Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Fri Nov 09 2018

Valid 12Z Fri Nov 09 2018 - 12Z Mon Nov 12 2018

...Great Lakes...

Days 1 and 2...

Impressive two-phase snow event likely across the Great Lakes
today through Saturday. A surface low pressure system will lift
northward through the eastern Great Lakes in response to a
shortwave which will tilt negatively and close off late Friday
into Saturday morning. This will initially create synoptically
driven snowfall in response to warm advection and mid/upr
diffluence and light accumulations are likely across portions of
Wisconsin and Michigan. During this warm advection regime,
high-res guidance is indicating that a convergence band/mesolow
will develop over Lake Michigan with enhanced snowfall rates. This
development is likely in response to both diabatic heating effects
of the warmer lake temperatures, as well as pressure drop through
ventilation aloft. As the mid-level low moves eastward this
afternoon, flow will pivot to the W/NW pushing this feature
onshore of the western L.P. of Michigan, likely enhancing snowfall
along the eastern shore of Lake Michigan.

More significantly, as this upper low moves eastward, strong cold
advection commences and an intense round of lake effect snow is
likely S/SE of the lakes Friday, and then east of the lakes on
Saturday. A surface trough dropping southward across the U.P. of
Michigan will further enhance snowfall potential, and the
combination of unidirectional sfc-700mb shear, CAPE of around 500
J/kg, and a very high inversion height suggests intense snowfall
rates within lake effect bands into the U.P., L.P., as well as
downwind of Lake Erie and Ontario into Saturday. High res models
suggest rates of up to 5"/3hrs, and despite winds generally
backing with time precluding a long duration lake effect event in
any one location, probabilities remain high for significant
accumulations. Across the U.P. and northern/western portions of
the L.P., WPC probabilities remain high for 8 inches of snow, with
locally 12 inches or more possible. Elsewhere across southern
Michigan and E/SE of Lake Erie and Ontario, more than 4 inches of
snow is likely before dry advection shuts off the snow from west
to east on Saturday.


...Northern New England and New York...

Days 1 and 2...

To the east of the developing low in the Great Lakes, significant
warm-air advection should lead to a broad area of precipitation in
the Northeast. Residual cold air across the region may be
sufficient for some snow initially, even at lower elevations, but
this warm advection will quickly allow the warm nose to exceed 0C
by Saturday morning bringing an end to any snowfall even in the
high terrain. Further complicating this forecast is an expected
dry slot which will rotate into the region, shortening the period
of potential snowfall. However, robust ascent is likely due to
increasing mid-level frontogenesis and synoptic lift within the
right rear quadrant of a departing upper jet max. Intense snowfall
rates nearing 1"/hr are possible at times, and the strongest omega
into the DGZ occurs before the warm nose climbs above freezing.
This suggests that much of the precipitation will fall as snow in
the high terrain of Maine and New Hampshire, before a changeover
to a brief period of sleet/freezing rain, followed by rain. The
accumulation forecast is challenging, but the heavy rates into
cold temperatures at elevation should allow for more than 6 inches
of snow in the mountains of north-central Maine, with high
probabilities for 4 inches occurring in the White Mountains of NH
and the Adirondacks of NY.


...Northern Rockies...

Days 1 and 2...

A shortwave and accompanying weak surface low will dig southward
from the Canadian Rockies into the Northern Plains spreading snow
from the Washington Cascades south and east into the Rockies and
into North Dakota. Despite a rapid increase in 1000-500mb relative
humidity and moderate forcing in response to height falls, the
system is progressive. This will limit snowfall accumulations to
less than 4 inches across most of the area. The exception will be
across the mountains of western Montana and the Big Horns in
Wyoming, where WPC probabilities feature a moderate risk for more
than 4 inches of snow.


...Southern Rockies and Southern Plains...

Day 3...

A shortwave and impressive jet streak will dig into the Four
Corners region on Sunday while a cold front drops into the
Southern Plains. The shortwave remains positively tilted into
Monday morning, leading to persistent mid-level SW flow atop
surface winds which will increasingly veer to the N/NE behind the
front. This produces intensifying isentropic ascent in conjunction
with low-mid level frontogenesis. The result will be an expanding
area of snowfall across eastern New Mexico into northwest Texas
and western Oklahoma, with a secondary area of snowfall across the
mountains of CO/NM beneath the best synoptic forcing due to jet
level diffluence. Snow in the terrain will likely exceed 4 inches,
with WPC probabilities increasing for 8 inches in the Sangre De
Cristos. Further SE, there is more uncertainty into the intensity
and location of the best snowfall, but a band of heavier snow is
likely to be oriented SW to NE with more than 4 inches possible
across the Panhandle of Texas. A stripe of lighter snow along the
best baroclinic boundary is possible NE into Kansas, but
confidence in the placement of is low at this time.


The probability for significant ice (0.25 inches or more) is less
than 10 percent outside of several valleys in interior New England.


Weiss