Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Wed Nov 20 2024
Valid 00Z Thu Nov 21 2024 - 00Z Sun Nov 24 2024
...Pacific and Interior Northwest...
Days 1-3...
Persistent moisture advection within an intense atmospheric river
/AR/ will drive widespread precipitation, including high elevation
snow, across the Northwest into the weekend.
The period begins with an amplified closed low positioned off the
British Columbia coast. This low will feature impressive height
anomalies of -2 to -3 sigma according to NAEFS ensemble tables, but
will gradually fill as it retrogrades back to the northwest. As
this occurs, a secondary shortwave pivoting around the base of this
amplified trough will deepen and pivot eastward, deepening to
feature renewed impressive height anomalies and an accompanying
surface low approaching the WA coast by Friday afternoon.
Downstream of this low, mid-level ridging blossoming across the
Rockies will result in pinched/gradient flow to enhance wind speeds
and warm/moist advection onshore, with robust divergence across
the Northwest overlapping with periods of strong jet dynamics to
produce strong ascent across the region. At the surface, a cold
front will be pushed towards the coast, but is expected to stall
just offshore until Friday when the secondary low will finally
advect it eastward. This indicates that the most impressive IVT
plume, for which both GEFS and ECENS feature high probabilities for
exceeding 750 kg/m/s, will persist into northern CA and push as
far east as the Great Basin and Northern Rockies as noted by PW
anomalies from NAEFS reaching above +1 sigma into Friday. During
this time, however, snow levels will rise dramatically, especially
within the core of the most intense IVT, surging to 4000 ft in the
Northern Rockies and as high as 9000 ft in CA. Although snow levels
will remain more modest in WA, they will still rise to generally
above pass level in most areas, turning snow to rain across a lot
of the region, and leaving the highest snowfall confined to the
higher peaks. WPC probabilities are above 70% for 6+ inches D1 in
the Shasta/Trinity region, the OR Cascades, and most impressively
from the Blue Mountains through the Salmon River/Sawtooth region.
On D2 this pivots to be focused from the highest terrain of the WA
Cascades (near Mt. Rainier) into the Northern Rockies near Glacier
NP.
During D3, the pattern begins to evolve as a wave of low pressure
develops across the interior NW along the advecting, finally,
baroclinic gradient to the east. This will interact with a cold
front digging out of Canada to enhance frontogenesis along the
Canadian border near the Northern Rockies, which will interact with
an upper level jet streak to push a swath of heavy snowfall from
eastern OR through eastern MT. Here WPC probabilities are renewed
above 50% for 6+ inches in the higher terrain, with some low
probabilities for 4 inches extending into the High Plains of MT
along the international border. Otherwise, snowfall is expected to
wane D3 across the West, at least briefly, as the most robust IVT
pivots south and east away from the area, but snowfall will develop
once again in the Sierra.
...Great Lakes, Appalachians, & Northeast...
Days 1-3...
Challenging forecast into the weekend across much of the Great
Lakes, Mid-Atlantic, and Northeast as a complex upper low evolves
across the region.
The period begins with a sprawling upper low centered over
Wisconsin forcing a longwave trough across much of the eastern half
of the CONUS. Vorticity lobes spinning around and within this large
gyre will cause the upper low center to wobble gradually eastward
into Friday while deepening to as much as -3 to -4 sigma across the
Mid-Atlantic according to NAEFS ensemble tables. Beneath this
trough, dual surface lows are progged to develop and retrograde in
response to the amplification of the upper pattern, one pivoting NW
and then eventually SE into the Ohio Valley, while a secondary low
develop along the triple point south of New England and rotates NW
into the Hudson Valley before finally advecting eastward to the
coast of Maine by Saturday morning. THere remains uncertainty into
the track and intensity of both of these features, but the general
trend in guidance has been for locally deeper lows driving more
intense ascent, with a track a bit farther south/west than previous
model runs.
This will result in two areas of heavy snow. The first, and region
of highest confidence, is across the Central Appalachians,
beginning Thursday aftn and persisting with rounds of snow until
Friday night. The onset of snow across this area will be due to
post-frontal upslope flow which will maintain saturation within the
deepening DGZ on CAA. This will result in heavy snow, especially
above around 1500 ft, from the Laurel Highlands southward along the
Appalachians and as far south as the Great Smokey Mountains/Blue
Ridge of NC. As the low from Michigan shifts southward Thursday
aftn into Thursday evening, it will begin to weaken, but additional
ascent, especially with any modest deformation on its south side,
could result in additional areas of heavy snow from Michigan,
through the Ohio Valley, and then enhance ascent into the
Appalachians. WPC probabilities across the Ohio Valley are
generally 10-30% for more than 4 inches, highest near Lake Michigan
where some enhancement may occur. In the Appalachians, WPC
probabilities are high on D2, and moderate on D3, for 6+ inches,
highest in WV where locally close to 2 feet of snow is possible.
The more challenging aspect of this forecast involves the secondary
low development progged to occur off of New England Thursday
afternoon which will then retrograde NW towards Upstate NY before
moving back off the NJ coast Saturday morning. This low will likely
deepen in a region of impressive synoptic ascent, and the setup
supports an intense deformation axis developing on the west side of
this low as it pivots to the NW Thursday evening into Friday. This
deformation will overlap with some impressively sloped 925-700mb
fgen noted in cross sections, driving ascent into the DGZ and into
areas of conditional instability reflected by pockets of low or
even negative SEPV to support convective snow rates. The forecast
soundings indicate this will be purely a rain/snow event, however,
a potent dry slot will attempt to rotate cyclonically around the
system as well, which could dry out the DGZ and cutoff the snow,
but latest guidance has backed off on the westward extent of this
dry slot, so confidence is increasing that periods of intense
snowfall will rotate across Upstate NY, PA, resulting in heavy snow
accumulations above 1500 ft as reflected by WPC probabilities for
more that 4 inches reaching 70% in the Catskills and Poconos, where
above 1500 ft as much as 8-10" of snow could fall. This snow is
expected to be heavy and wet as well, (low SLR), which could
produce power outages and damage to trees, as reflected by 40%
WSSI-P probabilities for moderate impacts, driven primarily by snow
load.
Farther south, as well as in lower elevations, the incoming models
have become a bit more aggressive with snowfall, but the marginal
thermal structure suggests precipitation will be generally a mix of
cold rain and snow, except during periods of more intense rates.
This creates a very low confidence forecast at lower elevations,
and this is additionally reflected by high standard deviations in
the PWPF snowfall. Currently, WPC probabilities for at least 1 inch
are above 10% across much of PA as far south as the MD border and
towards I-95 where some snow may occur as far southeast as
Philadelphia and Baltimore.
The probability of significant ice across the CONUS is less than
10 percent.
Weiss
...Winter Storm Key Messages are in effect. Please see current
Key Messages below...
https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png
