FOUS11 KWBC 220831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Fri Nov 22 2024

Valid 12Z Fri Nov 22 2024 - 12Z Mon Nov 25 2024

...Pacific Coast through the Interior Northwest and Great Basin... Days 1-3...

Persistent closed low centered off the Pacific Northwest coast will
remain energized by vorticity maxima swinging through the
associated trough this period. This will result in amplification of
the gyre, noted by an expansive region of 500-700mb height
anomalies reaching below -1 sigma, and approaching -3 sigma in the
core of the trough. Beneath this trough, deepening surface low
pressure will approach the WA coast today, but likely get pulled
back to the west on Saturday before landfall in response to
secondary shortwave energy digging to its south. This will result
in persistent onshore and divergent mid-level flow, accompanying
upper level jet energy to spread moisture onshore as a continuation
of the ongoing atmospheric river (AR) across northern CA through
early Saturday. While IVT values exceeding 500 kg/m/s wane and
pivot southward by the end of D1, this will still result in
plentiful moisture and widespread moderate to at times heavy
precipitation pushing onshore the Pacific Coast into the interior Northwest.

Snow levels within this AR will be quite high, generally 6000-8000
ft, except east of the WA Cascades on D1 where they will remain
entrenched around 4000 ft before rising. This indicates that most
of the heavy snow will be above pass levels, except in the higher
Cascade Passes D1, and the Sierra Passes D1-D2. WPC probabilities
D1 are high for more than 6 inches of snow in the northern WA
Cascades, the Salmon River Range, and portions of the Northern
Rockies near Glacier NP. On D2, snowfall is more robust and
expansive, with a greater than 90% chance of 6 inches again in the
Northern Rockies and Salmon River/Sawtooth Ranges. More
impressively, the Sierra will be favorably located to be impinged
upon by moisture by the end of D1, leading to pronounced upslope
snow. WPC probabilities are above 80% for 12+ inches through
Saturday, with 2 feet possible above 8000 ft.

By D2 onward, snow levels crash again as a cold front pushes
onshore, but this is accompanied by reduced moisture advection as
the column begins to dry out. Some moderate snow will persist
through onshore flow, especially in the Sierra where an additional
6-12 inches is possible, but otherwise WPC probabilities for more
than 6 inches are between 30-50% and focused across the Wasatch,
Uintas, and ranges of NW WY.

...Appalachians and Northeast... Days 1-2...

Impressive upper low over the Northeast will feature a complex
evolution through the weekend resulting in a couple surface lows and
varying areas of heavy snow.

The Central Appalachians, especially from the Laurel Highlands
southward across WV and as far south as the Great Smokey Mtns of
TN/NC, will benefit from prolonged upslope flow on the western
periphery of the upper low resulting in waves of heavy snowfall
through Saturday. WPC's snowband probability tracker depicts an
extended period of 1-2"/hr snowfall rates during the D1 period
across the Allegheny Mtns. WPC probabilities in this area are high

70%) on D1 for more than 8 inches of snow. Event total snowfall

could peak around 2 feet in central WV.

The ongoing potent and compact surface low churning near Long
Island is forecast to loop westward and eventually toward the south
today over eastern PA as it remains embedded within the large
500mb gyre overhead. As this occurs, an impressive plume of low-
level moisture will continue to wrap northwest into the system,
driving rich theta-e advection into a TROWAL to support some
elevated instability and enhance available moisture for heavy
precipitation early on D1. Increasing deformation is noted on
current radar trends on the W/SW side of the system as it becomes
exceptionally wrapped. This setup will support intense mesoscale
ascent and bands of heavy precipitation rotating W/SW as the low
retrogrades. Although the column will be marginally supportive of
accumulating snow outside of higher elevations (above about 1500
ft), the intense lift should dynamically cool the column and allow
efficient snowfall accumulation at times even in lower terrain.
SLRs for this event will likely be below climo due to near freezing
temps and warm soils/compaction, but still, rates of 1-2"/hr will
promote heavy accumulations, especially in the Poconos and
Catskills and surrounding areas early this morning. Most of the
ongoing snowfall is expected to occur before or right around the
start of the D1 period (12z Fri), so WPC probabilities don't
reflect the ongoing event. Nonetheless, the greatest snowfall
amounts are expected in northeast PA and neighboring areas of New
York and northern NJ, with 2-day snowfall potentially reaching
above 12" in a few highly elevated spots. This will likely result
in dangerous travel and impacts to infrastructure including
scattered power outages.

In the lower elevations of the Mid-Atlantic, some snow bands could
pivot as far SE as the I-95 corridor, so while accumulations in
the major urban areas from Washington, D.C. to New York City are
expected to be minimal, if any, these cities could see their first
"falling" snow of the season Friday morning/aftn.

Along the Adirondacks, Green, and White Mountains, the
upper low will draw moist easterly flow northward and into these
ranges Friday night through Saturday while another surface low
develops east of Maine and pivots northwest. Snow levels should be
high enough that only the highest terrain will receive significant
snow, but WPC probabilities have increased and now feature a 30-50%
chance on D2 of more than 4 inches, primarily in the White
Mountains of NH and the peaks of northern ME.

...Northern Rockies through the Northern Plains... Days 1-2...

A shortwave ejecting through the base of an anomalous trough off
the Pacific Coast will swing eastward into the Northwest Saturday
and then begin to amplify into a potent but fast moving impulse
across the Northern Rockies and into the Northern Plains Sunday.
This deepening system will drive downstream ascent through height
falls, PVA, and divergence. At the same time, a zonally oriented
jet streak will pivot south of the region, placing its favorable
LFQ atop the far northern High Plains, interacting with increasing baroclinicity as a front wavers near the international border. The
overlap of synoptic lift with the enhanced mesoscale ascent
through fgen will result in weak cyclogenesis, leading to an
expanding area of precipitation falling as snow. The column will be
plenty cold for wintry precipitation thanks to high pressure from
Canada extending southward, and as the WAA along the warm front
lifts north, it will result in an exceptionally deep DGZ to support
fluffy aggregate dendrites which should accumulate efficiently. The
biggest question remains whether or not this favorable environment
and most impactful snowfall will span south over the U.S.-Canada border.

The heaviest snowfall is likely in the terrain near the Northern
Rockies where upslope flow will enhance ascent, and WPC
probabilities support a high risk (>70%) for at least 8 inches near
Glacier NP on D2. However, heavy snowfall is also possible farther
east across northern MT along the international border D1-D2 where
the most robust synoptic ascent will materialize. A leading surge
of modest upper diffluence and WAA may lead to a burst of snow
today across northwest and northern MT. WPC probabilities for at
least 2" on D1 are high throughout northern MT. For the main
activity related to the approaching shortwave on D2, the guidance
has continued to trend a bit farther north from central MT eastward
to ND, with current WPC probabilities suggest around a 30% chance
for more than 4 inches right along the international border as far
east as north- central MT on D2, with higher probabilities for
heavy snowfall located in southern Alberta and Saskatchewan.

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

Snell

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 140817
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 14 2024

Valid 12Z Sat Dec 14 2024 - 12Z Tue Dec 17 2024

...Western U.S...

An active period will bring widespread winter weather to much of
the West through early next week.

A large trough just offshore the Pacific Coast to start the period
will shed two distinct compact vort maxima/shortwaves onshore on
D1. One of these will pivot northward into British Columbia, while
the second one ejects into central CA. Although the northern
impulse is a bit deeper relative to the CFSR climatology (500mb
heights below the bottom 2.5 percentile according to NAEFS), the
greater moisture/IVT will pivot into CA associated with the
southern system as the northern moisture channel lifts into Canada. Additionally, there will be a modestly coupled jet streak
downstream of the primary trough axis and in the vicinity of this
southern impulse, helping to enhance ascent to spread more
widespread moisture northeast. On D1, this will cause widespread
heavy snow from the Sierra and Shasta/Trinity region eastward into
the Salmon River/Sawtooth Ranges, Blue Mountains, and as far east
as the Tetons and Big Horns, WPC probabilities are high (>70%) for
above 12 inches in the CA ranges and into ID thanks to higher
moisture, strong synoptic lift, and impressive upslope flow driving
snowfall rates of 2+"/hr at times. 1-2 feet of snow is possible D1
across these areas.

More modest snowfall across the WA/OR Cascades and Olympics where
both ascent and moisture are more limited. Still, WPC probabilities
in the Cascades and Olympics are moderate to high (50-90%) for
more than 8 inches. Snow levels D1 will climb briefly to as high as
5000-6000 ft in the core of the max IVT/WAA/moisture plume, but
will fall gradually most of the day to as low as 2000 ft in the
Cascades and 3500 ft elsewhere. This will allow at least modest
accumulations below pass levels, but most of the accumulating snow
is likely during the period of higher snow levels.

On D2, the pattern evolves quickly as a narrow but amplified
longwave ridge builds over the Pacific Coast, forced between the
departing trough now moving into the northern High Plains and a
renewed closed low back over the Pacific Ocean. This will force a
respite in precipitation across the coast and into the Great Basin,
but wintry precipitation will continue into the Northern Rockies,
but in a slowly waning fashion. Snow levels will continue to fall
steadily D2, reaching below 1000 ft in the Northern Rockies, which
coincident with continuing (but easing) snow will result in heavy
snow accumulations across the NW WY ranges, with some light
accumulations down into most of the valleys. WPC probabilities D2
are moderate (50-70%) for an additional 6+ inches, highest in the Tetons.

Late D2 and then more substantially into D3, the large closed low
west of WA/OR will spin two distinct vorticity maxima/shortwaves
onshore, leading to increasing waves of precipitation spilling
eastward once again. With this next impulse, snow levels will
initially be low, only around 1500 ft, with modest rising
occurring in the axis of greatest IVT. Despite IVT progged to
remain only around 250 kg/m/s, this will lift snow levels to around
3500 ft west of the Cascades in WA/OR, and then potentially as
high as 4500 ft later D3 with the second wave. With impressive
ascent increasing, and upslope flow amplifying into the terrain,
heavy snowfall will result, and WPC probabilities D3 are moderate
(50-70%) for 6+ inches from the Shasta/Trinity region northward
along the Cascades.

Moisture spilling east into the interior Northwest will encounter
some trapped cold air and modest easterly flow ahead of the
accompanying WAA/IVT, such that some light freezing rain could
result on the eastern side of the Cascades. Potentially more
impressively, the accompanying WAA and associated fgen could result
in heavier snow rates farther east which could reach the valley
floors of interior WA and OR. Confidence is low this far out, but
WPC probabilities do indicate at least a low chance for 1" of snow
across much of WA east of the Cascades, with heavier snow likely
moving back into the Salmon River/Sawtooth Ranges before the end of
the period.

...Central Plains into the Upper Midwest... Day 1...

A mixed snow, sleet, and freezing rain event will be ongoing to
start the forecast period as a wave of low pressure moves steadily
from Kansas into the western Great Lakes. This low will be driven
by a compact but potent closed low moving over-top the surface
low, with downstream warm and moist advection expanding the
precipitation shield northeast through the region. As this
vertically stacked system moves east, it will run into some cold
and dry antecedent air as an expansive high pressure gradually
retreats towards New England. As WAA intensifies, it will push a
warm nose above 0C, while surface wet-bulb temperatures will remain
below freezing. The guidance has narrowed the corridor of
significant freezing rain, and despite what could be moderate
precip rates (supported by soundings) and a lack of dry advection
to offset latent heat release of freezing (and to prevent
substantial warming), there is high confidence in a stripe of
significant ice accretion, especially in eastern and central IA.
Here, WPC probabilities are moderate (50-70%) for an additional
0.1+ inches of ice, leading to local event totals exceeding 0.25".

...Central Appalachians... Days 1-2...

Sprawling high pressure with impressive central pressure
approaching 1050mb will gradually pivot east through New England
during the weekend. While this feature will retreat, it will extend
down the coast east of the Appalachians as a cold-air damming
scenario, at least initially reinforced by mid-level confluence
ahead of an approaching shortwave. This impulse will lift from the
Corn Belt late Saturday to off the New England coast, with the
associated moisture and ascent producing a swath of precipitation
extending from the Southern Appalachians through southern New
England. The WAA overrunning the cold surface layer will likely
result in an axis of moderate to heavy freezing rain in the higher
elevations from southern WV through the MD Panhandle and into the
Laurel Highlands where WPC probabilities for more than 0.1 inches
of ice reach as high as 70%, and locally in excess of 0.25" is
possible near the MD Panhandle and into the southern Laurel
Highlands which is where the greatest duration of freezing rain is
expected. Surrounding this, light freezing rain could result in
accumulating ice exceeding 0.01 inches as far south as the northern
escarpment of SC, and as far north as some of the higher terrain of
Upstate NY.

While the most significant winter impacts are likely due to
freezing rain, many areas from PA through MA will experience
precipitation at least starting a snow with some light
accumulations likely in the higher elevations of the Poconos,
Catskills, and Upstate NY/western New England. At this time
however, WPC probabilities are only 50-70% for 2+ inches in these
areas. In places that receive mostly rain, a brief mix of snow and
sleet is possible during precipitation onset even as far southeast
as the I-95 corridor, but no accumulation is expected.

...Northern Plains... Days 2-3...

Vorticity maxima leftover from a weakening shortwave will lift
northeast from the Great Basin, with secondary intensification of
this feature likely over eastern MT Sunday. The guidance has
continued to trend a bit deeper and farther south with this
secondary development, suggesting a higher potential for some
moderate to heavy precipitation from MT into ND.

As the lead shortwave weakens and pushes northeast, some leading
WAA will expand light precipitation into the northern High Plains.
Forecast soundings suggest considerable dry air to overcome, but
as this occurs it will cause some wet-bulb effects to keep temps
below freezing and an axis of light freezing rain is likely from
eastern MT into western ND Saturday night into Sunday morning.
Accumulations are likely to be modest, however, as reflected by WPC probabilities for 0.1+" of ice peaking around 30% in eastern MT on
D2. Still, any freezing rain can cause hazardous travel
conditions.

Thereafter, the secondary impulse deepens into a closed low which
should cause some enhanced deformation to the north and west of the
system. This deformation acting upon some higher theta-e air
wrapping cyclonically around the low (emerging from intensifying
290K isentropic ascent downstream) will support some banded snow
rotating through ND and then into MN Sunday night and Monday. At
this time total snowfall is still expected to be light however,
with WPC probabilities for 2+ inches of snow reaching just around
50% from eastern ND into northern MN.

Weiss
$$
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