594
FOUS30 KWBC 252021
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 PM EDT Wed Sep 25 2024

Day 1
Valid 16Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHEAST GEORGIA THROUGH FAR WESTERN SOUTH CAROLINA AND SOUTHWEST
NORTH CAROLINA...

..LIFE-THREATENING FLASH FLOODING INCREASINGLY LIKELY OVER THE
SOUTHERN APPALACHIANS..

Deep layer south to south-southwest flow will allow for the
advection of increasingly moist, unstable air to funnel poleward
ahead of Helene thanks to a closed upper low/trough centered over
the Mid-Mississippi Valley. Afternoon destabilization within the
budding warm sector along with sufficient left-exit region (LER)
jet dynamics from the trough will translate to more widespread
convective coverage across the Southeastern U.S into the
Central/Southern Appalachians as we move into the late-afternoon
and evening time frame. Areas across the Escarpment located over
far Northeast GA into the Western SC/NC border have seen 2" to as
much as 5" of rainfall in the last 24-36 hrs leading to a priming
of the top soil layer as indicated by the latest NASA SPoRT soil
moisture percentiles approaching 60-70% over the areas affected.
Recent CAMs and ensemble output have increased the QPF signal
across the Southeast with the axis of heaviest rainfall bisecting
Eastern AL up through Western GA into North GA and the Southern
Apps/Smokies bordering TN/SC/NC. HREF blended mean QPF indicates
widespread 3-6" of rainfall from this predecessor rainfall event
(PRE) that correlates with the strong upper forcing and deep
tropical connection to north of Helene as it wanders into the
Eastern Gulf. Probability fields are also very aggressive with the
signature for enhanced rainfall rates and totals with the EAS
signal for 3" settling at 50-80% within the Escarpment area into
Northeast GA. Neighborhood probabilities of >5" between 70-90+% and
>8" between 20-35% are more than sufficient for a very impactful
event expecting to unfold across the complex terrain in-of the
Southern Appalachians. Given the signals expressed via the hi-res
ensemble, along with ample support from the various ML output for
the heaviest rainfall, as well as coordination with the local
Greenville-Spartanburg WFO, a targeted High Risk was added for the
D1 period.

The MDT risk was expanded a bit further southwest and west based on
the latest trends with the axis of heavier QPF a touch further
west within the 12z deterministic and adjusted HREF mean QPF
output. This is likely due to the deeper trough axis to the west
and slower progression of the overall upper pattern placing the
best LER jet dynamics a bit more in the interior than what was
progged the last 24 hrs. The MDT still encompasses much of Central
and Northern GA, but does now expand down into a small area of
Eastern AL and Eastern TN along the Great Smoky Mountains. SLGT
risks still align within the Apalachicola region in the Panhandle,
as well as the Western coast of FL as the outer bands of now
Hurricane Helene begin approaching the area(s) overnight with
locally heavy rainfall (>3"/hr) in any banding structures.

Kleebauer

Day 2
Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE
SOUTHERN APPALACHIANS AND NORTHEAST GEORGIA...

..LIFE-THREATENING FLASH FLOODING LIKELY OVER THE SOUTHERN
APPALACHIANS AND PORTIONS OF THE SOUTHEAST..

The extensive period of heavy rainfall during the D1 period will
continue through D2 across portions of the interior Southeast U.S
with a primary focus across the Southern Appalachians where the
back end of our predecessor rainfall event (PRE) will still be
occurring. Further to the south, all eyes turn to Hurricane Helene
as the storm continues to move quickly through the Eastern Gulf
within the weakness created by the trough to the north. The
northern extent of Helene`s tropical moisture will begin nosing
into the Gulf coast with PWAT anomalies steadily rising from 1-2
deviations above normal to as high as 4 deviations above normal by
the time it makes landfall on Thursday evening. Despite a fast
forward propagation, copious amounts of rainfall will impact the
Florida Panhandle over to the Big Bend with the feeder bands on the
eastern flank of the cyclone impacting the coastal areas of the
Florida Peninsula. Rainfall amounts between Panama City to
Tallahassee will reach between 8-12" with local maxima of up to 15"
possible within the axis of Helene`s landfall as the storm moves
almost due north, perpendicular to the coastal plain. For this
reason, a High Risk was added to portions of the Florida
Panhandle/Big Bend, including the Tallahassee area, as well as
across Apalachicola.

By Thursday night into Friday morning, Helene will continue making
steady progress northward with a large rain shield capable of
dropping 3-6" of rainfall within the core of the cyclone in short
periods of time. A swath of 6+ inch totals will progress northward,
eventually cascading into the Atlanta metro and surrounding
locales generating a higher threat of flash flood potential within
the urban corridor. Areas in the path of Helene will undoubtedly
see significant flood potential with small rivers and streams
likely becoming inundated very quickly to exacerbate the flood
threat within the rural areas of Georgia. The recent HREF blended
mean QPF footprint is robust for the period with a large areal
extent of 5+ inches with the secondary maxima positioned across
Northeast GA where the next round of significant rains will
approach with Helene`s core making an approach.

The situation across the Southern Appalachians is becoming
increasingly severe with regards to the ongoing flash flood threats
from the PRE and eventually with the approach of Helene`s core
as the cyclone moves up through Western GA. This will place the
eastern quadrant of the cyclone across Central and Northern GA with
a strong, prevailing east southeast flow expected to bank against
the terrain situated over North GA through the Western Carolinas, a
mechanism for maximizing ascent within a deep tropical moisture
plume that will enhance rainfall rates for several hours moving
through late Thursday night into Friday morning. The 12z HREF mean
QPF output continued to signal for prospects of 3-6+ inches within
a short time frame as the core of higher PWATs advect into the
region and douse the Southern Appalachians in another round of
significant rainfall with rates generally steady between 1-2"/hr
with some small probabilities (10-15%) for hourly rates pushing
close to 3"/hr in the stronger convective cores embedded in the
precip shield. The HREF neighborhood probability for >5" has peaked
at 90+% for the Southern Appalachian front, a signal reserved for
only the most impactful events, correlating well with the EAS
signal of >90% as well for at least 3". The combination of multiple
periods of heavy rainfall with elevated rates likely thanks to
Helene will produce significant, life- threatening flash flooding
across the area over Northeastern GA through the Blue Ridge of SC
and NC, including areas in and around Asheville and to the west of
Greenville- Spartanburg in Upstate SC. Several landslides due to
the prolific rainfall are very likely with a history of these types
of events producing catastrophic damage from both flooding and the
prospects of land and mudslides within the complex terrain. For
these purposes, and in coordination with the GSP and FFC WFO`s, the
High Risk was maintained and expanded further southwest to just
northeast of the Atlanta metro to account for the most significant
signals for flash flood prospects during the period.

Considering the scope of Helene`s size, the flash flood threat will
not be relegated to a small corridor like we see in some tropical
events. The trough to the west will play a pivotal role in the
steering mechanism for Helene as the storm approaches the latitude
of North GA, eventually pulling the cyclone more northwest towards
the back end of the forecast period. Due to the progression, there
is a growing consensus within the deterministic of the heavy rain
threat also shifting into Northern AL with the area in and around
Huntsville to the TN state line becoming more favorable for flash
flooding potential within the western flank of the cyclone. The
signal is not as prolific as the Southern Apps or where Helene
makes landfall, but the probability and QPF fields certainly
depict a significant amount of rain (3-5") within a time frame
generally between 4-8 hrs prior to the end of the forecast period.
This swath will extend across Eastern AL and Western GA up through
Northern AL, following the western periphery of the cyclone as it
moves north then northwest. The heavy rain will expand into
Southern TN before the end of the forecast period with more rain
anticipated in the Tennessee Valley after D2. Because of the
growing consensus in the heavy rainfall across the above areas, the
MDT risk was expanded further west and northwest into Alabama and
over the southern fringes of TN.

A large SLGT and MRGL risk will encompass a large area of the
Southeast and Southern Mid Atlantic as well as the outer bands of
Helene are forecast to impact the region with locally heavy
rainfall possible as is customary with these types of tropical
evolutions.

Kleebauer


Day 3
Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024

...INTRODUCED A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF THE UPSLOPE REGION INTO THE SOUTHERN APPALACHIANS...

20Z Update: The recent forecast for Helene continues to signal the
cyclone weakening rapidly but maintaining an expansive QPF
footprint as it migrates northwestward into the Tennessee Valley as
it becomes captured within the 500mb trough. The storm will
eventually slow begin meandering around Western TN with a
continued heavy rain threat circumnavigating around Helene`s
general circulation. A SLGT risk still exists further west into the
Mid and Lower Mississippi Valley, as well as portions of the
Southern Ohio Valley, but the prospects for a higher risk are
slowly increasing given the recent ensemble QPF trends. With the
threat still some time away and a lack of any short range guidance
which can narrow down some of the banding/convective details, the
wording will remain a higher-end SLGT risk with a possibility of an
upgrade in future updates.

To the east, the heaviest rainfall will be coming to an end, but
still a prevailing easterly flow banked against the terrain in
Western NC will allow for a continued threat of locally heavy rains
on top of what will fall the prior period. The previous MDT was
maintained given the consistent signal for ongoing rains will occur
through early Friday afternoon before coming to an end, outside
some scattered shower threats through the evening. To outlined the
continued threat of rainfall, did not want to deviate from the
previous forecast and kept the previous MDT with a small expansion
to the south over Upstate SC where some guidance lingers the
rainfall through Friday morning where FFGs will likely be at their
lowest points.

Kleebauer

..Previous Discussion..

Based on latest NHC track guidance...Helene should be in northern
Georgia and tracking on a more northwestward course during the day.
This suggests the flow of deep moisture should be on-going to the
terrain of western Carolinas into southwest Virginia. Latest model
guidance shows several inches of rainfall in addition to what falls
in the predecessor 48 hours...felt a focused Moderate risk placed
where there was best model consensus was warranted. It was also a
multi-day rainfall total farther south along the NHC track in the
eastern Tennessee Valley that was the main reason to keep a Slight
Risk where model QPF be a little too light to support one. That
portion of the Slight Risk area extends westward to the deep-
layered low where steepening low level lapse rate and the
additional moisture streaming off of Helene could lead to some
showers and thunderstorms capable of producing locally heavy
downpours.

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