Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Sun Mar 12 2023

Valid 00Z Mon Mar 13 2023 - 00Z Thu Mar 16 2023

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

A series of shortwave impulses moving southeast across the Pacific
will gradually move onshore the coast beginning Monday night /late
D1/ with subsequent impulses continuing through the forecast
period. Although each individual shortwave will be of modest
amplitude, subtle backing of mid-level flow from westerly to
southwesterly to drive WAA, combined with the associated height
falls will produce periods of enhanced ascent across the West.
Additionally, a persistent nearly zonal Pacific jet streak
reaching 130 kts will surge into CA and the Great Basin, providing
additional synoptic lift through LFQ diffluence, and, more
importantly, increased PWs across the region. The prolonged W/SW
mid-level fetch across the Pacific combined with this persistent
upper jet streak will result in yet another atmospheric river (AR)
moving into CA beginning D2 as noted by high CW3E probabilities
for IVT exceeding 500 kg/m-s. The overlap of periods of enhanced
ascent into the gradually moistening column will yield widespread
precipitation across much of the West, with snow falling across
the higher terrain due to snow levels that will be generally
4000-5000 ft north, but as high as 7000-8000 ft south within the
core of the AR. As the first shortwave moves across the Pacific
Northwest early Tuesday, it will push a cold front eastward which
will help drive snow levels down to 1500-3000 ft in the Pacific
Northwest, but this will also be accompanied by weakening ascent
so only lighter snows are anticipated into D3 during the lower
snow levels. WPC probabilities each day are moderate to high for
6+ inches across much of the terrain from the Olympics of WA
though the Sierra and eastward into the Northern Rockies, but
gradually expand in coverage to encompass the Central Rockies and
eastern Great Basin ranges by D3. Snow totals will likely be
highest, reaching 4-6 ft, in the Sierra, with multiple feet of
snow also likely in the Cascades, Sawtooth, and Tetons through the
middle of the week.

Late D3, the guidance is beginning to suggest that the northern
stream and southern stream will begin to interact across the
Central/Northern Rockies leading to an amplifying full-latitude
trough with downstream jet energy beginning to pulse to the north.
This should result in more robust deep layer ascent, and could
yield a surface low developing in the lee of the CO/WY Rockies. At
the same time, the aforementioned cold front will begin to lay
more west to east across the Northern High Plains, along which
some elevated fgen, both due to the low-level baroclinicity and
the ageostrophic response to the upper jet evolution, should drive
more intense lift. Where this overlaps with better moisture
advection pivoting northward downstream of this amplifying trough,
a band of heavy snow may develop late for portions of the Northern
High Plains and foothills of the Northern Rockies, but current WPC
probabilities are less than 5% for 4 inches. However, this could
become an impressive storm system in to D4 farther downstream.


...Mid-Atlantic and Northeast...
Days 1-3...

...Major nor'easter likely for the interior Northeast. Uncertainty
continues for I-95...

An anomalously deep closed upper low digging across the Great
Lakes today will dive southeast across PA on Tuesday and then be
pulled off the NJ coast Tuesday night while reaching -2 to -3
standard deviations from the climo mean with respect to 500mb
heights according to NAEFS ensemble tables. This closed low will
be pulled eastward late Tuesday in response to a potent vorticity
lobe which will be shed around the closed feature and arc towards
Cape Cod, driving an impressive negative tilt to the already
closed mid-level feature. This will eventually phase into an even
stronger closed low south of New England with only very slow
movement eastward expected through D3. Aloft, initially modestly
coupled jet streaks will drive some enhanced upper diffluence off
the Mid-Atlantic coast D1, which will interact with the offshore
baroclinic zone to drive surface low development in a Miller-B
type evolution (as the primary low over the Great Lakes weakens.)
The trailing jet streak will eventually become dominant as it
strengthens to 130kts over the Southeast, but this will continue
to place favorable upper diffluence within the LFQ just south of
New England. The guidance is in good agreement that a rapidly
strengthening surface low will move up the coast, potentially
stall or even retrograde briefly near eastern MA, before then
drifting eastward Tuesday night and Wednesday. A powerful
nor'easter moving slowly near New England is a classic setup for a
major nor'easter, and it is becoming likely that major to extreme
impacts will occur across this area.

The challenge to this forecast will be related to the thermal
structure of the column. Temperatures ahead of the low will be
marginal for wintry precip, likely a few degrees either side of 0C
at the surface north to south, and as the impressive warm/moist
advection pivots northward ahead of the developing low, it will
overrun the slowly retreating high pressure to surge warm air
northward, with increasing onshore flow (characterized by 850mb
U-winds reaching -4 sigma into MA Tuesday) also pushing warm
Atlantic Ocean air onshore. This suggests that many places along
the coast and across southern New England will begin as rain,
while inland regions and higher elevations will begin as snow. The
difficulty to the forecast then involves the exact track and
intensity of the surface low which will demarcate where air will
be cold enough for snow, or at least allow for dynamical cooling
to support snow.

The models have continued to trend a bit S/SE the past several
runs, but there remains uncertainty into where exactly this low
will move due to multiple low centers being progged by the models,
and the potential for the rapid intensification to drive
downstream ridging to push the storm a bit farther NW than models
suggest. With such razor-thin column thermals in place, this will
make all the difference between heavy snow accumulations and a mix
of rain snow. What seems more certain is that this low will deepen
extremely rapidly, potentially "bombing out" just inside the 40/70
benchmark. While this will continue to flood warm air into eastern
New England, there are clear signals in model cross sections for
CSI and even some upright convection as impressive deformation
bands develop beneath the pivoting TROWAL NW of the system. Where
these bands occur, intense ascent and precip loading will likely
dynamically cool the column so that even lower elevations or areas
with warmer temperatures could switch back to snow, with intense
rates of 2-3"/hr possible. This makes for a very challenging
accumulation forecast as heavy snow is nearly certain in the
terrain, but heavy snow mixing with and/or changing to rain at
lower elevations and points east during less intense precipitation
is possible. Regardless, this setup will support SLRs that are
generally below climo, and a heavy wet snow is expected in most
areas, which is also reflected by the pWSSI snow load parameter
showing the potential for major to extreme impacts.

The heaviest snow will likely occur in the interior northeast
terrain including the Adirondacks, Catskills, Greens, Berkshires,
Worcester Hills, Monadnocks, and Whites where WPC probabilities
for 6 inches are above 80% on D2, and reach 50-80% for 12 inches
in these same areas. In the valleys, snow totals will likely be
less, but intense ascent could still prevent extreme shadowing on
the e/ne flow, and WPC probabilities for 6+ inches are above 50%
even in the lower elevations all the way up the coast of Maine
despite that area being impinged upon by drier air and being more
removed from the low center as it moves east instead of northeast
late. For the I-95 corridor, there is good potential for a period
of heavy snow to develop on the back side of the system near the
coastal front, and while WPC probabilities for NY to Boston are
pretty low for 2-4 inches, the guidance has trended a bit more
aggressively for this area, and there are several members in the
WSE plumes that suggest heavy accumulations even to the coast.
While confidence in major impacts and heavy snow accumulations is
greatest inland across the higher terrain, the potential still
exists for a significant snowstorm for NYC to Boston as well.

Behind this system, a long duration of moderate to robust upslope
flow will drive periods of heavy snow into the central
Appalachians. The guidance has trended stronger with the available
moisture today, and is now featuring better lift reaching into the
lower portions of a deep DGZ. This has led to an increase in
snowfall potential with higher SLRs leading to fluffy
accumulations for which WPC probabilities suggest a 20-40% chance
of exceeding 6 inches.


Weiss


...Key Messages for March 10-15 (Upper Midwest to Northeast)
Winter Storm...
--A coastal low pressure will strengthen rapidly into a major
norâ€easter that significantly impacts the Northeast Monday Night
- Wednesday.

-Heavy snow rates (2"+/hr) and strong winds will likely produce
dangerous to near impossible travel. The heavy-wet nature of the
snow, combined with max wind gusts up to 50 mph, will result in
scattered to widespread power outages and tree damage. Similar
impacts could be felt along the I-95 corridor from NYC to Boston.

--March norâ€easters tend to favor the more elevated terrain for
receiving the heaviest snowfall totals. Snowfall totals >12" are
most likely in the Catskills and southern Adirondacks in NY,
Berkshires and Worcester Hills in MA, Monadnocks and White
Mountains in NH, and southern Green Mountains in VT. Localized
amounts >24" are possible.

--Widespread minor coastal flooding and beach erosion may be
possible Monday night through Wednesday as the low pressure stalls
and deepens off the coast of New England.



...Key Messages for Mid-March (Western U.S.) Winter Storms...
--Occasional precipitation tonight and Monday Rain in low
elevations and foothills, and snow at high elevations, from
northern to central California, as well as northwest Nevada, and
Oregon tonight at a slightly lower intensity.

--Rain and snow to intensify late Monday into Wednesday
Considerable flooding impacts below 5000 foot elevation along the
central California coast, San Joaquin Valley, Sacramento Valley,
and southern Sierra Nevada foothills into midweek.

--Heavier Rain, Snow, and Snowmelt likely Midweek Additional heavy
rain and snowmelt 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.

--Heavy, Wet Snow at Higher Elevations Mid-Week Another round of
heavy snow, measuring in feet in the higher elevations in
California, particularly the Sierra Nevada, will further compound
snow load impacts and issues.