Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
452 AM EST Wed Mar 08 2023

Valid 12Z Wed Mar 08 2023 - 12Z Sat Mar 11 2023

...California, Great Basin...
Days 1-3...

A very active pattern continues for the West Coast with a focus on
California through this weekend as a major shift from low
elevation heavy snow (which continues today) to high snow level
heavy rain arriving with an atmospheric river Thursday. A closed
low along the Oregon coast will open into a trough and track
inland over the northern Great Basin today, directing a plume of
Pacific moisture at the Sierra Nevada with snow levels around
2000ft and heavy snow expected above that level. Day 1 snow probs
are moderately high for >8" over the northern Sierra Nevada, most
of which will fall between 12Z and 18Z with potential for a foot
to fall on the western slopes of that portion of the Sierra. Minor
ridging builds over CA behind this front this afternoon with a
calm before the (atmospheric river) storm.

The next upper low rotating around a cyclonic gyre over western
Canada has pushed off the northern BC coast and will further
amplify as it tracks off the WA coast through Thursday. At the
same time a disturbance ejecting out ahead of an upper low north
of Hawaii approaches California. This will create a strong
atmospheric river of subtropical moisture with origins as far
south and west as Hawaii that surges into north/central California
Thursday. By 00Z IVT >750 km/m/s is aimed at the central CA coast
with PWs ~1.50" moving ashore Thursday night. Both of these
parameters are above the 99th climatological percentile according
to NAEFS CFSR. As the nose of the AR approaches, snow levels will
rapidly rise over the Sierra Nevada from around 3000ft Thursday
afternoon to over 8000ft Thursday evening. So some snow will occur
on the western slopes of the Sierra Thursday afternoon/evening,
but most of this will melt as precip changes to rain and the
temperature rises as indicated by the moderate day 2 probs for >8"
over the western slopes of the Sierra. Snow levels peak around
9000ft late Thursday night as do precip rates. It should be noted
that multiple feet of snow are expected above about 10,000ft which
are generally limited to the southern Sierra.

While there is a flood threat for heavy rain over a deep and
melting snow pack at lower elevations (generally below 6000ft),
there are extreme impacts at the highest elevations of the Sierra
as happens in these strong atmospheric rivers which create such
high snow levels and low SLRs. 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 as well as
roof collapses continuing well below the snow level as heavy rain
is absorbed by the near record snow depths. WPC-led Key Messages
for this system can be found below.


...Pacific Northwest, Northern Rockies onto the Northern Plains...
Days 2/3...

Away from the potent atmospheric river are lower snow levels and
the upper low ejecting east over WA Friday, bringing heavy
mountain snow to much of the Northwest. Day 2 snow probs are high
for >6" over the length of the Cascades with snow levels generally
2000-3000ft. Lee-side cyclogenesis develops over WY Friday evening
with bands of heavy snow developing over MT into ND. Day 3 snow
probs are high for >6" again for the Cascades and now through the
northern Rockies. Moderate probs for >4" are along the northern
border of MT, arcing into central ND.


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

The upper trough crossing the northern Great Basin today will
produce diffluent flow over the North-Central U.S. allowing for an
inverted trough to form in lee of the central Rockies tonight that
shifts east to the Midwest through Thursday with surface
cyclogenesis delayed until around the eastern Midwest Thursday
night. The broad, positively-tilted toughing pattern in the west
and the large dome of high pressure over south-central Canada
spilling across the Great Lakes 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 late tonight,
particularly around the Black Hills with mostly moderate periods
of snow, but occasionally heavy bands are possible where the best
upslope flow is present. An 850mb low will develop over the Neb/KS
border Thursday and track over southern IA by Thursday evening
with periods of snow tracking farther east into the Midwest
Thursday afternoon. Snowfall bands will be heaviest just north and
west of the 850mb low, which has shifted south (and
progressive/east) a bit more with the 00Z consensus with the swath
of heaviest snowfall over the northern half of IA, far southern
MN, the southern half of WI and into far northern IL. The timing
is now such that most snow falls over the Midwest Thursday night
which avoids diurnal limits to accumulation.

With the surface low developing over the eastern Midwest Thursday
night there is a notable banded snow threat then east across the
northern Mid-Atlantic. As of now it appears the system will
continue to be progressive east as focus switches to a developing
coastal low Friday night, but there is a risk for enhanced snow
through the northern Mid-Atlantic coast.

Day 1 snow probs are moderately high for >4" over the Wasatch of
UT up through the Tetons in NW WY, along the central WY/CO border,
and over the north-central High Plains around the Black Hills up
into southeast MT. Day 2 snow probs are moderately high for >6"
over the aforementioned areas of MN/IA/WI/IL. A noted max in snow
is possible off the southwestern shore of Lake Michigan from
enhancement that could extend into/through Chicago. Day 2.5 snow
probs are moderate for >6" over southern MI and then currently
drop to low-moderate along the NY/PA border for Day 3. WPC has Key
Messages for this winter storm which 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 9-11 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 5500 feet elevation with some deep snow
areas melting rapidly. 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 will very heavy
snow through this morning, which could lead to
difficult-to-impossible travel. Then snow is expected at the onset
of the atmospheric river Thursday before changing to rain.


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

--A winter storm is set to track across the central High Plains
Wednesday night, pushing through the Midwest and Lower Great Lakes
Thursday night, before tracking across the northern Mid-Atlantic
Friday into Friday night.

--Periods of snow, falling heavily at times, are expected from the
North-Central Plains through the Midwest, and possible for
portions of the Northeast.

--The heaviest snowfall is currently forecast for southern
Minnesota, northern Iowa, and southern Wisconsin into northern
Illinois.

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


Jackson