Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Sat Nov 10 2018

Valid 12Z Sat Nov 10 2018 - 12Z Tue Nov 13 2018

...Great Lakes...

Days 1-3...

Broad generally cyclonic flow through the weekend and into early
next week will continue the potential for periods of Lake Effect
snow. The highest probability for significant snowfall is on Day 1
/Saturday/ east of Lake Erie where the best fetch and instability
will combine. WPC probabilities are high for 4 inches of snow just
south of Buffalo, NY, with the potential for 6 inches or more in
isolated locations where a robust band sets up east of the lake.
Lake effect snow potential falls on day 2 due to drier air and
more directional shear, but should renew on Monday as a more
significant trough digs towards the Great Lakes and combines with
a surface low moving up the coast to bring a return to deep NW
flow and cold advection atop still warm lake waters. On day 3, WPC
probabilities are 10-30 percent for 4 inches of snow E/SE of all
the Lakes.


...Northern Rockies into Northern Plains...

Days 1-2...

An Alberta Clipper will affect the Northern Rockies and Northern
Plains Saturday, with a second impulse dropping into the Rockies
late on Sunday accompanied by jet level diffluence. The air mass
during both events should be cold enough to support snow even into
the lower elevations surrounding the mountain ranges in the
region, however the heaviest snow is likely to be tied to the
higher elevations. The mean low-mid level flow through the entire
period should be out of the north/northwest, so the favored areas
for snow will be on the northern and western slopes, with the
highest probabilities for 4 inches of snow across the Big Horns of
Wyoming. Despite a saturated column across North Dakota on
Saturday, snowfall should remain below 4 inches as the 700mb
trough is progressive and forecast QPF is meager.


...Southern Rockies into Southern Plains...

Days 2-3...

A shortwave will dig into the Four Corners region early on Sunday,
followed by a second impulse Monday which will reinforce the
positively tilted trough across the Southern Plains. As this
occurs, a low-level cold front will drop southward into NM/TX.
Persistent SW 700-500mb flow within the longwave trough will begin
to overrun shallow 850mb E/NE flow producing enhanced isentropic
lift from NM eastward into TX/OK/KS. This isentropic lift will
moisten the column sufficiently for precipitation to develop
across the area, and will be enhanced both synoptically and on the
mesoscale to produce two areas of heavy snow.

The first will be in the terrain of southern Colorado into
northern New Mexico, specifically along the Sangre De Cristo
Range. Here, jet level diffluence and vorticity advection will
combine with upslope enhancement along the E/NE ridges to produce
heavy snow. WPC probabilities are high for 8 inches across the
Sangre De Cristos, with 12 inches possible. The remaining terrain,
including the San Juans, have a moderate chance for 4 inches of
accumulation.

The other area of heavy snow will be along an axis from eastern NM
through the Panhandle of TX through southern KS. Here, mid-level
deformation and frontogenesis will drive ascent, coupled with the
right entrance region to a departing but strengthening upper jet
streak. Guidance is in very good agreement that a stripe of
accumulations of more than 4 inches will occur, with a small
chance for 8 inches, focused near the Panhandle of Texas. Despite
relative short duration of high 1000-500mb saturation, intense
snowfall rates of 1"/hr are possible where upright convection
occurs beneath the best coupled forcing in a saturated DGZ. The
column is cold enough throughout for snow, and SLRs will likely
exceed climatological norms. Note that the NAM/SREF are displaced
NW of the majority of the guidance, and some uncertainty remains
into where this band will occur. However, good agreement between
the GFS/ECM and its ensembles along with recent runs of the NBM
support the current forecast.


...Interior Mid-Atlantic and Northeast...

Day 3...

Longwave positively tilted trough will gradually eject eastward
into the Ohio Valley Monday into Tuesday, spawning slow-developing
surface low pressure along the Gulf Coast. This low will move
northeast to be near the VA coast at the end of day 3. Significant
moist advection will occur ahead of this trough, with
strengthening baroclinic gradient producing the potential for snow
well northeast of the surface low track. A long swath of snowfall
is likely from eastern OK northeast through the Ohio Valley and
into interior New England. Although precipitation may be extensive
across the Southeast and into the Mid-Atlantic/New England,
guidance suggests only a thin ribbon of precipitation where
temperatures will be cold enough for snow. For this reason, WPC
probabilities for 4 inches are currently below 30 percent.


The probability for significant ice (0.25 inches or more) is less
than 10 percent.


Weiss