Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Wed Nov 14 2018

Valid 12Z Wed Nov 14 2018 - 12Z Sat Nov 17 2018

...Mid Mississippi Valley...
Day 1-2...

An anomalously deep upper low, 2-3 standard deviations below the
mean, will lift across the Bootheel of Missouri through tonight
before opening and ejecting towards New England on Thursday.
Intense height falls will accompany this feature as it lifts
across the area, with very cold temperatures aloft helping to
create an environment supportive of snow. As this low moves
northeast, strong warm advection within the WCB will lift into a
TROWAL which will rotate around the north side of the upper
feature. The models have converged on an intense deformation band
of snow which will likely setup and pivot across southern
Illinois, with intense snow rates likely as 850-500mb theta-e
lapse rates approach 0 and the DGZ remains saturated. There is
expected to be enough forcing during this time that dynamic
cooling will overcome any weak warm nose as low-level temperatures
approach 0C. This has led to an increase in forecast accumulations
since mixing is not anticipated to limit accumulations, especially
across southern IL where the deformation band will pivot,
extending its duration in that area. WPC probabilities are high
for 4+ inches of snow from extreme southeast MO into southern IL,
with 8 inches possible in a few locations. As the upper low opens
and ejects northeast, continued high 1000-500mb relative humidity
and synoptic forcing due to height falls combined with persistent
mid-level deformation will push a stripe of snow of greater than 2
inches into portions of Indiana.


...Appalachians and Northeast...
Days 1-3...

A strong closed upper low will eject from the Mississippi Valley
towards New England, with warm advection ahead of this feature
producing widespread precipitation from the Southern Appalachians
through Maine. At the surface, a cold high pressure centered over
New England will only slowly retreat to the east, but maintain a
wedge down the east side of the Appalachians. Significant forcing
and ample moisture will produce heavy precipitation along the East
Coast, with p-type the biggest challenge.

Across the Southern Appalachians from North Carolina northward
into the Panhandle of West Virginia, isentropic lift will produce
precipitation beginning tonight. Warm advection to the east of the
upper low will quickly lift a warm nose such that the 850-700mb
temps climb above 0C. After a possible very brief period of snow,
most of the precipitation across this area is expected to be
Sleet/Freezing Rain, with significant accumulation likely. There
still remains some uncertainty into how much sleet will mix with
the freezing rain, as dynamic cooling due to very strong ascent
may keep the warm nose cold enough for sleet. The other concern is
that the warm nose, even as it rises above +4C, is elevated such
that re-freezing may occur into sleet despite the very warm 700mb
temperatures. There remains very good agreement that significant
freezing rain accretion, 0.25-0.5 inches, will occur in the
terrain of the southern Appalachians, but sleet mixing in would
certainly lower these amounts. Still, WPC probabilities show a
high risk for 0.25 inches of freezing rain or more as far north as
extreme western Maryland.

Further north, sleet and snow are likely to be the primary weather
type, although freezing rain may extend as far north as southern
New England, but in lesser amounts. However, a colder overall
airmass and mid-level confluence will prevent the warm nose from
getting nearly as strong as it will further south. As the upper
low moves into the Mid-Atlantic and New England D2-D3, a surface
low will develop along the coast and lift northward towards New
Jersey. The surface low will move very close to the coast in
response to the upper trough tilting negatively, and then lift
northeast towards Downeast Maine at the end of the forecast
period. Significant QPF is likely on strong warm/moist advection,
with cold air NW of the surface low likely locking into the
terrain. A deformation band progged to setup NW of the surface low
will enhance snowfall from central PA into th Adirondacks of NY,
and here WPC probabilities are high for 4+ inches of snow, with 8
inches or more possible. Probabilities for heavy snow drop off
points southeast from there as well as with loss of elevation.

The major cities from Washington DC through Boston will likely be
too far southeast for significant snow or ice. There is moderate
potential that some snow/sleet/freezing rain will occur across
these cities with the initial warm advection precipitation, but
low-level winds becoming E/SE will quickly transition p-type to
rain.


...Northern Rockies...
Days 2-3...
A shortwave will dig into the Northern Rockies Thursday into
Friday accompanied by modest Pacific jet energy. At the surface a
cold front will dive southwards into the Northern Plains and bank
against the mountains producing a sharpening baroclinic gradient
and upslope low-level flow. A wave of low pressure will develop
along this temperature gradient, while Pacific moisture causes a
steady increase in column relative humidity. Lift associated with
upper diffluence and the developing surface low will produce
widespread elevation snows from the Northern Rockies near Glacier
National Park, southward towards the Laramie mountains of Wyoming.
The highest snowfall is expected near Glacier National Park as
well as the Absaroka and Big Horn ranges, where two-day snowfall
totals may reach 12 inches.


Weiss