Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Wed Feb 13 2019

Valid 00Z Thu Feb 14 2019 - 00Z Sun Feb 17 2019

...Northeast and Great Lakes...
Days 1-3...

A shortwave digging through Manitoba into Minnesota will interact
with a cold front to spawn cyclogenesis, and this surface low will
lift towards Lake Huron early on Friday. Along and ahead of this
feature, height falls, WAA, and mid-level diffluence will all
contribute to ascent, producing a swath of snow from extreme
northeast MN through the upper Great Lakes and across far northern
New England. The mid-level system moves relatively quickly east,
and the surface low occludes Friday to a triple point across New
England, behind which a dry slot will rapidly advect northeast.
This suggests only a brief period of ascent/moisture for snow, and
WPC probabilities for 4 inches are moderate only across the U.P.
of Michigan and northern Maine. South of the snow, a small window
exists for freezing rain as the warm advection tops cold surface
temperatures. The best chance for this is from Michigan, across
northern New York, and into Central Maine. However, WPC
probabilities are low for for even 0.1 inches of accretion.

Additionally during this period, Lake Effect snows may be robust
as cold advection occurs across the Great Lakes. The best overlap
of forcing, including moderate MUCape and strong unidirectional
wind shear will occur off Lake Ontario Days 1-2, and off Lake
Superior Day 2. Forcing appears to be favorable for briefly
intense snowfall, but the duration is limited, and WPC
probabilities are moderate for 4 inches in these bands.


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

An Atmospheric River event progged for much of the forecast period
as anomalously high tropical moisture advects into the West Coast
beneath a trough off the Pac NW. This trough will be reinforced by
two distinct mid-level lows dropping southward along the coast,
with sharp height falls and a strong Pacific Jet will drive PWAT
to as high as 5 standard deviations above normal into California
and the Southwest. While the mean trough will remain off the
coast, continuing to funnel moisture into the region, the first
wave will open and eject northeast towards the Northern Rockies on
Friday, spreading snow across much of the terrain from Washington
State to Montana and points south. Ahead of this feature snow
levels will be quite high, up to 7000 ft east of the Cascades as
far north as the Bitterroots, but will crash behind the associated
warm front later on Day 2 into Day 3, falling to less than 2000
feet north of 40N, and as low as 5000 feet down towards the
Mexican Plateau.

The heaviest snow through the 3 days is likely to be focused into
the Sierra of California where 3-day snowfall may reach 6 feet due
to the persistent moisture feed and orographic enhancement as the
700mb flow remains nearly orthogonal to the range. Heavy snows are
also likely in the ranges of northern California where 3 day
totals may reach 3 feet. Elsewhere, heavy snows are likely above
4000 ft in Colorado, Utah, WY and Idaho, with lesser amounts in
the lower elevations as far south as the Mogollon Rim of Arizona.
WPC probabilities are low for more than 4 inches of snow outside
of the high terrain however.



...Great Plains through the Mid-Atlantic...
Days 2-3...

The mid-level remnants of the trough moving into southern CA
Thursday will shift across the central Rockies on a potent zonal
jet Friday morning and reach the southern Appalachians by late
Friday night. This mid-level energy undercuts a ridge axis up the
central and northern Great Plains which allows low level
convergence of cold Canadian air and warm Gulf-sourced air to
produce an intensifying baroclinic gradient across the Upper
Plains. Isentropic lift atop this gradient will produce a band of
snow from eastern MT southeast through SD, NE, and into MO Friday,
with the heaviest snowfall possible across Missouri as the Pacific
Jet interacts with the baroclinicity to enhance ascent and spawn
cyclogenesis in the vicinity. WPC probabilities are moderate for 4
inches of snow across this band, but are slightly higher in
Missouri.

Thereafter, guidance diverges with the track of the developing
surface low. The trend has been southward today, with the Euro on
the north end of the guidance envelope, while the NAM is so far
south with the baroclinic gradient that a weak low remains near
the Gulf Coast. The NAM is discounted as of this time, and a track
near the GFS/GEFS/UKMET is preferred which races the low from
northern MS to off the Outer Banks Saturday night. Jet diffluence
will accompany this feature, so although forcing is of short
temporal duration, enough lift and precipitation is expected in a
marginally cold air mass to produce light snow accumulations
across the southern OH VLY into WV/VA/MD. WPC probabilities are
less than 20 percent for 4 inches outside of the Appalachians, and
some light snowfall may accumulate even into DC on Saturday.


Weiss