Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
212 PM EST Mon Mar 06 2023

Valid 00Z Tue Mar 07 2023 - 00Z Fri Mar 10 2023

...Northern Mid-Atlantic...
Day 1...

A narrow but potent 500mb shortwave trough exiting the Great Lakes
tonight has its sights set on the northern Mid-Atlantic tonight.
The setup features the left exit region of a 150 knot 250 mb jet
streak positioned over northern PA, northern NJ, and into southern
NY with the nose of a strong 100 knot 500mb jet aimed at these
same locations. At lower levels, a surface and 850mb low will
travel along a warm front that will be situated from the upper OH
Valley to the Chesapeake Bay. This will allow for the three
aforementioned regions above (northern PA, northern NJ, and into
southern NY) to be located just north of the 850mb low track. In
addition, along and north of the warm front, CAMs are indicating a
strengthening 850-700mb front via increasing WAA aloft, along with
a steady surge of 850-700mb moisture. These ingredients are ideal
for strong mesoscale-driven banding at the nose of the 500mb jet
and north of the warm front, which along with the bulk of the
snowfall occurring at night (daytime becomes more difficult for
accumulations with an ever rising sun angle this time of year),
gives rise to the concern for an intense band of heavy snowfall
that could setup from northern PA to both northern NJ and the NYC
metro area by early Tuesday morning.

It was telling that the WPC Snowband Probability Tracker showed a
good consensus for snowfall rates as high as 2-2.5"/hr starting
close to the Buffalo metro area this evening, then moving across
northern PA and along the PA/NY border between 00-06Z Tuesday.
These kind of rates can lead to rapid accumulation on all
surfaces, as well as near whiteout conditions for those traveling
in these areas. Where guidance begins to differ is by the early
morning hours Tuesday. Some guidance shows the 850-700mb front
beginning to undergo frontolysis or there is too much low-mid
level dry air at the onset to overcome as the shield of
precipitation approaches northern NJ and the NYC metro, which
despite the strong synoptic scale lift and sufficient moisture
present, would cause snowfall rates to wain and perhaps struggle
to get above 1"/hr. However, the favorable ingredients mentioned
does provide a floor for this event, or in other words, enough
lift and moisture is present to still cause locally heavy
accumulations between 06-12Z Tuesday. Due to the best mesoscale
drivers being positioned over northern PA, it is here where
guidance is coming into better agreement on heavier totals. Latest
forecasts suggest totals of 4-8" have grown in confidence north of
I-80 in northern PA with totals closer to that 4-6" range within
reach closer to the Delaware Valley in eastern PA. It is in these
areas where the experimental PWSSI shows 40-50% probabilities for
Minor impacts Monday night. Once you cross the Delaware River, the
waning frontogenetic support could lead 1-3" with localized
amounts >4" in northern NJ and could encroach into the NYC metro
area. The affected areas in eastern PA, northern NJ, and perhaps
into the NYC metro could see enough snow accumulation to make for
a slushy AM commute on Tuesday, making for what could be a slick
and treacherous AM rush hour for motorists.

...West/Rockies...
Days 1-3...

As one embedded shortwave trough leaves northern California for
the northern Rockies and northern Plains tonight and into Tuesday,
the next disturbance in the form of an upper level low will be
located off the Pacific Northwest coast tonight and plunge south
towards the northern California coast by Tuesday evening. The
approaching upper low will only continue to prolong this extended
period of heavy mountain snowfall in northern California. Latest
WPC PWPF shows 60-70% probabilities for 48-hr snowfall totals >12"
between Tues 00Z - Thurs 00Z in the northern Sierra Nevada while
there are 30-40% probabilities in the Shasta and along the coastal
range extending from far northern California to far southwest OR.
The experimental PWSSI does show 60-80% probabilities for Moderate
impacts in these ranges, which include hazardous travel conditions
and possible road closures. This upper level low will open up into
vigorous shortwave trough tracking into the northern Great Basin
by Wednesday afternoon. Meanwhile, the next robust upper level low
will be hot on its heels, tracking south from the British Columbia
coast to off the Pacific Northwest coast by Wednesday night. This
low will then direct another surge of 850-700mb moisture flux
towards the Northwest, this time setting up the Olympics and
Cascades for rounds of heavy snow on Thursday. Latest WPC PWPF
shows 60-70% probabilities for >6" of snow in the Olympics on
Thursday with 20-40% probabilities from the Washington Cascades to
as far south as the Oregon Coastal Range. This upper low will help
bring about the next round of heavy mountain snow to northern
California Thursday night, but it will be more confined to the
higher elevations >7,000 this time around compared to the
onslaught of heavy snow at lower elevations to end February.

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

Similar to the Pacific Northwest, the Northern Plains and Upper
Midwest reside in an active storm track thanks to their location
downwind of the longwave trough over the southwest Canada and the
northwestern U.S. Embedded shortwave troughs revolving around the
base of the trough will make their way towards the North-Central
U.S., providing sufficient lift and WAA aloft into an air-mass
plenty cold to support snow. The cold air-mass is anchored in
place through the short range thanks to an expansive dome of
1040+mb high pressure engulfing much of south-central Canada. The
first round of snow occurs this afternoon and into Tuesday as
steady PVA aloft, divergent flow at 250mb, and southeasterly 850mb
moisture flux give rise to periods of snow out ahead of a cyclone
forming in lee of the Rockies over southeast Montana. The best
moisture and lift will be located over the Dakotas where latest
WPC PWPF shows 20-40% probabilities for >6" of snowfall. Snowfall
rates themselves will be generally <1"/hr, although as the primary
band moves through tonight in eastern Montana and into the daytime
hours Tuesday in the Dakotas, some snowfall rates of 0.50-0.75"/hr
may occur. The experimental PWSSI shows a large swath of 60-80%
probabilities for Minor impacts through Tuesday, but Moderate
impacts are topping out around 10%, showing the setup may lead to
some treacherous travel conditions in the hardest hit areas, but
significant impacts are less likely to occur.

Most of Tuesday night and into Wednesday will feature easterly
850mb flow and residual moisture that may lead to ongoing periods
of light snow in the eastern Dakotas and Minnesota. By Wednesday
morning, the next and more potent in the series of upper level
disturbances looks to track through the northern Great Basin and
track into the Rockies Wednesday night. In response to the
lowering pressures over the Intermountain West, easterly upslope
flow will strengthen along the front range of the Rockies and the
Black Hills. The 500mb trough begins to take on a negative tilt
Wednesday night with strong diffluent flow over the northern
Plains causing pressures to fall over the central Plains. Guidance
continues to differ on the placement and track of the 850mb low on
Thursday with the ECMWF being slower and farther north and west
than most guidance. Looking at the WPC PEPF, the >6" probabilities
footprint from Wednesday evening through Thursday evening is
large, stretching from eastern MT and northeast WY to the Dakotas
and both northwest Iowa and southwest Minnesota. Focusing on the
>8" probabilities for the same time frame, there are moderate
(40-60% probabilities) chances for >8" in western South Dakota,
northwest Iowa, and southwest Minnesota. Presently, the
experimental PWSSI shows 20-30% odds for Moderate impacts in these
areas. Should ensemble guidance come into better agreement on
where the heaviest snowfall axis is to occur, it is likely the
Moderate impacts will increase in upcoming forecast shifts.
Potential impacts include slick roadways and heavily reduced
visibilities that could approach white out conditions.


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


Mullinax