Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
440 PM EST Tue Mar 07 2023

Valid 00Z Wed Mar 08 2023 - 00Z Sat Mar 11 2023

...West Coast...
Days 1-3...

A busy period for the West Coast thanks to a dynamic duo of
troughs working together to funnel rich Pacific moisture that will
result in significant impacts for California. Through this evening
and into Wednesday a closed low off the coast of Oregon will open
into a trough and track into Oregon by Wednesday, directing a
plume of Pacific moisture at the Sierra Nevada. Between 00Z Wed -
00Z Thurs, WPC PWPF shows 80-90% probabilities for >8" of snowfall
along the Coastal Range of northern California into the
Trinity/Shasta and northern Sierra Nevada. While this shortwave
trough tracks into the Intermountain West, snowfall rates will
diminish along the mountain ranges of the Pacific Northwest but
still continue to snow due to the prolonged fetch of moisture from
the longwave trough.

By Wednesday night, however, the next upper low dives south off
the Pacific Northwest coast at the same time as a disturbance
ejecting out ahead of an upper low north of Hawaii approaches
California. This will create a conveyor belt of subtropical
moisture with origins as far south and west as Hawaii. The result
is a robust atmospheric river aimed directly at California
Thursday afternoon and into Friday. By 00Z Friday, NAEFS shows an
IVT >750 km/m/s aimed at the central CA coast with PWs >1.00"
moving ashore Thursday night. Both of these parameters are above
the 99th climatological percentile according to NAEFS CFSR. The
biggest difference between this atmospheric river and the storm
systems in February is the warmer temperatures in the 850-500mb
layer. As the nose of the AR approaches, snow levels will rise
from 5,000' in the Sierra Nevada Thursday evening to considerably
higher levels for this event. NBM suggests snow levels Thursday
night look to rise as high as 8,000-9,000' Thursday night and into
Friday. Snow levels begin to fall in wake of a cold frontal
passage Friday night. Latest WPC PWPF showed 70-80% probabilities
for >12" of snowfall in the southern and central Sierra Nevada and
into the Trinity/Shasta of northern CA. Farther north, lower snow
levels will be much lower over the Cascade Range and farther east
into the Blue Mountains of OR, the Sawtooth and Boise of ID, and
the Tetons of western WY. Probabilities for >8" of snowfall
currently range between 60-80% in all these ranges.

With such high snow levels and lower SLRs for this event, the
combination of snowfall amounts, snow rate, and snow load are the
primary drivers in the experimental PWSSI in the CA ranges where
60-70% probabilities for Major impacts are depicted late Thursday
into Friday morning. Given the expected amounts and anticipated
snowfall rates, which could reach 2-3"/hr, hazardous to even
impossible travel is possible in these areas. The compounding
weight of the heavy/wet snow on tree limbs, along with 30-40 mph
wind gusts could also result in down tree branches and power
lines. WPC has initiated Key Messages for this system and they can
be found below.

...Northern Rockies/Plains & Upper Midwest...
Days 1-3...

The upper trough emerging out of the Intermountain West Wednesday
night will produce diffluent flow over the North-Central U.S.
allowing for an area of low pressure to form in lee of the central
Rockies by Thursday morning. The broad toughing pattern in the
west and the large dome of high pressure over southern Canada
spilling into the eastern U.S. will cause a deepening fetch of
850mb moisture flux to funnel northward into the Northern Plains.
Precipitation is expected to breakout across the front range of
the Rockies and the northern High Plains Thursday morning with
mostly moderate periods of snow, but occasionally heavy bands are
possible where the best upslope flow is present. An 850mb low will
emerge over southern NE and track into southern IA by Thursday
evening with periods of snow tracking farther east into the
Midwest Wednesday afternoon. Snowfall bands will be heaviest just
north and west of the 850mb low, which at this time based on
ensemble and probabilistic guidance, puts the swath of heaviest
snowfall over northern IA, southern MN, and into southern WI.
There have been adjustments to deterministic guidance in the past
24 hours for a more southerly and progressive track, so given
recent adjustments, it is possible that additional changes in
storm track occur. Should the trend south continue, it could
suggest more of northern NE, central IA, and northern IL could be
placed in the better areas north of the 850mb low to see heavier
snowfall. One other factor to consider is the combination of
snowfall rates during the late morning to mid afternoon hours.
Lighter snowfall rates with more direct solar input from a higher
March sun angle could limit some accumulations to primarily grassy
and untreated surfaces. However, where snowfall rates over 1"/hr
occur, snowfall can accumulate on all surfaces. Locations farther
east into southern WI, northern IL, and southern MI would be
better suited for accumulating snow with the shield of snowfall
associated with the storm forecast to move in Thursday evening and
into the overnight hours.

Latest WPC PWPF shows a large footprint of moderate probabilities
(40-60%) from from far southeast MT to eastern SD 00Z Thurs - 00Z
Fri. As the 850mb low consolidates and strengthens farther east,
the likelihood for >6" of snowfall increases over the Upper
Midwest. WPC PWPF depicts a large areal extent of 60-80%
probabilities for >6" of snowfall from southern MN and northern IA
to southern WI and far northern IL. It is worth noting the higher
end of snowfall totals could surpass a foot with WPC PWPF showing
low chances (10-30%) for >12" of snow fall in these aforementioned
areas. The locations with slightly higher odds at >12" of snow are
just inland from Lake Michigan in southeast WI and northern IL
where some lake enhancement is possible. The WPC experimental
PWSSI shows 40% probabilities for Moderate impacts in western SD
with an even larger 40-60% area from southern MN and northern IA
to the WI shores of Lake Michigan. Some urbanized corridors do lie
within these probabilities, including but not limited to
Minneapolis-St. Paul, Madison, Milwaukee, and the northern suburbs
of Chicago. With the strong synoptic and mesoscale factors at
play, it is the Snow Rate component that is driving the PWSSI
probabilities in these areas. Snowfall rates of 1-2"/hr is
possible within the most intense snow bands, and combined with
occasionally gusty winds, could lead to near whiteout visibility
and drifting snow. WPC is doign Key Messages for this winter storm
and they can be found below.


For days 1-3, the probability of icing greater than 0.1" is less
than 10 percent.


...Key Messages for the March 9-10 Winter Storm...

--Strong Winter Storm Arrives Thursday Night: A winter storm will
reach the West Coast on Thursday Night and provide a burst of
heavy precipitation into Friday.

--Heavy Rain and High Snow Levels Likely: Several inches of rain
are expected in the favored upslope areas of the Coastal Ranges
and Sierra Nevada in California. This will be a warm storm system
with rain falling on existing snowpack up to 8500 feet, with the
highest snow levels expected in central California.

--Rain and Snowmelt May Lead to Flooding: The combination of heavy
rain and snowmelt may lead to flooding. The most significant
snowmelt is expected below 5000 foot elevation, in areas with
shallow snowpack. Creeks and streams in the western foothills of
the Sierra Nevada will be most vulnerable to flooding from rain
and snowmelt.

--Difficult Travel in Snow at High Elevations: Higher elevations
in northern California and in the Sierra Nevada are likely to see
very heavy snow, which could lead to difficult travel.


...Key Messages for the March 8-12 Winter Storm...

--A winter storm is set to track across the central High Plains on
Thursday and into the Lower Great Lakes by Friday.

--Periods of snow, falling heavily at times, are expected from the
northern High Plains to the Upper Midwest.

--Some uncertainty remains with the timing and track of the
system, which will determine where the heaviest snow and more
significant impacts will occur.

--Currently, the heaviest snowfall appears most likely in southern
Minnesota, northern Iowa, and southern Wisconsin.

--Snow-covered roads and reduced visibility will make travel
difficult across impacted areas. Whiteout conditions are possible
within the heaviest snow bands.


Mullinax