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

Valid 00Z Thu Mar 09 2023 - 00Z Sun Mar 12 2023

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

A deep upper low off the Pacific Northwest coast will work in
tandem with a disturbance emanating out of an upper low north of
Hawaii to introduce highly anomalous moisture into California and
the Great Basin via a strong Atmospheric River (AR). By 00Z
Friday, NAEFS shows a powerful 750-1000 kg/m/s integrated vapor
transport (IVT) will be aimed at California. These IVT levels are
at or above the 99th climatological percentile from California
into the northern Great Basin according to NAEFS. This impressive
stream of moisture with origins as far south and west as Hawaii
will also coincide within a tongue of much milder temperatures
compared to previous winter storms that impacted California this
past February. Instead of low elevations snowfall, snow levels as
the slug of moisture arrives will rise from as low as 2,000-3,000
feet in the Sierra Nevada and Trinity/Salmon mountains Thursday
morning to 8,000-9,000 feet Thursday night into Friday morning.
The southern Sierra Nevada, where elevations are able to get above
9,000 feet can expect to remain snow for the duration of the
event, will measure snowfall accumulations in feet with snowfall
totals ranging between 4-7 feet. The latest WSSI shows Extreme
impacts late Thursday into Friday in elevations >8,000 feet in the
central and southern Sierra Nevada, as well as in the higher peaks
of the Trinity/Salmon mountains. Snow Amount and Snow Load are the
primary drivers in the WSSI in these areas, but for the elevations
>9,000 feet in the southern Sierra Nevada, strong wind gusts in
excess of 50 mph are also causing Extreme impacts for Blowing Snow
as well. Snow rates will also be impressive as well, highlighted
by the WPC Snowband Probability Tracker (via the 12Z HREF) showing
3-5"/hr snowfall rates Thursday night. With such heavy/wet snow in
these areas, along with the gusty winds throughout the region,
there is concern for downed tree limbs and power lines, as well as
the concern for roof collapses atop buildings with lingering
snowpack on roofs. Key Messages for this impending winter storm
can be found below.

Farther north, the Cascade Range from Oregon on north into
Washington can also expect to see heavy snow for Thursday night
into Friday as subtropical moisture from the south is drawn north
out ahead of the upper low off the Pacific Northwest coast. Snow
levels will be lower here and snowfall totals over a foot are
likely (WPC PWPF shows high probabilities of 60-80% for snowfall
totals >12"). The latest WSSI shows Major impacts along the
Cascade Range, which suggests considerable disruptions to daily
life are anticipated during this time frame.

...Northern Plains, Upper Midwest, Great Lakes, & Northern
Mid-Atlantic...
Days 1-3...

The upper trough in the northern Rockies today will advance east
into the northern High Plains by Thursday morning. The diffluent
flow aloft in the 500-250mb layer over the northern and central
High Plains will result in lee cyclogenesis over southeast
Colorado with several embedded surface troughs located near the
front range of the northern and central Rockies. A steady fetch of
southeasterly 850mb moisture flux and WAA will introduce
sufficient moisture tonight and into Thursday morning to prompt a
large shield of snowfall to envelope much of the northern High
Plains on east towards the lower Missouri River Valley. As the
850mb low consolidates and deepens gradually over central Nebraska
midday Thursday, heavier snow bands look to form across the
southern SD and will track east along the northern flank of the
850mb low, eventually reaching southern MN and northern IA
Thursday afternoon, followed by southern WI and northern IL
Thursday evening. Snowfall rates will increase in intensity by
late Thursday afternoon in the Upper Midwest as the surface low
deepens over the mid-MS River Valley. The 12Z HREF showed 50-65%
probabilities for 1"/hr snowfall rates over eastern IA, southern
WI, and northern IL which coincides during the afternoon rush
hour. While surface and road temperatures may originally be above
freezing, as 1"/hr rates occur and the sun sets, accumulations
will be able to more rapidly accumulate even on paved surfaces.
This same swath of 1"/hr snowfall rates will move east along the
northern periphery of the 850mb low into northern IN, southern MI,
and northwest OH.

Latest WPC PWPF shows 40-60% probabilities for >6" of snow in the
Black Hills and in east-central SD through Thursday morning, then
50-70% probabilities from northern IA and southeast MN to southern
WI and far northern IL. The area with th highest probabilities for
>8" of snowfall is just inland from the coast of southeast WI
where probabilities are up to 50-60%. The swath of 50-70%
probabilities for >6" of snowfall then moves into southern and
central MI through Thursday night and into early Friday morning.
Speaking of Friday morning, the primary low then heads for the
Upper OH River Valley where a ribbon of 850-700mb frontogenesis
and strong vertical velocities ahead of the 850mb low will cause
periods of moderate-to-heavy snow over northern PA and western NY.
WPC PWPF does contain moderate probabilities (40-60%) for snowfall
totals >6" here, but the shield of snowfall will extend as far
east as the Poconos and northern NJ by Friday evening. How long
snow sticks around in the northern Mid-Atlantic will be determined
from the transition of the primary low in western PA and a coastal
low forming off the DelMarVa Peninsula. This has the potential to
bring snow for areas along and west of the I-95 corridor Friday
evening, but most confidence in when this transition occurs,
duration of snowfall, and if temperatures through the depth of the
atmospheric column are cold enough to support snow remain low at
this time. Key Messages for this event can be found below.

...Northern & Central Rockies into the Northern Plains...
Days 1-3...

While the West Coast will see the heaviest snowfall totals over
the next few days, moisture from this rich fetch of subtropical
Pacific moisture will spill over into the northern and central
Rockies. Look for 1-2 feet of snow (locally up to 3 feet possible)
in mountain ranges that include the Sawtooth/Boise, the
Bitterroots, Lewis Range, Tetons, Wasatch, and Colorado Rockies.
Heavy snow arrives out ahead of the bigger Pacific storm system as
the best surge of 700mb moisture flux streams out over the region
and persists into the day on Friday. The WSSI highlights the
Sawtooth/Boise, Teton, and Wasatch likely to see Major impacts
from this system. As the upper low off the Pacific Northwest coast
tracks east into the northern Rockies Friday night, a deepening
surface low will form over eastern MT while a steady stream of
southerly 850mb moisture flux and 700mb moisture via the West
Coast storm bring about periods of snow north of a warm from
northern MT to ND and western MN. WPC PWPF shows a swath of 50-70%
probabilities for >6" of snow in these areas on Friday, but where
the corridor of >6" of snowfall occurs is still subject to change
due to differences in deterministic guidance's positioning of the
shield of snowfall along the warm front.


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 Later Thursday: A winter storm will
reach the West Coast later in the day on Thursday and provide a
burst of heavy precipitation into Friday.

--Heavy Rain and High Snow Levels Likely: Several inches of rain
will fall in the 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 most significant
snowmelt, and overall flooding threat, is expected below 5000 foot
elevation, in areas with shallow snowpack. Creeks and streams in
the foothills of the Sierra Nevada will be most vulnerable to
flooding from rain and snowmelt.

-- Heavy, Wet Snow at Higher Elevations: Higher elevations in
northern California and in the Sierra Nevada are likely to see a
heavy, wet snow. This will lead to difficult travel, and combined
with an already deep snowpack, may lead to increasing impacts from
the depth and weight of the snow.


...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 heavy snow are expected from portions of the northern
Great Plains through the Great Lakes tonight into Friday.

--Currently, the heaviest snowfall is forecast over southern
Minnesota, northern Iowa, southern Wisconsin and into northern
Illinois.

--The storm will then move into the northern Mid-Atlantic on
Friday where several inches of snow are possible.

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


Mullinax