Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Tue Mar 14 2023

Valid 00Z Wed Mar 15 2023 - 00Z Sat Mar 18 2023

...The West...
Days 1-3...

The robust atmospheric river (AR) that produced significant
impacts in portions of California today will extend from the
Southwest to the Intermountain West this evening. Latest NAEFS
guidance for 06Z Wednesday showed an expansive area of >99th
climatological percentile IVT values from southern California on
north and east into the central Rockies. This highly anomalous IVT
is delivering >90th climatological percentile PWs to the Four
Corners region with values >99th in northern Arizona. Given the
time of year featuring a more marginally cold air-mass and the
warm/moist characteristics of the AR, snow levels will remain
quite high, ranging from above 6,000 feet in the northern Rockies
to 8,000 feet in the southern Rockies. The higher terrain will
also be more favored for strong upslope flow, resulting in heavier
snowfall rates as well as gusty winds. Latest WPC PWPF through
Wednesday evening shows 80-90% probabilities for >12" of snowfall
in the southern Sierra Nevada, while 60-80% probabilities for >8"
of snowfall are found in the Wasatch, Sawtooth, Tetons, Wind
River, and San Juan mountain ranges. As an upper level disturbance
tracks into the Desert Southwest Wednesday night, the nose of a
130 knot 250mb jet will place its diffluent left-exit region over
the central Rockies, which combined with the residual 850-700mb
moisture flux will provide sufficient moisture and lift to produce
heavy snow in the Colorado Rockies and as far south as the San
Juans and Sangre De Cristo mountains of northern New Mexico. The
San Juans feature the highest probabilities (50-60%) for >8" of
snowfall between 00Z Thurs - 00Z Fri while the rest of the
Colorado Rockies feature 20-40% probabilities. By Thursday and
into early Friday, northeasterly upslope flow in wake of a cold
frontal passage into the Sangre De Cristo of Colorado/New Mexico
allow for an increase of probabilities for 8" of snowfall, rising
as high as 60-70% in these ranges. The Days 1-2 WSSI show the
southern Sierra Nevada expected to contend with Extreme impacts,
largely driven by Snow Amount and Snow Load. The more interior
West mountain ranges range anywhere from Moderate to Major with
the same two criteria (Snow Amount and Snow Load) driving the
WSSI. Expect difficult to even impossible travel in these ranges
through Thursday.

...Upper Midwest...
Days 2-3...

A pair of shortwave troughs; one emerging out of the Southwest and
the other out of the northern Rockies, will provide large scale
ascent throughout the northern and central Plains on Wednesday. At
the surface, a wave of low pressure organizing in lee of the
central Rockies on Wednesday will become the focus for a
developing storm system that coincides with an impressive 60 knot
850mb jet Wednesday night across the central Plains. Meanwhile, a
strong cold front racing south from the Dakotas will outrun the
rich 850mb moisture flux and wrap around the developing 850mb low.
This will allow for a warm conveyor belt of moisture to intersect
the cold front with a strengthening 850mb front to set up from
eastern Nebraska on north and east into southern Minnesota. There
remains a good deal of uncertainty on the evolution of this front.
Some guidance suggests the developing 850mb low is more
consolidated with a more prolonged period of precipitation in wake
of the front, allowing the shield of precipitation in wake of the
front to look more anafrontal in appearance. This could lead to
several hours of intense snowfall rates that could result in rapid
accumulation. However, should snowfall rates struggle to approach
1"/hr, with the heaviest period of snow in eastern Nebraska and
into northwest Iowa coming during the day, the higher sun angle
could make roadways warmer and boundary layer temperatures more
marginal due to the lack of dynamic cooling in the column. This
will continue to be closely monitored in the coming forecast
cycles. For now, WPC PWPF does show 20-30% odds for >4" of
snowfall in northeast Nebraska, northwest Iowa, and southern
Minnesota through Thursday afternoon.

By 18Z Thursday, the upper trough in the northern Plains will
begin to absorb the shortwave trough in the southern Plains. With
a steady diet of Gulf of Mexico moisture funneling out ahead of
the troughs, the warm conveyor belt will setup Thursday evening on
the northern flank of the 850mb low with a banana-shaped dome of
high pressure engulfing much of southern Canada and bleeding down
the spine of the Rockies' Front Range. Combined with strengthening
easterly flow over Lake Superior, heavy snow bands will be common
from the northern Minnesota Arrowhead, northern Wisconsin, and the
U.P. of Michigan. Hourly snowfall rates of 1-2"/hr are a good bet
in these areas Thursday night into Friday with areas strong wind
gusts of 30-40 mph helping to create whiteout conditions in the
heaviest snow bands. WPC PWPF has high chances (70-80%) for
picking up >8" of snowfall from northern Wisconsin to much of the
U.P. of Michigan. The impressive moisture fetch from the Gulf, the
lake enhancement from Lake Superior, and the sufficiently cold
temperatures throughout the column have good odds of producing
>12" of snowfall in the Michigan U.P. In fact, WPC PWPF even has
20-40% probabilities for >18" of snow in the central Michigan
U.P.. The experimental PWSSI shows 40-60% chances for Moderate
impacts along the Michigan U.P. coast late Thursday and into
Friday evening. Travel will be quite difficult in these portions
of the Upper Midwest and northern Great Lakes with significantly
reduced visibility and snow covered roads.

...Northeast...
Day 1...

A powerful nor'easter located east off the Massachusetts Capes
will position a conveyor belt of 850mb moisture flux over northern
New England while lingering 850-700mb moisture continues late
afternoon and into the evening hours. As the low gradually moves
east and the storm becomes vertically stacked, the storm's WAA
input across the region will wane and lingering snowfall rates
will dissipate by late Wednesday morning. Snowfall rates >1"/hr
are still possible from the Finger Lakes and Adirondacks on east
to the Berkshire and throughout much of the interior terrain of
northern New England. WPC PWPF feature 40-60% probabilities for
snowfall totals >6" in the northern Adirondacks and across
northern/western Vermont, northern New Hampshire, and northern
Maine. Wind gusts of 30-40 mph will still support near whiteout
conditions in these areas, highlighted by the Minor to Moderate
impacts from the WSSI through Wednesday morning. Snow looks to
taper off Wednesday afternoon.


...Key Messages for March 10-15 (Upper Midwest to Northeast)
Winter Storm...
--Heavy-wet snow, strong winds, and coastal flooding continue
tonight over New England.

--While the heavy snow rates of 1â€+/hr will diminish through
this evening, strong winds and the heavy-wet nature of the snow
that has fallen will result in further power outages and tree
damage.

--Additional snowfall of 3-8†can be expected over portions of
New England. Please be mindful of the increased danger for
injuries and heart-attacks when shoveling heavy-wet snow.

--Minor coastal flooding and beach erosion are forecast for high
tide tonight, mainly in Massachusetts as the strong low pressure
center lingers in the Gulf of Maine.


...Key Messages for Mid-March (Western U.S.) Winter Storms...
--Heavy Precipitation Thru Late Tonight
Considerable to locally catastrophic flooding impacts below about
5000 feet elevation for the South-Central and Southern California
coasts, and the southern Sierra Nevada foothills.

--Heavy Rain and Snowmelt Expected
Heavy rain, combined with snowmelt in terrain below about 5000
feet to result in more widespread flooding from Tuesday into
Wednesday, particularly in low elevations and areas with shallow
and warming snowpack. Creeks and streams will continue to be
vulnerable to flooding from additional rain and snowmelt.

--Sierra Nevada Snow Load Impacts Worsen
Heavy rain absorbed into the particularly deep snowpack in the
Sierra Nevada along with heavy snow, measuring in feet above about
7500 feet, will further compound ongoing snow load impacts and
issues.

Mullinax