Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
915 PM EST Tue Nov 13 2018

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

...Great Lakes...

Day 1...
Lake effect snow will continue on some of the northern lakes from
tonight into early Wednesday, with the greatest potential for
heavy snow in the eastern portion of the Upper Peninsula of
Michigan, and downwind of Lake Ontario. The lake effect potential
will begin to shut off from west to east late tonight and into
Wednesday morning as significant mid-upper level height rises
arrive, coinciding with a surface high building into the region.
This should lead to a drier air mass, large scale subsidence and
weak and variable winds -- all of which should contribute to the
diminishing of lake effect bands. Areas downwind of Lake Ontario
will have over a 50 percent chance of receiving at least 4 inches
of snow, and localized amounts could be higher.


...Mid Mississippi Valley...

Day 1 to 2...
A strong closed low will develop over the Mid Mississippi Valley
and lead to an early season winter storm. The 500mb heights are
forecast to be in the lowest 10th percentile of the climatological
distribution for November across Arkansas and western Tennessee,
and accompanying strong height falls will enable the development
of cold tropospheric profiles supportive of snow. The greatest
probability of heavy snow looks to be just north of the mid-level
low, where the ascending warm conveyor belt will lead to a
pronounced TROWAL from south-central Illinois into east-central
Missouri. WPC winter weather probabilities indicate a greater than
50 percent chance of 4+ inches of snow from near I-70 in Illinois,
south to the Ohio River border of Illinois and Kentucky. There was
good model agreement on the location of the heaviest QPF in this
region, and this lead to greater confidence in the placement of
the heavy snow. Many of these locations have seasonal total
snowfall thus far of zero or several tenths of an inch (per NOHRSC
analysis), so this would be the first significant snow of the
year. It's also worth noting that the average date of the first
inch of snowfall in southern Illinois is typically in late
December.

Around the periphery of the heavy snow area, there is less
confidence in wintry precip amounts due mostly to the expectation
of lighter precip rates and increasingly marginal temperature
profiles outside of the region of strongest height falls. However,
light snow, freezing rain, or freezing drizzle could spread to
other areas of the Mid South and Ohio Valley.


...Appalachians and Northeast...

Day 2 and 3...
As the closed low kicks out of the Mississippi Valley and toward
the East Coast, widespread precipitation is expected for much of
the Appalachians and Eastern Seaboard. Given the expectations for
a closed low relatively cutoff from the northern jet stream for
the first 36-48 hours of the forecast, the forecast was trended
closest to the ECMWF and NAM, which showed a slower progression to
the mid-upper level low as it opens into a strong wave. This had
implications for the location and intensity of a developing
coastal low along the coast of the Northeast US, with the ECMWF
and NAM stronger and closer to the coast, with slightly higher
precipitation amounts inland over the region.

The initial concern for wintry precipitation will be over the
Appalachians, where most of the uncertainty is related to
precipitation type. The models are generally in good agreement on
the placement of QPF as it will be tied to the terrain, but
temperature profiles are variable with the wave of precipitation
that will begin on Day 2 ahead of the primary height falls. Given
the timing of that round of precip, no significant cooling is
expected aloft, but there is a strong model signal for cold air
damming down the east side of the Appalachians with a pronounced
surface ridge. This has the potential to lead to freezing rain and
sleet from as far south as the mountains of western North
Carolina, to as far north as Pennsylvania. As the primary wave of
precip builds north in the Appalachians on Day 2, models show some
cooling of the warm nose aloft (from closer to +1.5C max wet bulb
temp, to closer to +0.5C). The increased vertical motion and
heavier precipitation rates could push the temperature profile
aloft closer to 0C and lead to a dominant precip type of sleet
when most of the QPF is falling. For this reason, accounted for a
greater possibility of sleet than model precip types would
indicate by default, and lowered the freezing rain amounts
slightly. Forecast users should note that WPC WWD probabilities
account for sleet in the snow probabilities. Precip types are
likely to be variable from location-to-location, but in general
the Appalachians region could expect a couple inches of sleet in
addition to some ice glaze from freezing rain (with more or less
sleet or freezing rain depending on nearby terrain features and
location of heavier precipitation bands).

Further north into the Northeast (on Day 3), the primary forecast
uncertainty was related more to model spread regarding the
intensity and placement of the developing coastal low. Strong
mid-upper level height falls, and increasing low-level cold air
advection in the western half of the coastal low's circulation,
should yield increasingly cold tropospheric profiles and
predominately snow over the Northeast. The greatest probabilities
for heavy snow lie inland from the coast, except for perhaps over
Maine, as a warm boundary layer should limit snowfall potential
right along the coast (barring a significant change in the low
track). WPC WWD probabilities show a moderate to high risk (60-80
percent) of 4+ inches of snow in 24 hours from interior New York,
through Vermont and New Hampshire, and into much of Maine.


...Northern Rockies...

Day 2 and 3...
A clipper shortwave will dig into the Northern Rockies on Thursday
and Friday, spreading chances of snow into the mountain ranges
from near Yellowstone, to western Montana, and the Panhandle of
Idaho. The heaviest snow is currently forecast in the Flathead and
Lewis Ranges in northwest Montana, the Little Belt and Big Belt
Ranges in central Montana, and the Bitterroots along the Idaho and
Montana border. There is a high risk of 4+ inches of snow (70+
percent) in these areas.


Lamers