacks of upstate New York
where some post-system Lake Effect snow will enhance totals on
Friday.


Weiss