Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 PM EST Tue Mar 09 2021

Valid 00Z Wed Mar 10 2021 - 00Z Sat Mar 13 2021

...Western U.S....

A deep layered closed low moving into the west coast will produce
areas of heavy snow through the period. The greatest accumulations
should be over the Sierra, with accumulations of 1 to 2 feet
expected tonight into Wednesday. Also looking at significant
snowfall over the higher terrain of southern CA. This will be a
pretty dynamic system with cold temperatures aloft...thus snowfall
will be showery and quite heavy at times...with hourly rates of
1-2" expected. Most of the accumulation in the Sierra and southern
CA mountains will be above 4000 feet, although some lighter
accumulations down to 3000 feet are expected in the Sierra. Snow
will spread eastward across the Great Basin and into the Rockies
Wednesday night into Friday. Amounts here through this period will
be lighter, and should generally remain below warning criteria
over most areas. While upslope snow will begin over
central/eastern CO by Friday...the big event does not look to get
going until the weekend...when significant snowfall is expected to
bring major impacts to portions of the region. Of course still
model uncertainty that far out, and will continue to provide
updates and details here as the event nears.


...North-central Great Plains through northern Minnesota...

An area of low pressure will bring a swath of moderate to heavy
snow from portions of WY and northwest NE, across much of central
SD and into northern MN. Guidance seems to be coming into better
agreement with this system...with an upward trend in snowfall
amounts noted. Temperatures will be pretty mild out ahead of
this...which will initially help cut back on accumulations.
However, the dynamics of this system will be pretty good...with a
strong mid level shortwave, strong low/mid level moisture
transport/convergence, and even some weak instability within the
warm sector of the low. Thus all in all, should end up with a
corridor of strong omega on the cool side of the system...which
will help dynamically cool the column, resulting in a transition
to a heavy wet snow. Accumulations may struggle closer to the low
track and near the rain/snow line...where marginal BL temps and
maximum lift below the DGZ suggests more of a rain/snow mix or
less efficient snowfall. However, a bit further north there should
be a narrower band of moderate to heavy snowfall...where lift is
maximized within the DGZ and BL temps cool to just below freezing.
This snowfall will be more efficient, and should see enough QPF,
for a swath of 4-8" of snow over the aforementioned areas.

Overall, not too much of a icing threat from this system...as it
seems more rain vs snow. The exception could be across northern WI
and into the western UP of MI. Here we did increase ice
accumulations a bit, and am seeing at least some probability of
exceeding 0.1". Several guidance members indicate enough of an
increase in low level northerly flow as frontogenesis increases to
result in temps dropping just below freezing in these areas.
Meanwhile strong 850mb WAA results in a robust elevated warm
layer. Still some question marks on the details here, and the
areal extent of any ice threat should stay pretty small.
Nonetheless something to keep an eye on as guidance is beginning
to depict this potential a bit more than previous runs...resulting
in the increase in WPC 0.1" ice probabilities. Still some
potential for totals to tick up even a bit more...but again the
spatial coverage should still be limited.

Chenard