Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Mon Nov 23 2020

Valid 00Z Tue Nov 24 2020 - 00Z Fri Nov 27 2020


...Pacific Northwest/Northern Rockies...
Another wave and associated front will move into the Pacific
Northwest Tuesday afternoon and night. Snow levels will initially
be quite high, but will drop a bit post frontal. Onshore upslope
flow and additional embedded shortwaves moving ashore, will
continue the mountain snow threat through Wednesday night.
Generally looking at 1 to 2 feet of snow across the WA and OR
Cascades, generally at or above 4000 feet.

...Central Rockies/Plains...
As an amplifying upper trough moves across the Great Basin and
Southwest, areas of heavy snow are expected to develop and
continue through Tuesday across portions of the western and
central Colorado ranges. Currently not looking like a significant
event for the foothills or plains of eastern CO, with thermals
borderline and the area of low pressure remaining rather weak.
However some light accumulations are likely across this region
during the day Tuesday as colder air works in from the north. We
will likely have some areas exceed 2", and a lower probability we
could see a narrow swath of 4" of snow just south of Denver.

As the system ejects eastward across the Plains, dynamical cooling
within a developing deformation band may be sufficient enough to
produce a swath of wet accumulating snow across portions of
Nebraska and Kansas Tuesday and Tuesday night. Currently not
expecting anything too significant, but a narrow swath of 1-2" is
possible. This swath of accumulating snow will likely be quite
narrow, as given the marginal thermal profiles, it will take all
the dynamical cooling we can get to result in any accumulating
snowfall. Thus would only expect it to occur within the narrow
axis of any heavier precip that is able to develop. Again this is
not a sure bet given the marginal thermal profiles...however the
most likely solution at this time has a swath of around 1"...but
certainly a lower potential of 2"+ amounts within a narrow swath
from western KS into southern NE should precip intensity get high
enough on the northwest edge of the precip shield.

...Upper Mississippi Valley/Great Lakes...
Low to mid level WAA, combined with some upper level divergence,
will allow for an expanding area of light to moderate snow tonight
over portions of eastern IA, spreading east northeast through the
overnight and into the day Tuesday. The general trend among the
guidance is for a weaker system, and less of a northern and
southern stream connection. This is resulting in a more suppressed
system, with lighter snow amounts to the north, and an increase in
forecast snowfall further south. Current forecasts show a moderate
to high probability of 2"+ from northeast IA, into far southeast
MN, much of southern and central WI, and into north central MI.
Embedded within this area we may see a narrower swath of 4-5",
with the greatest threat from far southeast MN into south central
WI. Thermal profiles are marginal across this corridor as well,
with some model differences noted. WPC did blend in some of the
colder NAM and other 12z high res models...with the 12z GFS
appearing a bit too warm within the boundary layer resulting in
less snowfall than we think will happen.

...Northeast...
Light overrunning snowfall will spread across NY and central and
northern New England Tuesday night into Wednesday. Models have
trended a bit wetter with this system...indicating we should be
able to get a pretty widespread swath of 1" with this event...with
some of the favored terrain likely exceeding 2-3".

After a brief break, another round of precipitation should
overspread the region Wednesday night into Thursday. By this time
warmer temperatures will likely have moved in aloft, however the
colder air will try to hang on in the lower levels aided by some
CAD. Thus seems like a decent setup for some light freezing rain
across portions of central and northern New England. Given we are
three days out, uncertainty sill exists with this threat, and not
looking like anything too significant. However enough of a signal
in some of the higher res guidance to suggest a light icing
threat, so we blended in some of these colder models to generate a
few hundredths of icing in our day 3 forecast across this region.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Chenard