Atmospheric Rivers

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Fri Sep 27 2024
Valid 12Z Fri Sep 27 2024 - 12Z Sun Sep 29 2024

...HELENE is forecast to continue to move inland to the northwestward to
the Ohio Valley by Saturday while slowly weakening...

...There is a High Risk of excessive rainfall over parts of the Southern
Appalachians on Friday and a Slight Risk over parts of the Ohio Valley on
Saturday...

...There is an Enhanced Risk of severe thunderstorms over parts of the
southern Mid-Atlantic on Friday...

...There are Excessive Heat Warnings over parts of Southeastern California
and the Southwest...

HELENE is forecast to move inland, heading northwestward to the Ohio
Valley, and slowly weaken by Saturday. Showers and thunderstorms with
heavy rain will develop over parts of the Southern Appalachians and
southern Mid-Atlantic. Therefore, the WPC has issued a High Risk (level
4/4) of excessive rainfall over the Southern Appalachians through Saturday
morning.  Severe, widespread flash flooding is expected. In addition,
areas that normally do not experience flash flooding will flood. Lives and
property are in great danger.

Moreover, showers and severe thunderstorms will develop over parts of the
southern Mid-Atlantic. Therefore, the SPC has issued an Enhanced Risk
(level 3/5) of severe thunderstorms over the southern Mid-Atlantic through
Saturday morning.  The hazards associated with these thunderstorms are
frequent lightning, a few tornadoes, and a minimal threat of severe
thunderstorm wind gusts and hail. Also, rain will develop over parts of
the Ohio/Tennessee Valleys on Friday.

On Saturday, the excessive rainfall threat from HELENE lessens. However,
heavy rain will develop over parts of the Ohio Valley. Therefore, the WPC
has issued a Slight Risk (level 2/4) of excessive rainfall over parts of
the Ohio Valley from Saturday through Sunday 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.

Further, the threat of severe thunderstorms ends on Saturday. Yet showers
and thunderstorms will develop over the Florida Peninsula. Additionally,
rain will continue over the Ohio/Tennessee Valleys and parts of the
Mid-Atlantic through Sunday morning.

Meanwhile, a front will move onshore over the Pacific Northwest and inland
to the Northern Plains. The system will produce rain over parts of the
Northwest ending by late Friday afternoon.

In addition, an upper-level high will remain over the Southwest through
Sunday. High temperatures will range from the upper 90s to 110s, and low
temperatures in the mid to upper 80s will provide little relief from the
heat overnight. The temperatures have prompted Excessive Heat Warnings
over parts of Southeastern California and the Southwest. 

alid 12Z Fri Sep 27 2024 - 12Z Sun Sep 29 2024 ...HELENE is forecast to continue to move inland to the northwestward to the Ohio Valley by Saturday while slowly weakening... ...There is a High Risk of excessive rainfall over parts of the Southern Appalachians on Friday and a Slight Risk over parts of the Ohio Valley on Saturday... ...There is an Enhanced Risk of severe thunderstorms over parts of the southern Mid-Atlantic on Friday... �There are Excessive Heat Warnings over parts of Southeastern California and the Southwest� HELENE is forecast to move inland, heading northwestward to the Ohio Valley, and slowly weaken by Saturday. Showers and thunderstorms with heavy rain will develop over parts of the Southern Appalachians and southern Mid-Atlantic. Therefore, the WPC has issued a High Risk (level 4/4) of excessive rainfall over the Southern Appalachians through Saturday morning. Severe, widespread flash flooding is expected. In addition, areas that normally do not experience flash flooding will flood. Lives and property are in great danger. Moreover, showers and severe thunderstorms will develop over parts of the southern Mid-Atlantic. Therefore, the SPC has issued an Enhanced Risk (level 3/5) of severe thunderstorms over the southern Mid-Atlantic through Saturday morning. The hazards associated with these thunderstorms are frequent lightning, a few tornadoes, and a minimal threat of severe thunderstorm wind gusts and hail. Also, rain will develop over parts of the Ohio/Tennessee Valleys on Friday. On Saturday, the excessive rainfall threat from HELENE lessens. However, heavy rain will develop over parts of the Ohio Valley. Therefore, the WPC has issued a Slight Risk (level 2/4) of excessive rainfall over parts of the Ohio Valley from Saturday through Sunday 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. Further, the threat of severe thunderstorms ends on Saturday. Yet showers and thunderstorms will develop over the Florida Peninsula. Additionally, rain will continue over the Ohio/Tennessee Valleys and parts of the Mid-Atlantic through Sunday morning. Meanwhile, a front will move onshore over the Pacific Northwest and inland to the Northern Plains. The system will produce rain over parts of the Northwest ending by late Friday afternoon. In addition, an upper-level high will remain over the Southwest through Sunday. High temperatures will range from the upper 90s to 110s, and low temperatures in the mid to upper 80s will provide little relief from the heat overnight. The temperatures have prompted Excessive Heat Warnings over parts of Southeastern California and the Southwest.

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Tue Sep 24 2024
Valid 12Z Tue Sep 24 2024 – 12Z Thu Sep 26 2024

…Potential Tropical Cyclone Nine is forecast to pass not too far from
the Florida Keys Wednesday night as a hurricane before skirting up the
West Coast of Florida by Thursday morning…

…Much needed rainfall from the Ohio Valley into the central to southern
Appalachians, Upper Tennessee Valley and Mid-Atlantic…

…Much above average temperatures continue across the West into the
Northern Plains and across the Gulf Coast into the Southeast…

A relatively benign mid- to upper-level trough that will likely bring
beneficial rainfall for the eastern U.S. is forecast to deepen when
another upper-level trough over the northern Plains drops southeast and
merges with the lead trough. The resulting trough amplification will be
instrumental in pulling the tropical moisture as well as Potential Cyclone
Nine (PTC9) northward into the eastern portion of the Gulf of Mexico by
the end of the short-range forecast period on Thursday morning.

The initial impacts from the lead trough will be in the form of an
expanding area of moderate to heavy rainfall potential from the Midwest
into the lower Great Lakes, Ohio Valley, upper Tennessee Valley, central
to southern Appalachians into the Mid-Atlantic near and ahead of a low
pressure wave. Much of these areas are currently experiencing severe to
exceptional drought conditions, with the expected 1 to 1.5 inch plus
rainfall amount over the next two days bringing some relief to these
drought conditions. With the dry conditions across these areas, river
flooding will be unlikely. However, there is still at least a marginal
risk of isolated flash flooding with the expected heavy rainfall amounts,
especially if they occur over urbanized regions.

As the above-mentioned amplifying pattern develops over the northern
Plains and dips toward the Deep South, PTC9 is forecast to track more
toward the north and intensify rapidly as it moves across the Yucatan
Channel on Wednesday and then pass not too far from the Florida Keys
Wednesday night. The Florida Keys can expect strengthening winds with
more frequent passages of squally downpours Wednesday night as PTC9 is
forecast to pass to the west as a hurricane. Computer models indicate
that the circulation of PTC9 will expand as it interacts with the
deepening upper trough in the Deep South. Given a more robust convective
structure concentrated more on the east side of the storm, the West Coast
of Florida could begin to see more frequent and intense squalls with winds
further strengthening by Thursday morning. Please refer to the National
Hurricane Center for the latest updates on this system.

The amplifying upper ridge across the interior West will be supporting
widespread much above average temperatures over the next few days across
nearly all of the West and into the Northern Plains. High temperatures
across these regions are forecast to be as much as 10 to 20 degrees above
average. Above average temperatures also likely across the Gulf Coast and
into the Southeast. While temperatures are forecast to be much above
across these areas over the next few days, there are not expected to be
many record highs. However, more numerous record high morning
temperatures are possible both today and Wednesday morning along the West
coast and from portions of the southern Mid-Atlantic into the Southeast
and Florida. Across the Pacific Northwest, the cold front associated
with a Pacific cyclone centered over the Gulf of Alaska will only bring
some rainfall into the region by Wednesday.

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Mon Sep 23 2024
Valid 12Z Mon Sep 23 2024 – 12Z Wed Sep 25 2024

…Threat of heavy rain and locally strong to severe thunderstorms will
slowly shift from the southern Plains this morning to the Ohio Valley,
central Appalachians and lower Great Lakes on Tuesday into early
Wednesday…

…Watching the western Caribbean Sea for tropical cyclone formation that
could bring strengthening winds and passing squally downpours into the
Florida Keys Tuesday night into Wednesday morning…

A low pressure consolidating over the southern Plains along a slow-moving
front is forecast to move northeastward through the Mid-Mississippi Valley
by tonight, then across the Midwest and Ohio Valley on Tuesday. Heavy
showers and some severe thunderstorms this morning across the southern
Plains are expected to taper off as today progresses but the threat of
heavy rain and locally strong to severe thunderstorms will increase from
the Mid-Mississippi Valley into the Midwest in the mean time. By Tuesday,
the center of the low will pass through the Midwest toward the lower Great
Lakes, the threat of heavy rain and embedded strong thunderstorms will
then develop well ahead of the low across the Ohio Valley, mainly from
Tuesday afternoon into Tuesday night. By early Wednesday, many areas
across the lower Great Lakes and the interior East Coast will be
enshrouded by showers and embedded thunderstorms. These showers and
storms will extend farther southwest near/along the trailing front across
the interior Gulf Coast states. Showers and periods of rain will also
move across the lower Great Lakes into the central Appalachians and
upstate New York in association with another wave of low pressure near the
front.

The upper-level low behind the slow-moving front will slide across the
central Plains, allowing much of the western U.S. to remain dry along with
a warming trend for the next couple of days. Meanwhile, colder air will
remain entrenched across the Plains behind the front but 90s will be
common in the afternoon across the South and into the interior Southeast
today and Tuesday ahead of the front. The outer edge of a Pacific cyclone
centered over the Gulf of Alaska will only bring some light rain into
northwestern Washington State this morning.

Farther south into the tropics, the National Hurricane Center continues to
watch the western Caribbean Sea for potential tropical cyclone formation.
Computer models now appear to be in good agreement for the system to move
northwestward and reach the Yucatan Channel by the time the short-range
forecast period ends Wednesday morning. This means that the Florida Keys
could experience strengthening winds along with passing squally downpours
from Tuesday night into Wednesday morning. Please refer to the National
Hurricane Center for the latest updates on this system.

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Sun Sep 22 2024
Valid 12Z Sun Sep 22 2024 – 12Z Tue Sep 24 2024

…A low pressure wave will bring showers and possible severe weather
across the central High Plains this morning…

…Rounds of heavy rain and embedded thunderstorms are expected to stretch
from the south-central Plains through the mid-Mississippi Valley, Midwest,
and into the Ohio Valley for the next couple of days…

…Locally heavy rain and strong thunderstorms moving across the interior
Mid-Atlantic this morning…

Much of the active weather for the next couple of days will continue to be
focused in the vicinity of a slow-moving frontal boundary stretching from
the south-central Plains through the mid-Mississippi Valley, Midwest, and
into the Ohio Valley. The strongest thunderstorms are expected to
traverse the Texas and Oklahoma Panhandles early this morning, and then
weaken during the day as they move farther east. Rounds of heavy rain and
embedded thunderstorms can also be found this morning across the central
Plains into the Midwest, followed by addition showers and thunderstorms
forming and moving across Lower Michigan during the day today.

By Monday, a low pressure wave is forecast to organize along the front and
move northeast across the south-central Plains. Showers and embedded
thunderstorms can be expected to accompany the low as well as the frontal
boundary. By Tuesday morning, showers and embedded strong thunderstorms
are expected to move across the Midwest toward the Ohio Valley as the low
pressure center tracks across the region.

Across the interior Mid-Atlantic, a subtle upper-level disturbance is
delivering a round of heavy rain with strong to locally severe
thunderstorms moving from north to south this morning. Meanwhile, recent
wet weather in the vicinity of Cape Cod is lingering into this morning.
As the slow-moving coastal cyclone begins to slide further out into the
Atlantic, the rain is forecast to end during the day today.

The high-elevation of central Colorado is waking up to some wet snow due
to an influx of colder air associated with the upper low. The upper low
will lift into the southern Plains, allowing much of the western U.S. to
remain dry for the next couple of days with near normal temperatures.
Colder air will surge down the Plains behind the front but 90s will be
common in the afternoon across the South into the interior eastern U.S.
ahead of the front.

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Sat Sep 21 2024
Valid 12Z Sat Sep 21 2024 – 12Z Mon Sep 23 2024

…Threat of heavy rainfall and severe weather will be focused across the
central U.S. and into Midwest for the next couple of days…

…A slow-moving coastal storm will bring another rainy day today across
southeastern New England and coastal flooding during high tides for the
northern Mid-Atlantic coast…

…Much below normal temperatures surge into the central High Plains on
Sunday as above average temperatures shift east from the Plains into the
Ohio and Mississippi Valley…

Most of the active weather for the next couple of days will be focused
across the central U.S. and will slowly shift east into the Midwest by
Monday morning. This is in response to a vigorous upper-level low
pressure system that will swing across the Southwest today and then
interact with a surge of cool air down from western Canada. Thunderstorms
are expected to develop quickly later today across the southern High
Plains where severe weather will be possible as a cold front surges south
and clashes with warm and moist air lifted northward by the upper low. By
Saturday night into Sunday, the main activities will then gradually shift
east across the central Plains toward the mid-Mississippi Valley as low
pressure waves form along the front. Sunday into Monday morning will see
the potential of heavy rain shifting farther east into the Midwest and
toward the Ohio Valley. Meanwhile, colder air behind the system will
change the rain into wet snow across the higher elevations of Colorado
Rockies Saturday night into Sunday morning. Showers and embedded
thunderstorms can also be expected farther north near/behind the cold
front across the northern Plains today, into the upper Midwest tonight,
followed by the upper Midwest and the Great Lakes Sunday into Monday
morning.

Temperatures will fall precipitously on the backside of the cold front
over portions of the southern High Plains and central Plains beginning
Saturday. High temperatures in the 50s will represent 15 to 30 degree
departures from average for the aforementioned areas. Above average
temperatures will be in place across the Midwest and Southeast this
weekend as upper-level ridging extending from Mexico into Texas remains in
place. Elsewhere, a slow-moving coastal storm will bring another rainy
day across southeastern New England with coastal flooding during high
tides for the northern Mid-Atlantic coast for today before the storm
slowly moves out into the Atlantic on Sunday.

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Thu Sep 19 2024
Valid 12Z Thu Sep 19 2024 – 12Z Sat Sep 21 2024

…Severe thunderstorms possible in the eastern Plains and Upper Midwest
today…

…Late-summer heat forecast from the southern/central Plains to the Upper
Midwest…

The main weather story for the next couple of days will be a strong
occluded low and frontal system bringing impactful weather to the Plains
and Midwest. The central low will gradually lift north into southern
Canada today while it pushes a strong cold front across the eastern Plains
and Upper Midwest. Precipitation will taper off in Montana and the
northern Plains by this afternoon as the low moves farther away, and the
focus for precipitation will shift to areas ahead of the cold front. A
line of showers and thunderstorms is forecast to develop ahead of the cold
front today, and a wave of upper level energy moving over the Upper
Midwest will provide support for scattered severe thunderstorm development
this afternoon into tonight. The Storm Prediction Center has highlighted
portions of the Upper Midwest and eastern portions of the central and
southern Plains with a Slight Risk of Severe Thunderstorms (level 2/4).
Potential storm hazards will include a couple of tornadoes, large hail,
and damaging wind gusts. Additionally, locally heavy rain in stronger
storms may result in isolated instances of flash flooding in the Upper
Midwest.

Showers and storms ahead of the cold front will push east into the Great
Lakes region on Friday, but the front will weaken as it becomes separated
from its parent low in Canada. Shower and storm chances will also linger
along the eastern seaboard as a low pressure system strengthens offshore
in the western Atlantic. The main low will remain parked southeast of Cape
Cod over the next few days while a slow-moving cold front extends
southwest to the Florida Peninsula. Strong gusty winds will be possible
over the coastal waters in the vicinity of the central low, which has
prompted the issuance of Small Craft and Coastal Flood Advisories along
portions of the Mid-Atlantic and Northeast Coasts and Gale Warnings for
the offshore waters south of Cape Cod and Long Island. This system will
finally pull away from the East Coast by Sunday, which will result in
decreasing winds and precipitation chances.

Calmer weather is expected for the West today, with some lingering showers
and storms under an upper low in the Great Basin and California, then the
next round of unsettled weather will arrive with a southward moving
frontal system Friday and Saturday. Precipitation will spread from the
Northwest and northern Rockies south to the Four Corners Region by
Saturday, and some wintry precipitation will be possible in the higher
elevations of the Intermountain West. Precipitation chances will also
expand again across the Plains and portions of the Midwest late Friday
into Saturday as the frontal system pushes east of the Rockies.

Temperature-wise, late-summer heat will stick around in the Central U.S.
through the end of this week. Warm southerly flow will keep high
temperatures in the 80s and 90s from the southern/central Plains to the
Upper Midwest. Some areas in the southern Plains could see near record
highs today and Friday as highs approach 100 degrees. Above average
temperatures are also forecast for the Great Lakes and interior Northeast
underneath an upper level ridge. Temperatures in the West will remain
below normal over the next few days in the wake of the Plains system and
the upcoming late week frontal system. Temperatures in the East and
Southeast will be near normal with highs generally in the 70s and 80s.

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Wed Sep 18 2024
Valid 12Z Wed Sep 18 2024 – 12Z Fri Sep 20 2024

…Very heavy rainfall and flash flooding possible in Montana today…

…Severe thunderstorms possible in the northern and central Plains and
Upper Midwest today and Thursday…

A strong occluded low will bring hazardous weather to Montana today as it
lingers over the state. Strong upslope flow along the northern Rockies on
the backside of the low will result in showers and thunderstorms with very
heavy rainfall totals. A Slight Risk of Excessive Rainfall (level 2/4) is
in effect for portions of Montana where scattered flash flooding will be
possible. Wintry precipitation will also be possible in the high
elevations of the northern Rockies. In addition to precipitation hazards,
a strong pressure gradient in the vicinity of the central low will result
in strong, gusty winds across the region. High Wind Watches and Warnings
are in effect for portions of Montana and Wyoming where winds are forecast
to be 30-40 mph with gusts up 60 mph. The low will gradually lift north
into southern Canada Thursday and Friday, and rain and winds will begin to
relax.

The strong low pressure system will also push a cold front across the
Plains and Upper Midwest through Friday. A line of showers and
thunderstorms will move east ahead of the cold front, and the environment
will support the development of scattered severe thunderstorms. The Storm
Prediction Center has highlighted portions of the Plains and Upper Midwest
with a Slight Risk of Severe Thunderstorms (level 2/5) today and again on
Thursday. Potential severe storm hazards will include damaging wind gusts,
large hail, and a few tornadoes. Shower and thunderstorm coverage will
expand into the Great Lakes and Mid-Mississippi and Ohio Valleys on Friday
as the cold front continues to trek east.

Elsewhere, two coastal lows will sandwich the continental United States,
one moving south along the West Coast and one moving north along the East
Coast. The West Coast low will bring precipitation to the Northwest this
morning, then to the Great Basin and California later today into Thursday.
Precipitation will fall mainly in the form of showers and thunderstorms,
but some wintry precipitation will be possible in the Sierra Nevada. The
low will weaken on Friday, and precipitation will taper off. The East
Coast low will gradually deepen offshore of the Mid-Atlantic over the next
few days with a slow-moving frontal boundary extending southwest from the
low. Showers and storms will be possible for the Mid-Atlantic, coastal
Northeast, Southeast, and Florida. This system will also produce gusty
winds over the coastal waters, which has prompted Small Craft and Coastal
Flood Advisories along the Mid-Atlantic Coast.

Temperature-wise, the West will experience well below average temperatures
in the wake of the strong frontal system in the Plains, while the Central
U.S. experiences well above average temperatures with southerly flow ahead
of the Plains system. Highs over the next few days will only be in the 60s
and 70s for much of the West, while highs in the Plains and Midwest reach
the 80s and 90s. A few near-record high temperatures will be possible in
the southern Plains where highs will approach 100 degrees. Above average
temperatures will also be observed in the Northeast under an upper level
ridge, and near to slightly below normal temperatures will be observed in
the Mid-Atlantic and Southeast.