312
FOUS30 KWBC 270115
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
915 PM EDT Thu Sep 26 2024

Day 1
Valid 01Z Fri Sep 27 2024 - 12Z Fri Sep 27 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FROM THE FLORIDA
PANHANDLE THROUGH THE NORTH CAROLINA BLUE RIDGE...

...WIDESPREAD AND CATASTROPHIC FLASH FLOODING LIKELY...

Widespread flash flooding will continue through the overnight
hours from the FL Panhandle into the western Carolinas. A broad
swath of moisture ahead of Helene continues to interact with the
front and upper low over the region resulting in a large area of
moderate to locally heavy rainfall.

The most significant rainfall impacts from Helene will likely be
across portions of the southern Appalachians from northeast GA
into northwest SC and western NC. This area has already picked up
significant rainfall from this event, with 8-12"+ totals reported.
Additional rainfall through the overnight hours will continue to
add to these totals. Ground conditions are saturated and this
additional rainfall will continue to result in areas of significant
flash flooding, with landslides becoming increasingly likely with
time. Unfortunately rainfall rates will increase across this area
late tonight into Friday morning as the center of Helene approaches
the area and the worst flooding conditions will probably be
between ~09z-15z over this region. What is left of the inner core
of Helene will push through the region and will likely still have
some convective elements within this core. On top of that,
increased easterly upslope flow will enhance rainfall efficiency.
Thus it seems likely that we will see rainfall totals upwards of
1-2" in an hour during this time frame late tonight into Friday
morning. This increase in rainfall rates over top saturated ground
conditions will likely result in catastrophic and life-threatening
flash flooding late tonight into Friday morning over these southern
Appalachian areas. Numerous significant landslides and fast moving
debris flows are possible. Additional rainfall totals from now
until the event winds down later Friday morning are expected to be
4 to 8", with up to 12" in the most upslope favored terrain. This
is in addition to the rain that has already fallen.

Over the FL Big Bend region to southern GA the heaviest rainfall
rates are expected through 06z as the core of Helene quickly moves
north northeastward across the area. Hourly rainfall as high as
2-3" is expected within this inner core of Helene as it moves
northward. The quick motion will limit the upper bound of
additional rainfall, but the 3-5" of rain it will produce will
likely cause some additional flash flooding given saturated ground
conditions over portions of this area.

Across central GA into western SC we will continue to see rain
bands pushing well ahead of Helene. While most of this rain is
peaking around 0.5"/hr, we will continue to see localized amounts
exceeding 1" within an hour. What is left of the inner core of
Helene is then expected to move across portions of central GA and
far western SC late tonight into early Friday. Even though Helene
will be weakening buy this time, still expecting 1-2" per hour
rainfall for a couple hours near and just east of the center.
Recent HRRR runs and radar trends suggest a slightly further east
track of this inner core rainfall, which may allow for this more
intense rain to move over portions of eastern GA and far western SC
that picked up 5-10" of rain earlier in the event. This will
likely result in significant flash flood concerns, and thus the
High risk was expanded a bit to the east to account for this.

North GA will continue to see embedded heavier rainfall rates and
a flash flood risk well ahead of Helene. Even if Helene tracks a
bit further east over GA there is an indication in the guidance
that a second corridor of enhanced rainfall could develop west of
the center where the system is interacting with the stalled front
enhancing convergence. This 2nd relative max in rainfall should
setup over portions of north to northwest GA into adjacent areas of
TN and northeast AL. An uptick in rainfall rates late tonight and
early Friday with this feature should result in an uptick in flash
flood coverage and severity.

Chenard


Day 2
Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR THE BLUE RIDGE OF
NORTH CAROLINA...

...CATASTROPHIC FLASH FLOODING WILL LIKELY BE ONGOING FRIDAY
MORNING...

...2030Z Update...

...Southern Appalachians...

Some of the heaviest rainfall in the Southern Appalachians is
expected to be impacting the Blue Ridge of North Carolina Friday
morning associated with the dissipating circulation center of
Helene as its forward speed slows. With likely well over 12 inches
of rain having fallen in this area prior to the start of the period
at 12Z/8am Friday, this quick hit of as much as 6 inches of
additional rain in 6 hours will likely push any flooding rivers,
streams and creeks into major/catastrophic flooding stage.

It`s important to note that the biggest component to the High Risk
in this area is the rainfall that will have already been falling
for 24 hours or more prior to the start of the Day 2 period, and
the rainfall expected Friday morning will be the "last punch" in
the round associated with the passage of the primary circulation
center. By this time Friday afternoon the rain will be long (by a
few hours, anyway) over, so the High Risk is exclusively for Friday
morning with the last of Helene`s rainfall...albeit again some of
the heaviest in terms of intensity. Once the circulation center
passes (and largely dissipates as it moves into the mountains), all
of the significant rainfall associated with Helene will finally be
over in this region...so less than 24 hours to go.

Into the Virginias, the rain will persist a bit longer as the broad
rain shield with Helene pivots northward. Southwest Virginia and
southern West Virginia are particularly flood sensitive, especially
following the rain seen in this area over at least the past 24
hours. Thus, despite slightly lower amounts of total rain expected,
numerous instances of flash flooding are expected. Thus, the
Moderate Risk was expanded well to the north.

...Ohio, Tennessee, & Mid-Mississippi Valleys...

The westward portion of Helene`s circulation will interact with a
strong upper level low whose nearly stationary movement has been
partially responsible for the northward surge of tropical moisture
associated with the PRE as well as Helene`s northward movement.
Once the circulation center nears Atlanta Friday morning, it will
turn more northwestward into central Tennessee, then stall out
there for an extended period of time, maybe even 2 days. On Friday
afternoon, the moisture plume that Helene is embedded within will
bifurcate at the southern Appalachians. A healthy amount of that
moisture will turn westward into the Ohio, Tennessee, and Mid-
Mississippi Valleys. This will support an extended period of heavy
rain across Kentucky, Tennessee, and their neighbors to the north
and west. The heaviest rain will be associated with the initial
turn towards the west and then south, resulting in a swath of 2-4
inches of rain from Louisville, KY south and west through Paducah,
KY and as far south as Memphis. This area from Louisville to
Memphis is highlighted in a higher-end Slight and consideration was
made for a Moderate Risk upgrade.

The Moderate Risk was forgone at this point primarily due to the
antecedent dry conditions that have impacted this area for an
extended period. Thus, at least the first half of this rainfall
event will be largely beneficial rain. Pockets of heavier rainfall
are certainly possible, and should they occur over urban or flood
prone areas then flash flooding is likely to develop, but at this
point given the rainfall deficit that Helene`s moisture will have
to make up, the area remains in a high-end Slight.

Helene`s circulation will become absorbed into the upper low, which
will potent will itself be weakening through Saturday. With much of
the rainfall on the side of the low with northerly flow, the
circulation will both be effectively cut off from the Gulf, and
ingesting drier air from the interior of North America. This will
result in steadily declining rainfall rates into Friday night. This
will be offset by the slow movement and looping back on itself that
the broader circulation center will be doing over central
Tennessee, keeping the heaviest rains all confined to one area.
Depending on how much rain can occur tonight, a Moderate Risk may
still be needed with future updates.

Wegman

...Previous Discussion...

The flow of moisture laden air into the east face of the
Appalachians should be on-going given the latest NHC guidance. With
an additional 2 to 3+ inches of rain to overlap with the footprint
of 12 to 15 inches of rainfall in the previous 48
hours...maintained a Moderate risk even though those amounts
typically do not trigger a Moderate risk, As the center of the
system continues into the Tennessee Valley...additional rainfall
amounts to the east of the mountains should be tapering off.

Farther west...convection should start to become better rainfall
producers over portions of the Mississippi and Ohio/Tennessee
Valleys as the deeper moisture encounters steepening low- and mid-
level lapse rates near the center of the deep-layered
cyclone...which has started to work in tandem with what had been
Helene. Only minor adjustments to the previously-issued Slight risk
area were needed based on the latest guidance.

Bann

Day 3
Valid 12Z Sat Sep 28 2024 - 12Z Sun Sep 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MIDDLE TO LOWER OHIO RIVER VALLEY...

...2030Z Update...

Lingering rain associated with the combined upper low and Helene
will continue mainly across the Ohio Valley into Saturday morning.
Considering that starting late tonight and throughout all of Friday
and Friday night this portion of the Ohio Valley will be getting
light to occasionally moderate rain, by Saturday the soils in this
region should be plenty saturated with additional rainfall being
converted almost entirely into runoff. Thus, any flooding impacts
may be realized to their greatest extent along this portion of the
Ohio River on Saturday. Given those antecedent conditions and
potential urban impacts...a Slight Risk was introduced with this
update. The heaviest rain during the period will be during the
daylight hours Saturday, as by Saturday night the rain will have
diminished to light nuisance activity.

Wegman

...Previous Discussion...

Rainfall from the Mississippi Valley/Tennessee Valley will begin to
spread eastward with considerably less moisture to work with than
before. As a result...expected rainfall amounts do not appear to be
enough to result in significant problems. In addition...the track
over during the latter part of the day should be north of areas
soaked in the Days 1 and 2 period. Made some adjustments to the
orientation from the inherited ERO but the overall forecast
rationale has changed little.

Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt