Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 AM EST Tue Nov 10 2020

Valid 12Z Tue Nov 10 2020 - 12Z Fri Nov 13 2020

...Central Plains to Upper Midwest...
Day 1...

Combination of strongly coupled jet streaks and interacting
mid-level shortwaves will spawn surface cyclogenesis along a
residual baroclinic zone behind a cold front today. This surface
low is progged to lift northeast from Kansas to Ontario today,
with rapid deepening likely. While initially the column may be too
warm for any snow or freezing rain this morning, as the low
deepens and CAA persists behind the cold front, a band of freezing
rain and heavy snow is likely to develop and spread northeast
through tonight. The guidance has shifted a bit NW with the 00z
suite tonight, leading to a subtle shift in snowfall and an
increase in freezing rain amounts.

As the low begins to deepen and lift northeast along the
baroclinic gradient, WAA and deep layer synoptic ascent will
expand the precipitation shield while also spreading moisture
further NW into the cold air. Along and just NW of the surface low
track, this WAA aloft will drive a warm nose >0C, while surface
temps fall to freezing or below behind the front. This will setup
a swath of freezing rain with moderate accretions likely. Road and
surface temps are likely quite warm due to antecedent temps well
above normal, but the WSE plumes and model runs have increased
their accretions for this event, likely due to a pronounced dry
slot progged to pivot across the region, which would dry out the
DGZ and limit snow potential while promoting more freezing rain.
The highest amounts will be on elevated surfaces, but WPC
probabilities now indicate a low to moderate risk for 0.25" of
freezing rain from north-central IA through the western U.P. of MI.

Northwest of the freezing rain, a band of heavy snow is likely.
Robust deep layer ascent through height falls and jet level
divergence will be enhanced by a potent deformation axis progged
to shift northward in response to the shortwave interaction. This
deformation axis on the NW side of the low will drive a period of
heavy snow with modest fgen and omega intersecting a saturated
DGZ. The HREF snowband probabilities indicate a chance for 2"/hr
snow rates, but this may be a bit overdone as cross sections show
limited potential for CSI. However, as the low wraps up, the WCB
will wrap cyclonically into the comma head, coincident with this
deformation axis, so 1"/hr snow rates seem likely at times. The
system itself will be moving pretty quickly northeast, so the
heavy snow should only last for 4-6 hours, with subtly longer
duration and more intense forcing expected invof Lake Superior.
WPC probabilities for 4 inches are high from NW Iowa through
southern MN and into NW WI and the Arrowhead of MN, with the
highest amounts which may approach 8 inches near the western side
of Lake Superior.


...Pacific Northwest and Northern Rockies...
Days 1-3...

Multiple shortwaves will rotate onshore the Pacific Northwest
through the end of the week, bringing rounds of snow to the
terrain of the West.

The first of these is moving onshore WA state this morning and
will move quickly eastward into the Northern Plains by Thursday.
This lead shortwave actually has dual vort maxima associated with
it, embedded within the mean cyclonic flow, with each vort max
causing an omega surge to enhance precipitation. Pacific jet
energy atop these shortwaves is minimal, any any PW surge is short
lived and weak. However, periods of ascent will produce moderate
to heavy snow at times today and Wednesday from the Olympics of
Washington eastward towards the Northern Rockies and NW WY ranges
on D1, with more across the Northern Rockies on D2 as forcing
shuts down and mid-level RH dries out across the PacNW. On D1, WPC
probabilities are high for 8 inches in the Cascades of WA and OR,
as well as into the Blue Mountains of OR and into the Northern
Rockies, with snow levels generally 2000-3000 ft, falling to
1000-2000 ft by the end of D1. As the forcing and moisture shift
eastward D2, snowfall should be a bit less, and as snow levels
rise again towards 3000 ft, WPC probabilities are moderate to high
only for 6 inches in the Tetons. 2-day snowfall could reach 2 feet
in the highest terrain of some of these ranges.

Yet another shortwave moves onshore the WA coast Thursday aftn
with sharp height falls. This shortwave will be driven eastward by
a much more powerful Pacific jet streak, providing better moisture
and more robust LFQ diffluence. Orthogonal 700mb flow into the
Cascades and Olympics will likely drive the heaviest snow amounts
in these ranges on D3, with snow levels forecast to be around 4000
ft. WPC probabilities on D3 are moderate to high for 6 inches in
the Olympics and WA/OR Cascades.


Weiss