Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Thu Nov 08 2018

Valid 12Z Thu Nov 08 2018 - 12Z Sun Nov 11 2018

...Central Plains...

Day 1...

Shortwave trough will move overhead the Central Plains today in
conjunction with the left exit region of a 120kt 300mb jet streak.
Low-to-mid level flow will initially be from the SW, causing an
increase in 1000-500mb relative humidity before the 700mb trough
axis shifts east of the region. Strengthening 850-600mb
frontogenesis will occur during a period of 700mb WAA and 500mb
CAA to produce a short duration of intense lift as lapse rates
steepen. There has been a noted southward shift in the heaviest
model QPF/snowfall, paired to the best frontogenesis axis, which
has resulted in a SE shift in the highest WPC probabilities for
significant snow. Column temperatures are marginal so SLR's may
end up below climatological normals, which combined with modest
QPF produces just moderate probabilities for 4 inches of snow
across east-central Kansas. Lower probabilities exist elsewhere
into Missouri.


...Great Lakes...

Days 1-3...

Shortwave and associated jet max will lift northeast into the
eastern Great Lakes late on day 1 /Friday morning/ producing some
light warm advection snows across the upper Great Lakes. More
significant snow is likely on day 2 into day 3 as a more robust
shortwave closes off at 500mb and moves through WI/MI before
lifting northeast into Canada on Saturday. Synoptic snows are
likely where best diffluence and vorticity advection combine with
steep lapse rates beneath the upper low across both the L.P. and
U.P. of Michigan.

However, more significant snow is likely to be featured in Lake
Effect bands, especially Friday through early Saturday morning. As
the upper trough swings across the Lakes, flow initially SW will
quickly turn to the NW behind the system, and then back to the
west as the low lifts into Canada. Strong CAA will produce steep
lapse rates atop water temperatures in the mid to upper 40s, which
combined with unidirectional sfc-700mb shear supports the
potential for strong lake effect bands SE of Lake Superior and
Lake Michigan on Friday, shifting to off Lake Erie and Lake
Ontario late Friday into Saturday. Despite modest QPF, persistent
forcing and high SLR's will create significant snowfall with more
than 4 inches likely downwind of all the lakes, with the highest
probability for 8 inches or more in the U.P. and northern L.P. of
Michigan.

...Northern New England...

Days 2-3...

Complex double-barrelled low pressure system will move in the
vicinity of New England Friday and Saturday bringing mixed
precipitation to the area. Initially the column will be cold
enough for snow across much of northern New England, but with much
of the precipitation being driven by WAA aloft, a warm nose will
gradually lift into the area causing p-type to transition to
sleet, freezing rain, and rain. A period of intense frontogenesis
will provide robust ascent Friday night, with omega focused in the
saturated DGZ. This is expected to be long enough in duration to
create significant accumulations in the high terrain above 2000
feet. By Saturday morning, 850mb temperatures climb above 0C
bringing an end to the snowfall locally. However, a second round
of snowfall is possible as CAA develops behind the departing
system and cold front Saturday as westerly flow upslopes into the
White Mountains of NH and ME potentially producing light
additional accumulations. WPC probabilities show a high risk for 4
inches or more of snow across far northern New Hampshire into
northern Maine, with the potential for 8 inches in Maine.

As warm air floods overhead into Northern New England, regional
forecast profiles suggest the potential for some light freezing
rain accretion, especially in the elevated valleys of the White
Mountains. Mid-levels of the column begin to dry coincidentally
with thermals appropriate for freezing rain, so accretions are
expected to be light, but WPC probabilities have risen to 30
percent for 0.1 inches of freezing rain.

...Northern Rockies...

Days 2-3...

Mid-level shortwave embedded within continued Pacific moisture
plume dropping southeast atop the ridge will advect into
Washington and through the northern Rockies. A surface reflection
of this feature will move across the area dragging a cold front
southward as well, while 700mb confluence and weak jet-level
diffluence will provide lift in a moistening atmosphere.
Temperatures are cold enough for snow in many of the mountain
ranges of ID/MT/WY, but the system is progressive so WPC
probabilities for 4 inches of snow are moderate only in the
Absarokas and northern Rockies near Glacier National Park.


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


Weiss