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

Valid 12Z Tue Mar 09 2021 - 12Z Fri Mar 12 2021

...Western U.S....
Days 1-3...

A deep-layer closed low currently centered off the OR coast tracks
southeast to the far northern coast of CA through tonight. The
next shortwave trough to round this low and track inland over the
CONUS crosses southern CA this afternoon and track across the Four
Corners and southern CO tonight.

A combination of improving upper-level jet support,
frontogenetical forcing, and orographics will favor areas of brief
heavy snow across the Uinta range of northeast UT, the high
country of northwest Colorado which have moderate Day 1.5 snow
probabilities for 6 or more inches. East of the Divide in WY,
increasing easterly flow from lee-side cyclogenesis over eastern
CO this evening allows the snow swath to expand with the southern
Wind River range, the Big Horns, the Laramie Range, and areas east
from there to across the Black Hills of SD where Day 1.5 snow
probabilities for 6 or more inches are 50 to 80 percent.

As the aforementioned closed low advances down across California
into Thursday expect increasing deep-layer Pacific moisture
transport along with a lowering of snow levels as the height falls
arrive. This coupled with rather strong dynamic ascent and
orographic forcing will favor heavy snow across the length of the
Sierra Nevada where locally 1 to 2+ feet of new snow are expected
through Wednesday. Moisture and energy will spread down through
the mountains of southern CA tonight through Wednesday night with
snow levels around 4000ft and Day 2 snow probabilities moderate
for 8 or more inches, especially for the San Gabriel and San
Bernardino mountains.

This moisture influx shifts inland over the Desert Southwest
Wednesday and continues through the rest of the week as the parent
low stalls over southern CA. Pacific moisture and good orographic
forcing will yield accumulating snow for the interior of the
southern Great Basin, the Wasatch of UT, northern AZ where Day 2
and 3 have probabilities for 6 or more inches. This rather
positively tilted trough promotes southern Rockies lee side
troughing by Friday which should bring about an extended period of
snow for CO, some of which could be quite prolonged and heavy for
the CO slopes east of the Divide this weekend.


...North-central Great Plains through northern Minnesota...
Days 2-3...

The shortwave trough that crosses CO tonight refocuses lee-side
surface cyclogenesis to the central Plains/KS with a low that
quickly tracks northeast to the eastern IA/MN border Wednesday,
then to the U.P. of MI Wednesday night before quickly tracking to
northern Quebec Thursday. This fast moving, but elongated wave in
the rather positively tilted western CONUS trough with the low
center between the left exit region of an intensifying
southwesterly jet under the trough and the right entrance region
of the southwesterly jet that crosses James Bay Canada. Moisture
influx from across the entire Gulf of Mexico streams up the MS
Valley to the Great Lakes while moderately cold Canadian air
(Arctic-sourced air does not arrive  into ND until Thursday) is
drawn down the northern Great Plains. Locally enhanced and banded
snow develops north of the low track across northwest Neb and SD
Wednesday then tracks across north-central and northeastern MN
through Wednesday night before lifting into Ontario Thursday
morning. Dynamical cooling in the banded snow looks to be required
to get the column cold enough for accumulating snow, limiting the
breadth of the snow swath. Overall there is good Day 2 agreement
on placement among global guidance. The 00Z regional CMC (which
goes out 84hrs) tracks farther south than the 00Z CMCnh which is
the southernmost track (ECMWF/UKMET/NAM are on the north side with
GFS in between. The Day 2 snow probabilities contain the Neb/SD
(east of the Black Hills)/MN snow well with a swath of low to
moderate probabilities for 4 or more inches from northeastern WY
across SD and north-central/Northeast MN. One note is the
southwestern extent of the wrap-around snow looks to linger in
northeast MN into Thursday morning.

The surface baroclinic zone looks to drift south enough to have
some overrunning and a stripe of freezing rain and sleet potential
across north-central MN into the western U.P. of MI. The ice
accretion risk is rather low with low probabilities for a tenth
inch of ice or more limited to the western U.P. for Day 2.


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

Jackson