FOUS11 KWBC 250813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Mon Mar 25 2024

Valid 12Z Mon Mar 25 2024 - 12Z Thu Mar 28 2024

...Southern Rockies... Day 1...

An inverted 500mb trough over the Southern Rockies will be the
focus for additional heavy snow today. The Sangre De Cristo and
eastern San Juans both sport high chances (>70%) for additional
snowfall totals >8" with the tallest peaks of the Sangre De Cristo
in northern New Mexico even having moderate-to-high chances
(50-70%) for >12" snowfall accumulations through Monday afternoon.
The axis of moisture and lift associated with this upper trough
will even result in heavy snow over the Gila Mountains in eastern
Arizona. WPC PWPF shows moderate chances (40-60%) for snowfall
accumulations >8" through Monday evening. The 500mb trough will
eject east into the Southern Plains Monday night, thus all but
concluding the lingering periods of snow by Tuesday morning.

...Northern Plains & Upper Midwest... Days 1-2...

The major winter storm will be well underway with the worst impacts
being felt from the central High Plains on north to north-central
Minnesota. The deformation axis on the northern and western flank
of the storm will be capable of producing 1-2"/hr snowfall rates to
go along with wind gusts that are exceeding 40 mph in northwest
Kansas, central Nebraska, and southeast South Dakota. Farther east,
the impressive 850mb WAA in the Upper Mississippi Valley that was
in part responsible for the heavy snowfall overnight will lead to
a burgeoning warm nose at low levels that causes snow to
changeover to a wintry mix this morning in southern Minnesota,
northern Wisconsin, and the Michigan U.P. By this afternoon, any
lingering wintry mix is expected to transition over to plain rain.
Meanwhile, the storm will continue its trek northeast into the
Upper Mississippi Valley Monday evening with heavy snow enveloping
eastern South Dakota, northern Minnesota, and the Minnesota
Arrowhead.

Snow will continue to fall heavily over north-central Minnesota
Monday night with some residual snow as far south as eastern
Nebraska and northern Kansas as a TROWAL pivots overhead. By
Tuesday morning, the low will be tracking into northern Wisconsin
with heavy snow across northern Minnesota and a mix of heavy
snow/wintry mix along the Minnesota Arrowhead. The storm will then
move over the U.P. of Michigan Tuesday afternoon, then finally into
southern Ontario Tuesday night. Snow will continue to fall across
northern Minnesota and portions of far northern Wisconsin and the
western Michigan U.P. Heavy snow will effectively come to an end by
Wednesday morning with just lingering snow showers along the
Minnesota/Canada border and along the northern coast of the
Michigan U.P..

WPC PWPF shows moderate-to-high chances (50-70%) for additional
snowfall >6" from north-central Nebraska and eastern South Dakota
to far southeast North Dakota, northern Minnesota, and the
Minnesota Arrowhead. South-central South Dakota and the Minnesota
Arrowhead even show low chances (10-30%) for additional localized
snowfall amounts >8". Farther southwest, WPC PWPF shows moderate-
to-high chances (50-70%) for additional snowfall totals >4" in
southwest Nebraska, northwest Kansas, and southeast Colorado.
Nebraska and northwest Kansas, in particular, could contend with
blizzard conditions due to prolonged high wind gusts and
significantly reduced visibilities.

...Pacific Mountains Ranges... Days 2-3...

Outside of some diurnally driven snow showers and weak onshore
flow in the Pacific Northwest through Tuesday night, little in the
way of heavy snowfall is likely to occur for many of the mountains
ranges along the West Coast. That finally changes by early
Wednesday morning as the next Pacific storm system approaches and
directs a slug of Pacific moisture first at the Pacific Northwest,
then into the Northern California ranges and interior Northwest
ranges Wednesday night. The IVT is a quick mover but a fairly
potent IVT sporting values >400 kg/m/s just off the California
coast. Overall, the peak of the IVT tracking into northern
California Wednesday afternoon is likely to top the 97.5
climatological percentile, supplying not only abundant moisture to
the region but strong upslope enhancement (especially in the Sierra
Nevada). Snow levels in the Sierra Nevada will initially start out
as high as 6,500ft, but gradually fall to 5,000ft by Thursday
morning as the cold front pushes through. Farther north, snow
levels will start out around 5,000ft in the northern California
ranges (Salmon/Siskiyou/Trinity) and closer to 4,500ft in the OR/WA
Cascades, but those too will fall an additional 2,000ft by
Thursday AM. Latest WPC PWPF shows high chances (>70%) for snowfall
amounts >8" in the Northern Sierra Nevada >6,000ft and in the
northern California ranges, while only the volcanic peaks of
southern Washington and the Olympics sport similar high chances
for >8" of snowfall through early Thursday morning. Some snow will
spill over into the Blue Mountains and the Boise/Sawtooth late
Wednesday night into Thursday with low-to-moderate chances (30-50%)
for snowfall totals >6" through early Thursday morning.

Mullinax


***Key Messages for Plains-Upper Midwest Winter Storm***

--A high-impact winter storm unfolding

A powerful storm system will impact the Northern/Central Plains
into the Upper Midwest through Tuesday. Periods of snow and gusty
winds will continue from the Central Plains to northern Minnesota,
along with some sleet and freezing rain in parts of the Upper
Mississippi Valley.

--Widespread heavy snow expected

Snow accumulating at rates of 1-2"/hr in heavier bands are
expected from central Nebraska and eastern South Dakota to northern
Minnesota. Additional snowfall totals between 6-12" are forecast
from central Nebraska to northeastern Minnesota.

--Hazardous impacts due to snow, snow load, and wind

Heavy snow and gusty winds approaching 50 mph will produce
blizzard conditions with near zero visibility into Tuesday morning.
Travel could be nearly impossible at times. Power outages and tree
damage are likely in some areas due to the heavy and wet snow
combined with icing and strong winds.


$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)

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/LatestKeyMessage_1.png

$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)