Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Sun Nov 17 2024

Valid 00Z Mon Nov 18 2024 - 00Z Wed Nov 20 2024

...Heavy snow over parts of the Olympic Peninsula/Cascades and moderate to
heavy snow over the Northern Intermountain Region/Northern Rockies...

...There is a Slight Risk of excessive rainfall over parts of the Southern
High Plains on Sunday, Central Gulf Coast on Monday, and Central Gulf
Coast/Southeast on Tuesday...

...There is a Slight Risk of severe thunderstorms over parts of the
Southern High Plains on Sunday and Southern Plains on Monday...

A front over the Pacific Northwest into Northern California will move
eastward to the Ohio, Middle/Lower Mississippi Valleys, and Southern
Plains by Tuesday evening. A second front extending from the Great Lakes
to the Southern High Plains will move eastward, and the front s eastern
end will dissipate overnight Monday over parts of the Northeast. A wave of
low pressure will develop along the western end of the boundary over the
Southern Plains overnight Sunday and moves to northeastward to Central
Canada by Tuesday evening while the boundary moves to the Great Lakes to
the Lower Mississippi Valley. In addition, a second wave of low pressure
will be over the Central Gulf Coast on Tuesday.

The Northwest front will create coastal rain over parts of the Pacific
Northwest into Northern Central California through early Monday. The
system will produce heavy snow over the Olympic Peninsula/Cascades through
Monday afternoon, followed by moderate to heavy snow on Tuesday. As the
front moves eastward, moderate to heavy snow will develop over parts of
the Northern Intermountain Region/Northern Rockies overnight Sunday into
Monday, then moderate snow over the region on Tuesday. Further, weak
onshore flow will aid in producing rain over parts of coastal Northwest
and Northern California on Monday into Tuesday.

Meanwhile, as the wave of low pressure moves northeastward over the
Southern Plains, moisture will stream northward over the area, producing
heavy rain. Therefore, the WPC has issued a Slight Risk (level 2/4) of
excessive rainfall over parts of the Southern High Plains through Monday
morning. The associated heavy rain will create mainly localized areas of
flash flooding, with urban areas, roads, small streams, and low-lying
areas the most vulnerable.

Moreover, showers and severe thunderstorms will develop along and ahead of
the front over the Southern High Plains. Therefore, the SPC has issued a
Slight Risk (level 2/5) of severe thunderstorms over the Southern High
Plains through Monday morning. The hazards associated with these
thunderstorms are frequent lightning, severe thunderstorm wind gusts,
hail, and a few tornadoes. Additionally, there will be an increased threat
of severe thunderstorm wind gusts of 65 knots or greater over parts of the
Southern Plains.

Furthermore, as the front moves eastward, showers and severe thunderstorms
will develop along and ahead of the boundary over the parts of the
Southern Plains. Therefore, the SPC has issued a Slight Risk (level 2/5)
of severe thunderstorms over the Southern Plains from Monday through
Tuesday morning. The hazards associated with these thunderstorms are
frequent lightning, severe thunderstorm wind gusts, hail, and a few
tornadoes.

Also, on Monday, as tropical moisture streams over the Central Gulf Coast,
heavy rain will develop over the region. Therefore, the WPC has issued a
Slight Risk (level 2/4) of excessive rainfall over parts of the Central
Gulf Coast from Monday through Tuesday morning. The associated heavy rain
will create mainly localized areas of flash flooding, with urban areas,
roads, small streams, and low-lying areas the most vulnerable.

Likewise, as the axis of tropical moisture moves eastward, heavy rain will
develop over parts of the Central Gulf Coast and Southeast. Therefore, the
WPC has issued a Slight Risk (level 2/4) of excessive rainfall over parts
of the Central Gulf Coast and Southeast on Tuesday. The associated heavy
rain will create mainly localized areas of flash flooding, with urban
areas, roads, small streams, and low-lying areas the most vulnerable.
Moreover, rain and thunderstorms will develop over parts of the Upper
Midwest and Ohio Valley overnight Monday into Tuesday.


Ziegenfelder


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EST Sun Nov 17 2024

Valid 12Z Wed Nov 20 2024 - 12Z Sun Nov 24 2024

...Strong, long-duration atmospheric river will take aim at
northern California/southwestern Oregon for midweek into late
week...

...Pattern change for east-central to eastern U.S. with cooler
temperatures and cold rain and high elevation snow...


...Overview...

A long duration atmospheric river is forecast to affect the West
Coast this week as upper and surface lows/frontal systems direct a
strong moisture plume into northern California/southwestern Oregon
in particular, where several inches of rain and mountain snow are
likely. Meanwhile a deep upper low developing across the Great
Lakes and Northeast U.S will meander slowly in place later this
week with a blocking high to its north across central Canada. This
will result in cooler temperatures, a cold rain from the Ohio
Valley to the East Coast, and early season accumulating snow for
the central Appalachians and the higher terrain of the interior
Northeast. Between these two regions, an upper ridge is likely to
build across the Rockies and eventually across the Plains by next
weekend, warming up temperatures in the interior West and southern
Plains.


...Guidance/Predictability Assessment...

Model guidance begins the medium range period in fairly good
agreement for important features, including a deep surface low
somewhere in the 940s to 960s in the eastern Pacific offshore of
Vancouver Island Wednesday, with its cold front directing the AR
into the West Coast. Models show this low gradually weakening as it
moves back northwest into Thursday, giving way to another low with
more spread in depth (weaker than the first one though) and
placement approaching the Pacific Northwest Friday into the
weekend. The newer 12Z guidance suite seems to be coming in with
better agreement for this secondary low fortunately. A model blend
seemed reasonable initially, but there was increasing spread by
late week into the weekend. Namely, the 00Z CMC was farther
offshore with an upper low and directed the AR farther north into
the Pacific Northwest given its mean flow from the southwest rather
than west. Meanwhile, the 06Z GFS brought in a notable shortwave
into the Northwest Saturday and tracking into the north-central
U.S. by next Sunday. This was dismissed as an outlier; however, the
newer 12Z GFS held on to an evolution like this and the 12Z EC is
not too off either, so this feature will continue to be monitored
and may necessitate changes in future forecasts.

Farther east, models are reasonably agreeable with the upper and
surface lows moving across the Midwest to East. The 00Z GFS became
the main outlier by late week as its lows were displaced to the
southeast of consensus. Otherwise a model/mean blend seemed
reasonable.

The WPC forecast began with a blend of the 00Z ECMWF/CMC/UKMET and
06Z GFS. Decreased the proportion of the deterministic runs
(especially those other than the ECMWF) with time as model spread
increased especially in the West, in favor of the GEFS and EC
ensemble means.


...Weather/Hazards Highlights...

Confidence remains high for a potentially major and prolonged
atmospheric river event setting up across portions of northwestern
California and into southwestern Oregon during the Wednesday
through Friday time period. Enhanced moisture flux ahead of a
meandering front and at least two low pressure areas will result in
enhanced orographic lift and likely produce some 4 to locally 8
inch rainfall totals during this time. Slight Risks remain in place
for both Days 4/Wednesday and 5/Thursday in the ERO. Burn scars
will be of particular concern for flooding. By Thursday/Day 5, snow
levels are forecast to rise with an influx of warm air, so
extended Thursday's Slight Risk farther inland/into higher
elevations where rain on snow may exacerbate flooding concerns. The
long duration of the AR stalling could make this event be on the
higher end of the Slight Risk range, with multiple days of rainfall
compounding flooding impacts. Additionally, at least marine areas
should see strong winds through Wednesday given the deep eastern
Pacific low. The AR should finally start to move and weaken into
late week and next weekend, bringing precipitation farther south
while snow levels lower into the Cascades/Sierra Nevada, and
farther east with some chance for snow to extend into the northern
High Plains.

Impactful weather is also expected for a large portion of the
eastern U.S. for the latter half of the work week. This will be
the result of a large upper low closing off over the Midwest and
slowly making its way across the Great Lakes/Ohio Valley and then
Northeast as the week progresses. Surface low pressure will track
through the Great Lakes region and give way to surface cyclogenesis
near the Mid-Atlantic and southern New England in a Miller B type
evolution. Some strong winds are likely in the northern Plains
Wednesday behind the first surface low, with breezy to windy
conditions broadly from the Midwest to East in this pattern.
Precipitation is also likely with these features. Some locally
heavy rain is possible in Florida for midweek with moisture
streaming in ahead of the tail end of associated cold fronts.
Coastal areas of the Mid-Atlantic and Northeast including the
Interstate 95 corridor should also see some modest to perhaps
locally heavy rain Wednesday-Thursday. This did not seem to
necessitate any ERO areas as lack of instability should put a cap
on heavy totals, and ongoing drought will mean most areas are not
particularly sensitive to flooding. Cold air aloft will be
sufficient to support accumulating early season snow for the higher
terrain near the Great Lakes and interior Northeast, and
especially for the central Appalachians where some totals in excess
of six inches are a good possibility. There may also be some lake
effect snow in the wake of this storm system, although temperatures
will likely be marginal and this could mix with rain at times.

In terms of temperatures, warm and slightly humid conditions can
be expected from the Mid-Atlantic southward to Florida on Wednesday
ahead of the approaching cold front that will bring a return to
much cooler conditions to close out the work week. Highs should
generally be about 5-10 degrees below average across the interior
Eastern U.S. by Friday, mainly because of the cold upper low
overhead keeping widespread cloud cover in place, and overnight
lows closer to average. A gradual warming trend is likely for the
Intermountain West and the Rockies as the ridge builds in after
the cold conditions expected in the first half of the week.


Tate/Hamrick


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EST Sun Nov 17 2024

Valid 00Z Mon Nov 18 2024 - 00Z Mon Nov 25 2024

The models agree well with a rather strong surface high situated
to the northeast of Hawaii through tonight, with a tight pressure
gradient over the state and resulting in windy conditions and
winds generally out of the east. The high should gradually weaken
some going into Tuesday and beyond as a trough to the north pushes
it to the east. The ensemble means are now in better agreement
with timing of the trough. In any case, there will likely be a
col in the subtropical surface ridge, and thus lighter winds going
into the early to middle part of the week.

In terms of precipitation prospects, there will likely be more
trade wind based showers across the east-facing terrain through
today, especially across the Big Island. Other than that, no
major heavy rainfall events appear likely over the next seven
days.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Sun Nov 17 2024
Day 1
Valid 16Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

1600Z Update...

The 12Z HREF guidance along with 06Z RRFS/REFS output continues to
support a locally heavy rainfall event across areas of the southern
Plains by this evening and extending into the overnight period. There
has been some shift farther west with some of the swaths of heavy
rainfall potential across west TX as some of the individual CAMs
are supporting the heavier rainfall/convection along the immediate
cold front and in close proximity to the deepening surface low
that will be evolving across the TX High Plains tonight. However, a
complication to the rainfall forecast is the downstream evolution
of convection along the front ahead of the surface low involving
areas of northwest TX through southwest to central OK. This is
where strong moisture convergence and forcing may foster a threat
for some training areas of convection that become aligned in a
southwest to northeast fashion. Overall, still expecting some
swaths of 2 to 4 inch rainfall totals, with isolated potential for
heavier amounts and especially across northwest TX into southwest
OK where the better convective training threat will exist. Given
the latest CAM guidance, the Slight Risk area has been expanded
somewhat to account for the varying modes of convective evolution
that are expected this evening and overnight.

Elsewhere, no changes have been made to the previous D1 outlook
including the Marginal Risk area across western OR where there
continues to be some lingering, but modest atmospheric river
activity.

Orrison

Previous Discussion...

...Southern Plains...

A southern stream cutoff low is expected to eject into the Plains
today with very impressive IVT and accompanying anomalous PWs and
forcing as it moves across Texas and Oklahoma by this evening into
the overnight hours. This looks to be enough to overcome the
limited forecast instability and result in deep convection and
heavy rainfall rates across portions of the Southern Rockies and
the adjacent Southern/Central Plains. Guidance continues to suggest
2-4" of rain (per latest HREF and WPC QPF), but would expect
isolated swaths as high as 4-6" within this setup (indicated by
some of the heaviest CAMs). The overall progressive nature of the
system and limited instability will cap the upper bound of rainfall
totals, but this system is dynamic enough with plentiful moisture
to suggest at least localized flash flooding is likely. A Marginal
Risk area remains in effect for central/eastern New Mexico,
northern Texas, most of Oklahoma and southern Kansas. The inherited
Slight Risk area saw only minor adjustments to reflect the latest
trends and WPC QPF and spans from northern Texas/panhandle to
western Oklahoma.

...Coastal Oregon and northwest California...

A fairly progressive, weak/moderate atmospheric river will be
ongoing across the region. Total rain of 2-4" (through 12z Mon)
and hourly rainfall rates possibly approaching 0.5" within a narrow
southward shifting axis suggest that some minor flooding issues
could arise along coastal areas/mountains. A Marginal Risk area
remains in effect along the coastline.

Churchill/Campbell


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Sun Nov 17 2024
Day 2
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND THE PLAINS/UPPER MIDWEST...

20z Update: A Slight risk was added across portions of southeast
Louisiana into coastal Mississippi including New Orleans and
Biloxi. The higher QPF with this event is still forecast to fall on
day 3 over areas further east. However, while heavy QPF coverage
may not be as high on day 2, the ingredients will still be there
for intense rainfall rates over southeast LA. The 12z HREF is not
overly aggressive, but given the setup and ingredients would not be
surprised to see an upward trend with the 00z guidance. Overall,
the environment in place is conducive for a conditional flash flood
threat over urban locations within the Slight risk. A slower
evolution (as indicated by the ECMWF and AIFS) would bring Baton
Rouge more into play, but will let the Marginal risk cover that for
now and keep the Slight focused over the corridor from New Orleans
to Biloxi where the probability of some training convection is
higher Monday night.

Chenard

...Previous Discussion...

...Central U.S. ...

The precipitation shield will shift eastward as the cold front
advances further into the Plains. Moderate to locally heavy
rainfall is expected from northern Texas to southern Minnesota with
areal average of 1 to 3 inches. The higher amounts will likely
concentrate over eastern Kansas, eastern Nebraska and western Iowa.
A Marginal Risk area was maintained once again with only minor
adjustments for this issuance (not enough confidence for 2"+ totals
to introduce a Slight Risk).

...Gulf Coast...

A plume of enhanced tropical moisture will likely advect northward
through the Gulf of Mexico well in advance of tropical cyclone
Sara; which will increase the potential for heavy rain and
localized flooding concerns along portions of the Gulf Coast. A
Marginal Risk area is in effect from southeast Texas to southern
Mississippi (changed very little from inherited with 3" localized
totals possible).

Churchill/Campbell


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Sun Nov 17 2024
Day 3
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND CENTRAL GULF COAST...

20z Update: Only minor changes to the inherited risk areas, as the
forecast generally looks on track. Expecting a backbuilding
convective complex to push eastward across the central Gulf Coast
Tuesday. Anticipate rainfall rates with this complex will be
intense enough to overcome the high FFG on an isolated to scattered
basis. 3 to 5" totals remain most likely, although localized
swaths exceeding that are probable. The progressive nature of the
system will put a cap on the upper bound of rainfall
magnitudes...although still think locally significant flooding is a
possibility given the intense rates and potential for backbuilding
into the strong southerly low level inflow.

Chenard

...Previous Discussion...

...Central Gulf Coast and Southeast...

By Tuesday significantly more tropical moisture will likely advect
northward through the Gulf of Mexico in association with the
remnants of Tropical Storm Sara; which will increase the potential
for heavy rain and localized flooding concerns along portions of
the Gulf Coast. An inherited Slight Risk area was maintained for
southern AL and surrounding portions of FL/GA/MS/LA, given the
potential for localized 3-5" totals (per ensemble guidance and WPC
QPF).

...Coastal Washington, Oregon, and northwest California...

Another weak/moderate atmospheric river is expected for region for
Day 3. Total rain of 2-4" (similar to Day 1) are expected with
hourly rainfall rates possibly approaching 0.5" once again suggest
that some minor flooding issues could arise along coastal
areas/mountains. An inherited Marginal Risk area remains in effect
along the coastline.

Churchill

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EST Sun Nov 17 2024


Confidence remains high for a potentially major and prolonged
atmospheric river event setting up across portions of northwestern
California and into southwestern Oregon during the Wednesday
through Friday time period. Enhanced moisture flux ahead of a
meandering front and at least two low pressure areas will result in
enhanced orographic lift and likely produce some 4 to locally 8
inch rainfall totals during this time. Slight Risks remain in place
for both Days 4/Wednesday and 5/Thursday in the ERO. Burn scars
will be of particular concern for flooding. By Thursday/Day 5, snow
levels are forecast to rise with an influx of warm air, so
extended Thursday's Slight Risk farther inland/into higher
elevations where rain on snow may exacerbate flooding concerns. The
long duration of the AR stalling could make this event be on the
higher end of the Slight Risk range, with multiple days of rainfall
compounding flooding impacts. Additionally, at least marine areas
should see strong winds through Wednesday given the deep eastern
Pacific low. The AR should finally start to move and weaken into
late week and next weekend, bringing precipitation farther south
while snow levels lower into the Cascades/Sierra Nevada, and
farther east with some chance for snow to extend into the northern
High Plains.

Impactful weather is also expected for a large portion of the
eastern U.S. for the latter half of the work week. This will be
the result of a large upper low closing off over the Midwest and
slowly making its way across the Great Lakes/Ohio Valley and then
Northeast as the week progresses. Surface low pressure will track
through the Great Lakes region and give way to surface cyclogenesis
near the Mid-Atlantic and southern New England in a Miller B type
evolution. Some strong winds are likely in the northern Plains
Wednesday behind the first surface low, with breezy to windy
conditions broadly from the Midwest to East in this pattern.
Precipitation is also likely with these features. Some locally
heavy rain is possible in Florida for midweek with moisture
streaming in ahead of the tail end of associated cold fronts.
Coastal areas of the Mid-Atlantic and Northeast including the
Interstate 95 corridor should also see some modest to perhaps
locally heavy rain Wednesday-Thursday. This did not seem to
necessitate any ERO areas as lack of instability should put a cap
on heavy totals, and ongoing drought will mean most areas are not
particularly sensitive to flooding. Cold air aloft will be
sufficient to support accumulating early season snow for the higher
terrain near the Great Lakes and interior Northeast, and
especially for the central Appalachians where some totals in excess
of six inches are a good possibility. There may also be some lake
effect snow in the wake of this storm system, although temperatures
will likely be marginal and this could mix with rain at times.

In terms of temperatures, warm and slightly humid conditions can
be expected from the Mid-Atlantic southward to Florida on Wednesday
ahead of the approaching cold front that will bring a return to
much cooler conditions to close out the work week. Highs should
generally be about 5-10 degrees below average across the interior
Eastern U.S. by Friday, mainly because of the cold upper low
overhead keeping widespread cloud cover in place, and overnight
lows closer to average. A gradual warming trend is likely for the
Intermountain West and the Rockies as the ridge builds in after
the cold conditions expected in the first half of the week.


Tate/Hamrick

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EST Sun Nov 17 2024


Confidence remains high for a potentially major and prolonged
atmospheric river event setting up across portions of northwestern
California and into southwestern Oregon during the Wednesday
through Friday time period. Enhanced moisture flux ahead of a
meandering front and at least two low pressure areas will result in
enhanced orographic lift and likely produce some 4 to locally 8
inch rainfall totals during this time. Slight Risks remain in place
for both Days 4/Wednesday and 5/Thursday in the ERO. Burn scars
will be of particular concern for flooding. By Thursday/Day 5, snow
levels are forecast to rise with an influx of warm air, so
extended Thursday's Slight Risk farther inland/into higher
elevations where rain on snow may exacerbate flooding concerns. The
long duration of the AR stalling could make this event be on the
higher end of the Slight Risk range, with multiple days of rainfall
compounding flooding impacts. Additionally, at least marine areas
should see strong winds through Wednesday given the deep eastern
Pacific low. The AR should finally start to move and weaken into
late week and next weekend, bringing precipitation farther south
while snow levels lower into the Cascades/Sierra Nevada, and
farther east with some chance for snow to extend into the northern
High Plains.

Impactful weather is also expected for a large portion of the
eastern U.S. for the latter half of the work week. This will be
the result of a large upper low closing off over the Midwest and
slowly making its way across the Great Lakes/Ohio Valley and then
Northeast as the week progresses. Surface low pressure will track
through the Great Lakes region and give way to surface cyclogenesis
near the Mid-Atlantic and southern New England in a Miller B type
evolution. Some strong winds are likely in the northern Plains
Wednesday behind the first surface low, with breezy to windy
conditions broadly from the Midwest to East in this pattern.
Precipitation is also likely with these features. Some locally
heavy rain is possible in Florida for midweek with moisture
streaming in ahead of the tail end of associated cold fronts.
Coastal areas of the Mid-Atlantic and Northeast including the
Interstate 95 corridor should also see some modest to perhaps
locally heavy rain Wednesday-Thursday. This did not seem to
necessitate any ERO areas as lack of instability should put a cap
on heavy totals, and ongoing drought will mean most areas are not
particularly sensitive to flooding. Cold air aloft will be
sufficient to support accumulating early season snow for the higher
terrain near the Great Lakes and interior Northeast, and
especially for the central Appalachians where some totals in excess
of six inches are a good possibility. There may also be some lake
effect snow in the wake of this storm system, although temperatures
will likely be marginal and this could mix with rain at times.

In terms of temperatures, warm and slightly humid conditions can
be expected from the Mid-Atlantic southward to Florida on Wednesday
ahead of the approaching cold front that will bring a return to
much cooler conditions to close out the work week. Highs should
generally be about 5-10 degrees below average across the interior
Eastern U.S. by Friday, mainly because of the cold upper low
overhead keeping widespread cloud cover in place, and overnight
lows closer to average. A gradual warming trend is likely for the
Intermountain West and the Rockies as the ridge builds in after
the cold conditions expected in the first half of the week.


Tate/Hamrick

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Sun Nov 17 2024

Valid 00Z Mon Nov 18 2024 - 00Z Thu Nov 21 2024

...Western U.S....
Days 1-3...

Active winter pattern for the Northwest at least through the next
week.

A leading wave over the PacNW coast this afternoon will track east
over the northern Rockies through Monday before rapidly developing
into a deep low over North Dakota on Tuesday. A deep low currently
over the Gulf of Alaska is rounded by a powerful shortwave Monday
night allowing for particularly rapid development to a surface low
well off Vancouver Island on Tuesday. This low then stalls/becomes
a gyre off the PacNW coast through at least Friday which will
result in prolonged moisture-potent onshore flow into the PacNW
starting later Tuesday.

The cold front from the leading wave currently crossing the
Cascades crosses the northern Rockies tonight with low levels
plummeting by this evening to 2,000ft in the WA Cascades and
2,500ft in the OR Cascades. These snow levels then gradually drop
an additional 500ft through Monday with moderate precip rates
persisting from continued onshore flow.

Snow levels fall over the northern Rockies from 5000-7000ft now to
2000-2500ft overnight. The unusually cold air-mass is in place at
the same time as persistent onshore flow, courtesy of the longwave
trough to the north, strikes mountain ranges such as the Cascades
at an orthogonal angle to maximize the snowfall potential through
Monday. Day 1 PWPF for >6" are categorical for the Cascades above
about 3000ft and the northern Rockies from the Tetons to the
Bitterroots above about 4000ft.

Following a brief lull in minor ridging Monday night, a rapidly
strengthening storm system (the 12Z GFS/ECMWF both have a sub 950mb
low by 21Z Tuesday!) will direct a stronger atmospheric river at
the Pacific Northwest with not only exceptional moisture advection
(>750 kg/m/s IVT on NAEFS would be above the 97.5 climatological
percentile Tuesday afternoon), but intense winds along the
mountainous terrain. Warm air advection will raise snow levels
to above 2,500ft in the WA Cascades, around 4,000ft in the OR
Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern is for a heavier/wet snow type to
rapidly accumulate, while powerful wind gusts potential lead to
tree damage and power outages Tuesday night and Wednesday.

All told, there is no shortage of snow in the forecast for the
Pacific Northwest and Northern Rockies. Snow is likely to be
measured in feet along the Cascades above 3,000ft, the
Salmon/Shasta/Siskiyou of northern California, and well inland
through the Blue, Sawtooth, Boise, Bitterroot, Tetons, and Lewis
Mountain Ranges. The latest WSSI shows Moderate to Major Impacts in
the Cascades and Bitterroots with impacts including not just
dangerous travel conditions (including notable Cascade passes), but
very heavy/wet snow and strong winds by Tuesday night that could
result in tree damage and power outages in some areas.

As of now Day 3 PWPF for >6" are categorical in the Shasta/Siskiyou
of CA (which is the center of the prolonged moisture plume, but
where topographical lift should keep snow levels from rising too
far above 4000ft) and the WA Cascades where a burst of snow is
expected from the arrival of the occluded low.


...Northern Plains...
Days 2/3...

A powerful late November cyclone will track from the Southern
Plains tonight to Minnesota Monday night. The 12Z ECMWF is not as
far east as previous runs and a bit more in line with the 12Z GFS
in the track north over MN. This exceptionally strong storm system
tracking over Minnesota coupled with high pressure building in
over the Northern Rockies will make for a very windy setup over the
Northern Plains. Snow is expected to fall across much of North
Dakota and winds of 30-40 mph would result in whiteout conditions
for areas like central and eastern ND would experience their first
round of accumulating snowfall this season. The Day 2.5 PWPF for
>6" is 20-70% over much of ND west of the Red River Basin and north
of I-94.


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


Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EST Sun Nov 17 2024

Valid 12Z Wed Nov 20 2024 - 12Z Sun Nov 24 2024

...Strong, long-duration atmospheric river will take aim at
northern California/southwestern Oregon for midweek into late
week...

...Pattern change for east-central to eastern U.S. with cooler
temperatures and cold rain and high elevation snow...


...Overview...

A long duration atmospheric river is forecast to affect the West
Coast this week as upper and surface lows/frontal systems direct a
strong moisture plume into northern California/southwestern Oregon
in particular, where several inches of rain and mountain snow are
likely. Meanwhile a deep upper low developing across the Great
Lakes and Northeast U.S will meander slowly in place later this
week with a blocking high to its north across central Canada. This
will result in cooler temperatures, a cold rain from the Ohio
Valley to the East Coast, and early season accumulating snow for
the central Appalachians and the higher terrain of the interior
Northeast. Between these two regions, an upper ridge is likely to
build across the Rockies and eventually across the Plains by next
weekend, warming up temperatures in the interior West and southern
Plains.


...Guidance/Predictability Assessment...

Model guidance begins the medium range period in fairly good
agreement for important features, including a deep surface low
somewhere in the 940s to 960s in the eastern Pacific offshore of
Vancouver Island Wednesday, with its cold front directing the AR
into the West Coast. Models show this low gradually weakening as it
moves back northwest into Thursday, giving way to another low with
more spread in depth (weaker than the first one though) and
placement approaching the Pacific Northwest Friday into the
weekend. The newer 12Z guidance suite seems to be coming in with
better agreement for this secondary low fortunately. A model blend
seemed reasonable initially, but there was increasing spread by
late week into the weekend. Namely, the 00Z CMC was farther
offshore with an upper low and directed the AR farther north into
the Pacific Northwest given its mean flow from the southwest rather
than west. Meanwhile, the 06Z GFS brought in a notable shortwave
into the Northwest Saturday and tracking into the north-central
U.S. by next Sunday. This was dismissed as an outlier; however, the
newer 12Z GFS held on to an evolution like this and the 12Z EC is
not too off either, so this feature will continue to be monitored
and may necessitate changes in future forecasts.

Farther east, models are reasonably agreeable with the upper and
surface lows moving across the Midwest to East. The 00Z GFS became
the main outlier by late week as its lows were displaced to the
southeast of consensus. Otherwise a model/mean blend seemed
reasonable.

The WPC forecast began with a blend of the 00Z ECMWF/CMC/UKMET and
06Z GFS. Decreased the proportion of the deterministic runs
(especially those other than the ECMWF) with time as model spread
increased especially in the West, in favor of the GEFS and EC
ensemble means.


...Weather/Hazards Highlights...

Confidence remains high for a potentially major and prolonged
atmospheric river event setting up across portions of northwestern
California and into southwestern Oregon during the Wednesday
through Friday time period. Enhanced moisture flux ahead of a
meandering front and at least two low pressure areas will result in
enhanced orographic lift and likely produce some 4 to locally 8
inch rainfall totals during this time. Slight Risks remain in place
for both Days 4/Wednesday and 5/Thursday in the ERO. Burn scars
will be of particular concern for flooding. By Thursday/Day 5, snow
levels are forecast to rise with an influx of warm air, so
extended Thursday's Slight Risk farther inland/into higher
elevations where rain on snow may exacerbate flooding concerns. The
long duration of the AR stalling could make this event be on the
higher end of the Slight Risk range, with multiple days of rainfall
compounding flooding impacts. Additionally, at least marine areas
should see strong winds through Wednesday given the deep eastern
Pacific low. The AR should finally start to move and weaken into
late week and next weekend, bringing precipitation farther south
while snow levels lower into the Cascades/Sierra Nevada, and
farther east with some chance for snow to extend into the northern
High Plains.

Impactful weather is also expected for a large portion of the
eastern U.S. for the latter half of the work week. This will be
the result of a large upper low closing off over the Midwest and
slowly making its way across the Great Lakes/Ohio Valley and then
Northeast as the week progresses. Surface low pressure will track
through the Great Lakes region and give way to surface cyclogenesis
near the Mid-Atlantic and southern New England in a Miller B type
evolution. Some strong winds are likely in the northern Plains
Wednesday behind the first surface low, with breezy to windy
conditions broadly from the Midwest to East in this pattern.
Precipitation is also likely with these features. Some locally
heavy rain is possible in Florida for midweek with moisture
streaming in ahead of the tail end of associated cold fronts.
Coastal areas of the Mid-Atlantic and Northeast including the
Interstate 95 corridor should also see some modest to perhaps
locally heavy rain Wednesday-Thursday. This did not seem to
necessitate any ERO areas as lack of instability should put a cap
on heavy totals, and ongoing drought will mean most areas are not
particularly sensitive to flooding. Cold air aloft will be
sufficient to support accumulating early season snow for the higher
terrain near the Great Lakes and interior Northeast, and
especially for the central Appalachians where some totals in excess
of six inches are a good possibility. There may also be some lake
effect snow in the wake of this storm system, although temperatures
will likely be marginal and this could mix with rain at times.

In terms of temperatures, warm and slightly humid conditions can
be expected from the Mid-Atlantic southward to Florida on Wednesday
ahead of the approaching cold front that will bring a return to
much cooler conditions to close out the work week. Highs should
generally be about 5-10 degrees below average across the interior
Eastern U.S. by Friday, mainly because of the cold upper low
overhead keeping widespread cloud cover in place, and overnight
lows closer to average. A gradual warming trend is likely for the
Intermountain West and the Rockies as the ridge builds in after
the cold conditions expected in the first half of the week.


Tate/Hamrick


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw