FOUS11 KWBC 210829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Thu Nov 21 2024

Valid 12Z Thu Nov 21 2024 - 12Z Sun Nov 24 2024

...Northern Mid-Atlantic... Days 1-2...

***The first winter storm of the season for the interior Mid-
 Atlantic begins this evening and lingers into the day on Friday**

In the northern Mid-Atlantic, a dynamic storm system will take
shape this afternoon as a vigorous 2PVU anomaly working in tandem
with the left-exit region of a 250-500mb jet streak will strengthen
low pressure off along the triple point just off the Jersey Shore.
As this takes shape and an 850mb low consolidates near the NYC
metro area, a plume of rich 850-700mb moisture off the Atlantic
will wrap around the northern and western flanks of the storm. At
the same time, this moisture advection is a byproduct of intense
WAA at those same levels. As the moisture ascends along an area of
850-700mb frontogenesis, a comma head of heavy precipitation will 
take shape as early as midday today from eastern PA and the 
Southern Tier of NY to the Lower Hudson Valley and northern NJ.
Once precipitation rates increase via increasing vertical 
velocities and the cold temperatures above 800mb arrives that 
afternoon, the profile will be sufficiently cold enough to produce 
heavy snow initially in the Poconos and Catskills Thursday 
afternoon, followed by lower elevations across the Southern Tier of
NY through Thursday night. 

Even 24 hours out from the peak of the event, there are still some
lingering question marks in this setup that will limit the 
potential for a more expansive snow storm. The first is the air-
mass leading up to this storm has been mild and soil temperatures 
remain rather warm. The other is these compact and quickly evolving
storm systems tend to generate a mid- level dry slot. This dry 
slot should make its presence known by 06Z Fri with central NY 
likely getting influenced by the dry slot first, then possibly as 
far west as the Binghamton area Friday morning. While this should 
limit the areal coverage of heavy snow in central NY, the 
impressive rates along the PA/NY border and on south into the 
Poconos will have an expansive areas of wrap around moisture to 
work with. In addition, HREF guidance shows up to a 9 hour window 
where these areas could see 1-2"/hr snowfall rates late Thursday afternoon
and into Thursday night. Even with warmer soil temps, while the 
first inch or so may melt or compact quickly, eventually these kind
of rates should win out and quickly cool the surface to freezing. 
This would then support rapid accumulations, especially in the 
higher terrain of the Poconos. But even some of the Valleys could 
see some heavier totals beneath the deformation zone Thursday night.

By Friday morning, the wrap around moisture at 700-300mb will
rotate as far south as the DC/Baltimore metro area. Some latest
CAMs show an almost squall-line type evolution Friday morning that
could bring a burst of snow to parts of northern MD and possibly
into the DC/Baltimore metro areas closer to midday. In PA, moist
westerly flow beneath this behemoth of a 500mb low (NAEFS show 
700mb heights over southeast PA that are near the lowest observed 
heights for this time of year in the CFSR database), will keep 
periods of snow around until the strong vertical velocities 
subside. Accumulations beyond midday Friday will be difficult with 
the lone exceptions being the higher terrain of the Poconos and 
Catskills. WPC PWPF shows moderate-to-high chances (50-70%) for 
snowfall >8" in the Catskills, Poconos for elevations >1,500ft. 
Similar probabilities are present for >4" for elevations >1,000ft 
in Northeast PA and along the NY/PA border. Note that these totals 
and probabilities are tied strongly to elevation, with the one 
disclaimer that some lower elevations in northeast PA could 
overperform as a result of this deformation band Thursday evening.

Lastly, there is the potential for the first sight of snow in parts
of the NYC/Philly/Baltimore/DC metro areas. While most snow in the
immediate metro areas will be primarily conversational, there are
low-to-moderate chances (30-50%) for >1" of snowfall to the west 
of these metro centers across northern MD, the Lower Susquehanna 
Valley, and into northern NJ.

...Pacific and Interior Northwest... Days 1-3...

The massive closed upper low over the northeast Pacific will
continue to support a robust atmospheric river into the the West
Coast and as far east as the Northern Rockies through Friday night.
On the heels of the historic cyclone that produced a myriad of
hazards in the Pacific Northwest the past 24-48 hours, another
potent storm system will strengthen rapidly Thursday night off the
Pacific Northwest coast. By 06Z Fri, the ECMWF SAT shows an IVT
topping 1,000 kg/m/s aimed at the northern California coast. While
this would be a recipe for heavy snow more often than not, but the
intense WAA over the past 24 hours as all but left snow levels to 
mainly above 5,000ft in the Cascades and Olympics, above 6,000ft in
the Shasta/Trinity mountains, and above 7,000ft in the Sierra 
Nevada. Of the West Coast mountain ranges, the Sierra Nevada will
become the primary focus for heavy snow late Friday and through
Saturday morning as the atmospheric river trajectory becomes 
positioned farther south into the Golden State. The atmospheric
river finally relents later in the day on Saturday and a lull in
the active pattern should settle in by Saturday night. WPC PWPF
shows high chances (>70%) for snowfall >12" for elevations
>7,000ft. In totals, most of the central and southern Sierra Nevada
>7,000ft can expect an additional 1-2ft of snowfall with the 
tallest peaks above 9,000ft possibly approaching 3ft of snow.

Areas with better opportunities for heavy snowfall at lower 
elevations are farther north into the Columbia Basin and even east
of the Northern Rockies in northern Montana where colder 
temperatures locked in place by high pressure to the north will 
allow for precipitation to fall in the form both snow and freezing 
rain. WPC PWPF shows moderate-to-high chances (50-70%) for >4" of 
snowfall along the Montana/Canada border through Sunday morning. In
the mountains, however, snowfall jumps up dramatically with 
portions of the Lewis Range, Sawtooth, Boise, and Blue mountains 
measuring as much as 1-2 feet of snow with localized amounts approach 30".

...Great Lakes & Appalachians... Days 1-2...

This morning, a massive upper low over the Great Lakes containing a
pair of potent 500mb vorticity maxima will be responsible for heavy
snow in the Central Appalachians and across the northern Mid-
Atlantic. Periods of lake enhanced snowfall will continue over the
Michiana area, while the one of the 500mb vort maxima tracking
south from the northern most portion of Michigan's Mitten aids in a
band of moderate-to-heavy snow in southeast Wisconsin and across
the greater Chicagoland metro area. 

As the leading 500mb vort max tracks over the Central Appalachians
Thursday afternoon, the large 500mb low will continue to deep with
heights at 500mb, 700mb, and 850mb that are near the lowest 
observed 06Z Nov 22 heights observed in the CFSR climatological 
record (1979-2009). As the upper low tracks east, brisk and moist 
westerly 850-700mb winds allows for periods of moderate to heavy 
snow to break out along the Laurel Highlands, Allegheny Mountains, 
and even as far south as the Smokeys of NC/TN. Snowfall increases 
in intensity in the Laurel Highlands and Allegheny Mountains 
Friday morning when NW flow not only strengthens but advects a 
greater concentration of 850-500mb moisture into the Central 
Appalachians. Snowfall rates of 1-2"/hr are possible during the day
Friday. Snowfall will gradually subside Friday night with only 
some very light snow in parts of east-central WV leftover by 
Saturday morning. WPC PWPF shows high chances (>70%) for snowfall 
>12" from the Laurel Highlands on south through east-central WV. 
Some guidance shows low chances (10-30%) for snowfall totals >24" 
in the highest terrain of east-central WV by the time the event 
concludes Saturday morning. The WSSI shows Moderate to Major 
Impacts in these areas driven largely due to the sheer amount of 
snow in the forecast, as well as a higher Snow Load component given
the more wet/heavy snow-type expected. This could result in some 
localized power outages and tree damage, especially when paired 
with 40kt 850 mb winds on Friday. Travel will be hazardous to 
impossible in areas with Major Impacts.

 ...Northern New England... Days 2-3...

Along the Adirondacks, Green, and White Mountains, the
aforementioned upper low will draw moist easterly flow northward
and into these ranges Thursday night and into Friday. Only the
peaks of these ranges are likely to see measurable snowfall (Mt.
Washington included) but by Saturday, a new wave of low pressure
well off the East Coast will send another plume of moisture towards
northern New England. Uncertainty in amounts is high given the
spread in guidance, but the WPC PWPF currently shows low-to-
moderate chances (30-50%) for snowfall totals >4" from the White
Mountains to the western most border of Maine. Travel conditions
could be hazardous in some areas at minimum, but dependent upon
trends in guidance the next 24-36 hours, it is possible for 
snowfall to come up in these areas.

The probability of significant ice across the CONUS is less than 10 percent.

Mullinax

...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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