FOUS11 KWBC 052143
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 PM EST Fri Jan 05 2024

Valid 00Z Sat Jan 06 2024 - 00Z Tue Jan 09 2024

...Southern Appalachians through New England...
Days 1-3...

Major winter storm to bring combination of freezing rain and heavy
snow from the Southern Appalachians through New England.

Overall, the forecast remains much the same with impactful ice
expected along and east of the southern to central Appalachians
into the Piedmont; followed by heavy snow impacting portions of
the northern Mid-Atlantic and the Northeast. Low pressure now
developing along the central Gulf Coast is forecast to move east
through the overnight, directing moisture over an in-situ cold air
wedge centered over the southern-central Appalachians/Piedmont.
This will support the development of mixed precipitation across
the area Saturday morning, with areas of significant icing still
expected. WPC PWPF continues to show a solid stripe of at least
moderate probabilities (greater than 40 percent) for ice
accumulations of 0.10 inch or more extending along much of western
North Carolina and Virginia, with the highest probabilities
(greater than 70 percent) centered along and east of the Virginia
Blue Ridge. Some of these areas may see accumulations exceeding
0.25 inch.

As the system continues to evolve, with a deepening surface low
forecast to move to the Delmarva coast by late Saturday, guidance
continues to show heavy snow developing well northwest of the low
within a left-exit upper jet region centered over the interior Mid
Atlantic. The heaviest amounts through late Saturday are forecast
to center over central Pennsylvania, where the WPC PWPF indicates
that widespread accumulations of 4 inches are likely, with locally
heavier amounts of 8 inches or more are possible.

As the storm continues north along the Northeast coast it is
expected to continue to strengthen ahead of an approaching
shortwave moving across the eastern U.S. on Sunday Heavy snow
will spread from the interior Mid Atlantic across much of interior
southern to central New England. WPC PWPF shows at least moderate probabilities for accumulations of 8 inches or more extending from
northeastern Pennsylvania to coastal Maine. Moderate
probabilities for a foot or more extend from western coastal Maine
back into the Worcester Hills of Massachusetts, as well as over
portions of the Berkshires, Greens, and Catskills.

..Pacific Northwest/Great Basin/Sierra/Southwest....
Days 1-2...
The region will remain active with an amplifying shortwave and an
associated well-defined front moving into the Northwest on
Saturday. Strong forcing, afforded in part by left-exit region
upper jet forcing will support heavy precipitation along the
Cascades, where snow levels are expected to drop below 2000 ft for
much of the Washington and Oregon Cascades. Heavy accumulations
well over a foot are likely across the favored higher peaks, with
impactful amounts likely in the passes as well. WPC PWPF shows
probabilities above 70 percent for accumulations of 8 inches or
more falling within many of the major passes by late Saturday.

The system will continue to amplify, carving out an anomalously
deep 500 mb trough (2 to 3 standard deviations below normal)
across the Southwest/Four Corners region by late Sunday. This
will bring areas of heavy snow along the Sierra Nevada into the
Great Basin and Southwest, along with a strong cold front pushing
southeast across the region. In addition to the Sierra Nevada,
areas impacted by heavy snow are expected to include the higher
elevations of the central and northern Nevada mountains,
northwestern Arizona southeastward along the Mogollon Rim to the
White Mountains, and the Utah southwestern ranges. The WPC PWPF
shows at least moderate probabilities (40 percent or greater) for
accumulations of 6 inches or more across portions of these areas.
Portions of the Sierra Nevada are likely to see some of the
heaviest amounts, with a foot or likely for areas above 6000 ft.

A well-defined ridge will follow the trough will dry conditions
spreading across the Intermountain West and the Rockies on Monday.
A warm front associated with an approaching shortwave will bring
precipitation back into the Northwest on Monday. Snow levels will
begin to increase but remain low at the onset, potentially
bringing some additional impacts to the Cascade passes.

...Four Corners into the Southern/Central Plains...
Days 2-3...

A significant storm system is forecast to develop this Sunday (Day
2), initially taking shape over the Intermountain West as a
digging 140 kt 300 mb jet results in vigorous upper-troughing and
subsequent closed low formation over the Four Corners. Moderate to
heavy mountain snows are expected to overspread the San Juans
early Sunday beneath a focused region of left exit region forcing
and accompanying height falls, with southwesterly 700 mb flow
supporting appreciable moisture transport and orographic ascent
with this system. The latest WPC snowfall probabilities of
snowfall exceeding 4 inches sit squarely in the 80-90% range
across the San Juans through this weekend, with 50-80%
probabilities of exceeding 8 inches in the peaks of the terrain.

As the ejecting closed low and lead shortwaves eject eastward into
the Plains states, rapid leeside cyclonegenesis is expected to
take shape by Monday (Day 3) with a rapidly expanding
precipitation shield over the Heartland. An initial lead shortwave
and focused area of low level WAA will support an initial bout of
snowfall (with 40-50% probabilities of over 4 inches of snow) over
portions of the Corn Belt early Monday morning along an inverted
trough axis, while a focused axis of snowfall fills in upstream
across the Central Plains along a deformation axis on the back
side of the surface low. Guidance continues to differ somewhat as
to the exact track of the surface low center, with ramifications
for precip type and placement east of the Red River. However,
there is a general consensus within the 12z deterministic guidance
for a focused area of heavy blowing snowfall within the
deformation axis across southwest Kansas into the Oklahoma/Texas
Panhandle. The combination of enhanced snowfall rates and strong
gradient wind within the rapidly intensifying system could result
in blizzard conditions Monday over these areas, where the latest
WSSI-P advertises a 40-50% chance of at least Moderate Winter
Storm Impacts, primarily forced by blowing snow across portions of
the Central/Southern High Plains through Day 3.

Asherman/Pereira


***Key Messages for Jan 6-7 East Coast Winter Storm***

--Heavy Snow in the Northeast--
Expect heavy snow in interior portions of the northern
Mid-Atlantic through New England Saturday afternoon into Sunday.
There is a high chance (50-90%) of exceeding 8.. of snow from
central Pennsylvania east through much of south-central New
England. Local snowfall over 12 inches of snow is possible under
the heaviest bands.

--Impacts from Heavy, Wet Snow and Wind--
The combination of heavy, wet snow and gusty winds from northeast
Pennsylvania through much of southern and central New England will
result in difficult travel with some power outages and tree damage.

--Minor Coastal Flooding Possible in the Northeast--
Gusty onshore winds may lead to minor flooding along the
Mid-Atlantic and southern New England coasts during the Sunday
morning high tide cycle.

--Ice Impacts to Travel in Southern Appalachians--
The probability for ice accumulations in excess of 0.25.. along
the Blue Ridge of North Carolina and Virginia are 30-50%. This
icing, along with some areas of sleet, may produce hazardous
travel conditions on tonight into Saturday.

***Key Messages for Jan 6-11 Major Winter Storm***

--Weather Impacts From Coast-to-Coast--
The next winter storm shifts down the West Coast Saturday crossing
the Desert Southwest Sunday before rapidly strengthening over the
southern Rockies/Plains Sunday night/Monday and then shifting
through the Midwest/Northeast Tuesday/Wednesday.

--Heavy Snow Expected; Blizzard Conditions Possible--
Expect heavy snow in the higher elevations near the West Coast and
the Four Corners states this weekend before a swath of heavy snow
develops over parts of the Southern/Central Plains Monday. Heavy
snow then shifts through the Midwest Monday night and Tuesday.
Given the intensity of the storm, strong winds may create areas of
blizzard conditions over the Plains into the Midwest.

--High Winds Ahead of the Storm--
Gusty winds on the warm side of the storm may exceed 50 MPH and
cause damage over the Appalachians to the East Coast. Organized
severe thunderstorms are possible along the Gulf Coast and
Southeast Monday and Tuesday.

--Flooding and Coastal Flooding Risk in the Eastern U.S.--
Heavy rain Tuesday into Wednesday is likely to lead to river and
possibly flash flooding from the central Gulf Coast through the
Northeast. Powerful onshore winds are also likely to lead to
coastal flooding along much of the East Coast.

$$
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FOUS11 KWBC 060847
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Sat Jan 06 2024

Valid 12Z Sat Jan 06 2024 - 12Z Tue Jan 09 2024

...Southern Appalachians through New England...
Days 1-2...

Major winter storm to bring combination of freezing rain and heavy
snow from the Southern Appalachians through New England.

A positively tilted longwave trough enveloping much of the eastern
CONUS will shed two shortwaves and accompanying vorticity maxima
northeast through the weekend. The first of these will lift from
the TN VLY this morning into New England tonight and will combine
with the LFQ of an intensifying subtropical jet streak to drive
cyclogenesis across the Southeast. This low will lift rapidly
northeast as a Miller-A type system, with impressive moist
advection ahead of it surging northward to produce an expanding
shield of precipitation. As this low approaches New England, a
second, more impressive, but still positively tilted, shortwave
will again lift out of the Southeast, and interact with the
surface low to stall and strengthen it more rapidly south of the
New England coast Sunday morning. After a brief slowing, this low
will eject eastward to become south of Nova Scotia by Monday
morning. While in general this low will be fast moving and of
modest intensity, the interaction of this second shortwave with
the favorably positioned intense upper jet streak and a strong
baroclinic gradient offshore will result in a rapid deepening of
this low on Sunday, which should enhance snowfall across the
interior northern Mid-Atlantic and New England.

On D1, as the precipitation expands to the north, it will begin to
overrun retreating high pressure on moist 290-295K isentropic
upglide. Robust WAA aloft will surge a warm nose above 0C well
north, but at the surface this cold high will remain entrenched
and wedged against the Appalachians, with some enhancement of this
wedge likely during precipitation onset. Weak ageostrophic
drainage from the high pressure could additionally enhance the
wedge and lengthen the period of a favorable environment for
freezing rain, but eventually the WAA should overwhelm the airmass
to change precip over to all rain. While a brief period of
snow/sleet at onset is also possible, most of the p-type in the Southern/Central Appalachians and along the Blue Ridge should be
freezing rain. The duration is modest, wet bulb temperatures are
only around -1C, and precip rates may at times produce some runoff
instead of accretion, but significant ice accumulations are
likely. WPC probabilities for 0.1 inches are generally 40-60%,
with probabilities for 0.25" as high as 20-30% across the Blue
Ridge of VA.

Late this afternoon and into tonight, as the precipitation expands
northward, an axis of moderate to heavy snow driven by WAA leading
to waves of robust 850-600mb fgen will develop from the Central
Appalachians and across the interior Mid-Atlantic states. Snowfall
rates just inland from I-95 from Washington, D.C. to NYC could
reach 1-2"/hr in many areas, albeit very briefly closer to the
fall line before the strong SE flow pushes 850mb temps above 0C
all the way into central MD and eastern PA. SLRs will likely be
below climo in the modest thermals, but an impressive DGZ depth
noted by SREF probabilities for 100mb of depth reaching 50% in
central MD/central PA coincident with the best fgen should still
result in a few hours of heavy wet snow despite the progressive
nature of the storm. Higher elevations from eastern WV through
northern PA have a high chance (70-90%) for 4+ inches according to
WPC probabilities, with snow winding down late D1.

The most significant snowfall and related impacts from this event
are expected from the Poconos and northwest NJ through the
Catskills and into much of southern and central New England. While
there is still some uncertainty into the R/S line along the south
coast of New England, locations just inland, and especially across
higher terrain, will likely receive a long duration of heavy
snowfall. This will be aided by the developing TROWAL which will
pivot back into New England, and the setup should support an
intensifying deformation band which will lift through eastern
PA/eastern Upstate NY and then may transition to a pivoting band
across southern New England, and the presence of likely CSI in
cross-sections suggests snowfall rates could be 2"/hr within this
band despite the low SLR. The guidance has again trended a bit NW
and warmer tonight, but the overall forecast remains on track with
the greatest uncertainty along the south and east coasts of New
England due to strong onshore flow advecting warmer air onshore.
As the low occludes and shifts east on Sunday, the cold air may
still collapse back to the southeast resulting in an axis of heavy
snow dropping back into southeast MA late, although accumulations
still should be modest there. Just NW of I-95 however, the intense
band and long duration of heavy snow will result in the greatest
accumulations, reflected by WPC probabilities for more than 6
inches exceeding 80% from the Poconos and much of eastern Upstate
NY through nearly all of SNE and into southwest ME. Locally,
12-15" of snow appears likely, with the best chance being in any
higher elevations or across northeast MA due to pivoting band
potential.

..Pacific Coast, Great Basin, Northern and Central Rockies...
Days 1-3...

A series of shortwaves, the last one being accompanied by an
atmospheric river (AR) will spread widespread precipitation across
the West into early next week. The first of these shortwaves will
move onshore the WA/OR coast early today and then amplify as it
drops southeast into the Great Basin by Sunday morning. This
shortwave will rapidly deepen into a closed low over the Four
Corners by Sunday night, with lingering vorticity streaming
southward out of Canada through the weekend. The associated
longwave trough which develops during this time will steepen lapse
rates and combine with pockets of upper diffluence and an eastward
advancing cold front to drive ascent supportive to wring out any
available moisture from the Cascades through the Northern Rockies
and southward into the Central Rockies. Although moisture will be
modest, noted by PW anomalies from NAEFS that are generally normal
to slightly below normal, the widespread and robust synoptic
ascent will result in light to moderate snow across much of the
West with snow levels falling from around 3000 ft to as low as 500
ft beneath the core of the upper trough and behind the surface
cold front. The heaviest accumulations D1 are likely in the higher
terrain of the Cascades and Sierra where WPC probabilities for
more than 6 inches of snow exceed 80%, with pockets of >80%
probabilities also extending into the Northern Rockies, and higher
ranges of the Great Basin. In the Cascades and Sierra, due to
periods of impressive upslope and higher moisture with onshore
flow, locally 1-2 feet is likely in the higher terrain. During
this time, light snow, generally less than 1 inch however, may
occur into many of the western valleys as well.

The most intense forcing drops southeast out of the West D2 with
just some lingering moderate snow likely in the OR Cascades,
before an even more significant shortwave approaches during D3.
This shortwave may not actually reach the coast until just beyond
this forecast period, but impressive confluence of the mid-level
flow downstream of this trough will surge strong IVT eastward and
onshore WA/OR characterized by high probabilities in both the EC
and GFS for 250+ kg/ms. The best moisture will likely be acted
upon by downstream mid-level divergence to drive ascent, with
omega also being provided via upslope flow into the Cascades and
more direct diffluence within the LFQ of a zonally oriented
Pacific jet streak. WPC probabilities for more than 6 inches of
snow on D3 exceed 80% in the WA Cascades once again, with
widespread 50-80% probabilities appearing in the Olympics, OR
Cascades, and much of the Northern Rockies, and locally more than
1 foot is likely in the higher terrain. With snow levels rising to
around 4000-5000 ft by D3, the heaviest snow should remain above
most of the area passes.

...Four Corners through the Upper Midwest...
Days 2-3...

An intensifying shortwave digging out of the Pacific Northwest
today will close off over the Four Corners Sunday morning. This
feature will continue to deepen as it progresses to near the TX
Panhandle Monday morning, producing height anomalies at 500mb of
near -4 sigma according to NAEFS. This extremely anomalous trough
will drive impressive downstream ascent through height falls and
divergence to the northeast, with intensifying upper jet streaks
coupling efficiently overhead the greatest height falls to result
in rapid cyclogenesis beginning late Sunday night over eastern NM.
As this surface low deepens and tracks E/NE, it will drive
increasing moisture advection downstream from the Gulf of Mexico
noted by mixing ratios of 6 g/kg on the 300K isentropic surfaces
surging northward into the Southern Plains Monday. This is also
reflected by NAEFS PW anomalies reaching as high as +2 sigma
Monday from TX into KS, with some of this theta-e ridge rotating
cyclonically into a TROWAL back into the High Plains. The
combination of moist mid-level ascent on confluent SW flow
downstream of the primary closed low and this increasing low-level
moisture, in a region of impressive synoptic ascent will likely
result in an expansive area of heavy precipitation from the Four
Corners through the Upper Midwest by the end of the forecast
period.

During D2 as the closed low digs across the Four Corners and
interacts with the strengthening coupled jet structure, moisture
within the mid-level SW flow will be wrung out across the terrain
of the Southwest, especially from the Wasatch through the San
Juans and southward along the Mogollon Rim. WAA downstream of the
surface low will surge snow levels to around 4000 ft in NM/CO, but
these will quickly drop back to just around 500 ft behind the
accompanying cold front as the low advects eastward. Most of the
precipitation should wane behind the cold front so the heaviest
snow is still likely above 3000-4000 ft, which is reflected by WPC probabilities of 70+% for more than 6 inches across parts of the
Wasatch and southeast to include much of the Mogollon Rim, the
White Mountains of AZ, and across the San Juans. Locally more than
12 inches is possible in the highest terrain, and with the falling
snow levels as the event pushes east, light accumulations of
generally less than 1 inch are possible through most of the
valleys as well.

As the evolution becomes more intense into D3, the shield of
precipitation will become both more expansive and intense.
Synoptic forcing remains robust across the region, but begins to
interact with strong mesoscale ascent as WAA overlaps the best
mid-level divergence, and waves of fgen develop both downstream of
the low as well as upstream as a comma-head band tries to develop.
Within the WAA snow downstream of the surface low, DGZ depths
according to the SREF probabilities and through evaluation of
regional soundings are modest, which could result in more moderate
snowfall intensity. However, a long duration of snowfall is likely
which could still accumulate significantly, and WPC probabilities
for more than 6 inches have increased, reaching 50-60%, highest
across eastern NE. Farther to the west, as the low consolidates
and moves east, there is potential for a strong deformation band
developing to the NW near the TX/OK Panhandles Monday morning,
aided by the fgen response to the favorably placed upper jet
streak. This could result in more intense snowfall rates combined
with stronger winds nearer the low center. WPC probabilities at
this time are still relatively modest for more than 4 inches in
this area, at 30-40%, but have increased with this iteration.
Additionally, the probabilistic WSSI has shown an increase in
possible impacts due to blowing snow and snowfall, especially in
the upslope regions around the Raton Mesa and where this band may
develop near the TX/OK Panhandles.

Weiss


***Key Messages for Jan 6-7 East Coast Winter Storm***

--Heavy Snow in the Mid-Atlantic and New England
Heavy snow will overspread the Mid-Atlantic this afternoon and
then lift into New England tonight. Snow will end from west to
east by Monday morning. This will result in widespread snowfall
accumulations of 6-12 inches, with locally as much as 15 inches
possible in higher elevations.

--Impacts from Heavy Snow and Wind
The combination of heavy snow rates exceeding 1"/hr, and gusty
winds will lead to snow covered roads and limited visibility to
create dangerous travel. In some areas, especially southern New
England, the snow may be heavy and wet which could cause isolated
power outages and tree damage.

--Minor Coastal Flooding Possible in the Northeast
Gusty onshore winds may lead to minor flooding along the
Mid-Atlantic and southern New England coasts during the Sunday
morning high tide cycle.

--Ice Impacts to Travel in the Southern Appalachians
The probability for ice accumulations in excess of 0.25 inches
along the Blue Ridge of North Carolina and Virginia are 30-50%.
This icing, along with some areas of sleet, may produce hazardous
travel conditions today.


***Key Messages for Jan 6-11 Major Winter Storm***

--Weather Impacts From Coast-to-Coast--
The next winter storm shifts down the West Coast Saturday crossing
the Desert Southwest Sunday before rapidly strengthening over the
southern Rockies/Plains Sunday night/Monday and then shifting
through the Midwest/Northeast Tuesday/Wednesday.

--Heavy Snow Expected; Blizzard Conditions Possible--
Expect heavy snow in the higher elevations near the West Coast and
the Four Corners states this weekend before a swath of heavy snow
develops over parts of the Southern/Central Plains Monday. Heavy
snow then shifts through the Midwest Monday night and Tuesday.
Given the intensity of the storm, strong winds may create areas of
blizzard conditions over the Plains into the Midwest.

--High Winds Ahead of the Storm--
Gusty winds on the warm side of the storm may exceed 50 MPH and
cause damage over the Appalachians to the East Coast. Organized
severe thunderstorms are possible along the Gulf Coast and
Southeast Monday and Tuesday.

--Flooding and Coastal Flooding Risk in the Eastern U.S.--
Heavy rain Tuesday into Wednesday is likely to lead to river and
possibly flash flooding from the central Gulf Coast through the
Northeast. Powerful onshore winds are also likely to lead to
coastal flooding along much of the East Coast.

$$
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FOUS11 KWBC 080910
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Mon Jan 08 2024

Valid 12Z Mon Jan 08 2024 - 12Z Thu Jan 11 2024

...Southern High Plains through the northern Great Lakes...
Days 1-3...

A strong winter storm will bring heavy snow and blizzard
conditions beginning this morning in the High Plains of southeast
CO/northeast NM and then slowly expand northeast into the Upper
Midwest by Tuesday morning, and into the Great Lakes Tuesday
night, before waning from west to east during Wednesday. Heavy
snow rates, significant snow accumulations, and wind gusts of
50mph or more in some areas will create widespread moderate to
major impacts through mid-week.

Closed mid-level low over the Four Corners to start the period
will feature anomalous 500mb heights as low as -4 sigma according
to NAEFS, and these anomalies are progged to become even more
negative into Tuesday morning as this closed low continues to
intensify. This exceptionally deep low will drive pronounced
height falls and downstream divergence for impressive ascent
shifting across the region, aided by increasingly coupled jet
streaks to overlap the most intense upper diffluence with the
greatest height falls across the Plains. At the same time, the
amplified trough will force intense downstream meridional moisture
advection on unidirectional southerly flow out of the Gulf of
Mexico, which will surge PWs to +2 sigma according to NAEFS as
moist isentropic upglide maximizes in the presence of robust 6-8
g/kg mixing ratios. The accompanying theta-e ridge/WAA wing will
lift cyclonically into a TROWAL, and this will help increase
instability and likely result in intensifying snowfall rates, and
the WPC prototype snow band tool already shows high probabilities
for 1-2"/hr snowfall across much of the area. Additionally, the
setup has always looked favorable for a potent deformation band
developing NW of the cyclone and then translating eastward through
Tuesday, and tonight's cross-sections indicate a strong likelihood
for this to occur with coincident CI (folded theta-e in the
presence of EPV*<0) to support pockets of thundersnow and possibly
rates to 3"/hr across parts of astern CO through parts of MO/IA.
The exact placement and speed of this band is still uncertain, but
impacts will be substantial where it advects due to the
combination of these convective snow rates and gusty winds. While
immediate impacts may be most intense within this band, the
heaviest snowfall will occur where the WAA snow, which in itself
could be significant, will transition to the more convective snow
as the low pulls away.

D1 WPC probabilities for more than 6 inches of snow are above 80%
from the Raton Mesa due to a combination of this deformation band
and ascent aided via upslope northeast flow, into far NW KS which
is where this band may pivot the longest, and through eastern NE
where the most intense WAA snow is likely today, followed without
a break by the pivoting deformation band tonight Total snowfall in
some of these ares could exceed 12 inches. On D2 as the storm
accelerates more to the northeast, the highest WPC probabilities
for more than 6 inches extend from extreme northern MO towards the
Door Peninsula of WI where they exceed 70%, and again, locally
more than 12 inches is possible where the WAA snow and the
pivoting band occur without a break. This is most likely across
parts of southeast Iowa. By D3, the low pulls away but some lake
enhanced snow may develop across the U.P. and northern L.P. of MI
where WPC probabilities for more than 6 inches reach 30-40%,
bringing storm total across that area also up to 8-12 inches.

Finally, there is a challenging potential for some over-performing
snowfall within robust WAA bands surging across northern IN/IL and
into lower MI late tonight into Tuesday. Temperatures will be
marginal for heavy snow, but intense ascent could dynamically cool
the column and allow for rapid accumulation despite low SLR. This
could include the Indianapolis and Chicago areas, where, although
total snowfall accumulations are likely to be modest as snow
changes to rain by Tuesday morning, there could be a few hours of
very heavy snowfall rates accumulating to several inches, despite
low WPC probabilities.

..Northwest...
Days 1-3...

Nearly continuous precipitation into the Pacific Northwest during
the next 3 days will manifest via waves of heavier precipitation
in between bouts where it is less intense. The result of this will
be widespread heavy snowfall in the terrain from the Olympics,
through the Cascades, down into the Sierra, and as far east as the
Northern Rockies and maybe even to the Tetons and Wasatch. Snow
levels during the event will waver considerably, starting low but
rising to around 5000ft by Tuesday morning behind a leading warm
front, but then crashing almost as quickly back to 1000-3000 ft
(lowest north) by Wednesday morning, and continuing to collapse
thereafter behind a strong cold front.

The primary driver of this extended period of unsettled weather is
a series of shortwave impulses dropping southeast from the Pacific
and onshore the Northwest before settling into the Great Basin.
The first of these is progged to approach the PacNW coast late
tonight before surging to the Northern Rockies on Tuesday,
followed almost immediately by an event stronger impulse with more
intense vorticity streaming into OR/CA by Thursday morning. These
impulses will be embedded within otherwise confluent W/NW
mid-level flow, and beneath the approaching LFQ of a robust
Pacific jet streak impinging on the coast by Wednesday. This
suggests that the most impressive deep layer ascent through height
falls and upper diffluence will occur within a rapidly moistening
column noted by IVT probabilities exceeding 80% for 500 kg/ms.
Where the best synoptic lift can be aided by intense upslope flow
through orthogonal mean wind, intense precipitation rates are
likely. Additionally, regional forecast soundings beneath the
amplifying upper trough indicate steep lapse rates up from the
surface, supporting some elevated instability to further enhance
precipitation rates, and where this is all snow, the WPC prototype
snow band tool suggests snowfall could be 2-3"/hr at times, aided
by SLR that is likely to be slightly above climatological means.

The heaviest precipitation begins this evening, with one surge
occurring on Tuesday, followed by a secondary surge on Wednesday,
possibly a bit farther south than on Tuesday. For D1, this results
in WPC probabilities for more than 6 inches reaching above 80%
across most of the WA Cascades and some of the higher terrain of
the OR Cascades. Farther east, WPC probabilities for more than 6
inches are 50-70% in the Blue Mountains and Northern Rockies. The
first surge on Tuesday will then drive an expansion and
intensification of the snowfall, with snow levels plummeting later
in the day behind the cold front. WPC probabilities exceeding 80%
for more than 12 inches along the Cascades, the Olympics, and
Northern Rockies, with high probabilities for more than 6 inches
extending into the northern CA ranges, and much of the terrain as
far southeast as the Wasatch Front and Tetons. During D3 as the
cold front sags farther to the southeast, and a final surge of
moisture/ascent pivots onshore, WPC probabilities for more than 1
foot again eclipse 70% in the OR Cascades, and also extend into
the northern Sierra. More than 6 inches of additional snow on D3
is likely in the Olympics, WA Cascades, Northern Rockies, and
terrain of the Great Basin into the Wasatch. Additionally, with
snow levels continuing to crash, light accumulations are possible
even into most of the valleys across the Intermountain West as far
southeast as the Four Corners.

Storm total snowfall in the Cascades and Olympics could exceed 5
feet in some areas, with major to extreme impacts likely even at
the passes due to blizzard conditions.

Additionally, although confidence is quite low at this time, the
CIPS snow squall parameter is highlighting the potential for snow
squalls behind the cold front dropping into the inter-mountain
West. If these convective snow showers or snow squalls develop,
they could produce briefly extremely limited visibility and heavy
snowfall rates which could severely impact travel Wednesday into
Thursday. It will be worth monitoring how this evolves during the
next few forecast cycles.

...Southern Appalachians through New England...
Days 1-3...

An extremely anomalous mid-level low centered over the
Mid-Mississippi Valley Tuesday morning will continue to deepen as
it shifts northeast towards the eastern Great Lakes by Wednesday
morning. During this time, height anomalies are progged to reach
the minimum climatological percentile in the database according to
NAEFS, and while the anomalies weaken with time to the northeast,
this system will remain exceptionally intense into D3. Downstream
from this closed low, height falls and divergence will overlap
with a strong coupled jet structure to help deepen an associated
surface low as it tracks into the Great Lakes, with an occlusion
to a triple point over New England likely thereafter. The result
of this evolution will be exceptional moisture transport out of
the Gulf of Mexico and up the eastern seaboard, noted by IVT
anomalies reaching as high as +8 sigma near the DelMarVa Tuesday
night, exceeding the maximum climatological percentile for the
rolling 4-week period inherent to the NAEFS tables. The overlap of
this extreme moisture and intense synoptic ascent will result in
expanding heavy precipitation across the east, but with intense
WAA accompanying the moisture surge, much of the area will be too
warm for wintry precipitation.

There will be two exceptions to this. The first is into the
Southern/Central Appalachian foothills, along the Blue Ridge of
NC/VA, and into the Allegheny Mountains/Laurel Highlands where the
overrunning precipitation will initially encounter sub-freezing
surface wet-bulb temperatures. This will result in an axis of
sleet, transitioning primarily to freezing rain with some light to
moderate icing likely. The rapid erosion of the cold high pressure
combined with heavy rain rates which tend to runoff more easily
than they can accrete ice will limit ice accumulations, but WPC
probabilities for 0.1 inches of ice are 10-30% D1 into D2, highest
along the VA Blue Ridge, before changing to rain. The other
concern will be an area of very heavy snowfall in the highest
terrain of from central PA through New England. Snow levels are
progged to rise to as high as 4000 ft within the strong WAA/best
IVT, so eventually nearly all of the area will turn over to rain
below those levels. However, even as low as 2000ft precipitation
will likely begin as heavy snow with rates of 1"/hr or more within
the pronounced fgen driven by strong WAA before transitioning to
rain in the Catskills, Adirondacks, Berkshires, and Greens and
Alleghenies. Snowfall accumulation in these ranges should be
modest as reflected by WPC probabilities for 4+ inches reaching
30-70% before changing to rain. However, above 4000 ft in the
White Mountains of NH/ME, and even some of the lower elevations
across northern/central ME, the event may stay primarily snow and
accumulate significantly despite below-climo SLR in the warm/moist
environment. WPC probabilities for snowfall are above 80% for 6+
inches, with local amounts as high as 1-2 feet in the highest
terrain likely. Additionally, this will likely be very heavy and
wet snow as reflected by WSSI-P probabilities for major impacts
due to snow load eclipsing 50%, which could result in power
outages in some areas, especially when combined with the
increasing winds.

...Northern Plains...
Day 3...

A wave of low pressure will skirt along the Canada/North Dakota
border late Tuesday into Wednesday, accompanied by modest WAA
along a leading warm front. As this advects eastward, weak
enhancement of moisture will be wrung out by the accompanying lift
within the WAA and downstream of height falls associated with the
driving shortwave aloft. This feature is likely to be transient
and progress rapidly to the east, but a band of moderate snow is
likely from central ND through northern MN. WPC probabilities are
10-20% for 4+ inches of snow.

Weiss


***Key Messages for Jan 7-11 Major Winter Storm Southern Rockies
through Midwest and Eastern U.S.***
--Hazardous Travel Expected in Areas of Heavy Snow
Snow will develop in the Plains on Monday, and advance into the
Midwest on Monday Night and Tuesday. Bursts of heavy snow may
accumulate up to 2 inches in an hour, and result in hazardous
travel. A heavy, wet snow will also develop in the interior
Northeast Tuesday Afternoon and Night.

--Extremely Dangerous Plains Blizzard
A blizzard is expected in the plains of northeast New Mexico,
eastern Colorado, the Oklahoma and Texas Panhandles, western
Kansas, and southwest Nebraska. Wind gusts as high as 60-70 MPH
will create ferocious blizzard conditions with whiteouts. Travel
will become extremely dangerous to impossible. If you must travel,
pack a winter survival kit as wind chills will plummet below zero.

--Significant River, Flash, and Coastal Flooding
Widespread and potentially significant river and flash flooding
are likely for the central Gulf Coast and Eastern U.S. Monday
through early Wednesday. Powerful onshore winds will lead to
widespread coastal flooding along the eastern Gulf Coast and much
of the East Coast. Significant coastal flooding is likely,
especially on the Mid Atlantic coastline, including tidal rivers
and bays.

--Damaging Winds May Lead to Power Outages
Widespread wind gusts in excess of 50 MPH are likely in the
eastern Gulf Coast, central Appalachians, much of the East Coast,
and New England. Prepare for power outages.


***Key Messages for Jan 8-11 Northwest Winter Storm***
--Major Winter Storm for the Cascades
Back-to-back storms will cross the Pacific Northwest Monday night
and Tuesday night, with several feet of heavy snow and strong
winds across the Cascades. Blizzard conditions are likely. Snow
levels rise to around 5000 feet Monday night with an atmospheric
river but quickly fall to between 1500 and 2500 feet behind a cold
front on Tuesday, leading to considerable impacts for many
mountain passes with the second storm.

--Heavy Snow Elsewhere in the West
Over a foot of snow is likely (70-90%) in the higher elevations of
the Northern Rockies on Tuesday and Wednesday. Snow levels falling
to below 1000 feet later on Wednesday and early Thursday should
lead to some snow accumulations in valleys as well.

--Storm Will Move to the Central U.S.
The storm system is forecast to dig across the Western U.S.
through Thursday and reach the Plains late in the week. People in
the Plains and Midwest should prepare for another winter storm and
stay tuned for updates.
$$
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