Flash Flood Warning 12, NWS Charleston, SC

Area Forecast Discussion
National Weather Service Charleston SC
610 AM EDT Tue Aug 6 2024

.SYNOPSIS...
Tropical Storm Debby will impact the region through much of the
week. Potentially historic rainfall amounts will likely result
in severe to catastrophic flash flooding.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today and Tonight: Weather impacts experienced locally will remain
highly dependent on the track/position of Tropical Storm Debby,
which is anticipated to track very slowly northeast across Southeast
Georgia this morning, before shifting off the Georgia coast this
afternoon, then slowing across the Gulf Stream overnight while
encountering the Atlantic ridge. The latest forecast track for Debby
will continue to place much of the local area in the northeast
quadrant today, with a warm onshore flow and a large plume of
tropical moisture advecting across the area. The environment will
remain primed for heavy rainfall, with modest CAPE and PWATs
remaining between 2.5-2.8 inches supporting bands of showers and/or
thunderstorms wrapping around the northern extent of Debby. Given 5-
10 inches of rainfall occurring overnight and many areas expected to
see additional rains today (some 3-5 inches), flash flooding is
likely as rainbands shift onshore, particularly across Southeast
South Carolina, although additional rains will also attempt to
develop with moisture that wraps around the western edge of Debby as
it begins to shift offshore early. A Flash Flood Watch will continue
for the entire area given the setup (including across Southeast
Georgia counties). See the Hydrology section below for additional
hydrologic details. Heading into the overnight period, some drier
air should begin to wrap into the western side of Debby as the
system shifts offshore, reducing rain/shower coverage and the
potential for flash flooding, especially across inland areas of
Southeast Georgia.

Outside of heavy rainfall concerns, there will continue to be an
isolated tornado risk across parts of Southeast South Carolina as
bands of showers and thunderstorms push onshore from the nearby
Atlantic within a pronounced axis of instability and shear that
shifts north with time and eventually out of the local area. This
environment has already been responsible for producing a few
tornadoes overnight and could continue to support tornadoes this
morning into perhaps early afternoon hours, which is aligned fairly
well with the latest HREF updraft helicity tracks/ensemble
paintball. Currently, a Tornado Watch remains in effect for parts of
Southeast South Carolina until 7 AM. Additional Tornado Watches
could eventually be needed.

A Tropical Storm Warning remains in effect for the Southeast Georgia
and Southeast South Carolina coast, and also includes much of
Southeast Georgia. Refer to the latest Hurricane Local Statement
(HLS product) for the latest on current tropical headlines.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
The latest NHC track indicates the center of Tropical Storm Debby
will be off the Georgia and South Carolina coasts at the start of
the period. Debby is expected to meander offshore and then slowly
track northward and potentially strengthen until making landfall
somewhere along the South Carolina coast on Thursday.

The primary concern will continue to be the rainfall. On top of the
several inches of rain that has fallen already, periods of heavy
rainfall is expected to continue into at least Thursday. Refer to
the hydrology section below for more information.

Based on the current track, the tornado threat Wednesday and beyond
will be much lower. Otherwise, potential for tropical storm force
winds persist and Tropical Storm Warnings remain in effect for all
coastal counties as well as southeast Georgia inland counties.

Still some uncertainty late week, but as we move into Friday, Debby
should be lifting further away from the local area into North
Carolina. If this track holds, heavy rain threat should start coming
to a close.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
Less active weather is expected as we head into the weekend as Debby
pulls away. Should transition back to a more typical summertime
pattern with isolated to scattered diurnal shower/thunderstorm
chances. Temperatures are forecast to be within a few degrees of
normal.

&&

.AVIATION /10Z TUESDAY THROUGH SATURDAY/...
KCHS/KJZI/KSAV: Steady rains will continue to impact all terminals
into Tuesday morning as bands of showers and thunderstorms wrap
around the northeast edge of Tropical Storm Debby drawing slowly
closer to the Georgia coast. Prevailing MVFR/IFR conditions will
persist through the night, with perhaps some improvement in cigs
heading through Tuesday afternoon. Vsbys remain limited to 1/2SM at
times as rain/showers directly impact the terminals, especially at
the JZI and CHS terminals tonight. There is also a risk for low-
level wind shear at all terminals tonight into through much of
Tuesday afternoon.

Extended Aviation Outlook: Prolonged periods of flight restrictions
are expected through much of the week as Debby impacts the region.

&&

.MARINE...
Today and Tonight: Tropical Storm conditions will likely persist for
much of the day and overnight as Debby makes way to the Georgia
coast early afternoon, then shifts offshore and slows across the
Gulf Stream overnight while encountering the Atlantic ridge. This
track continues to place much of the South Carolina coastal waters
in the northeast quadrant of Debby this morning into early
afternoon, producing a risk for waterspouts with bands of showers
and thunderstorms along a pronounced axis of instability and shear.
A Tornado Watch remains in effect for until 7 AM for all South
Carolina waters, including the Charleston Harbor. Additional Tornado
Watches could be needed as the threat is slow to move north of local
waters this morning.

Wednesday through Saturday: The latest NHC forecast indicates
Tropical Storm Debby will be over or near the coastal waters on
Wednesday. Debby should slowly track northward while potentially
strengthening before making landfall somewhere along the South
Carolina coast on Thursday. Hazardous marine conditions are expected
to continue with tropical storm force winds and very elevated seas.
A Tropical Storm Warning remains in effect for all coastal waters,
including the Charleston Harbor. Conditions should improve as we
head into the weekend and Debby pulls further away.

Rip Currents/High Surf: Hazardous beach conditions are expected
through much of the week as Debby impacts the region. A High Risk of
rip currents is forecast today through Wednesday, and will likely
continue beyond. In addition, large breaking waves will impact
the beaches.

&&

.HYDROLOGY...
Debby remains on target to potentially produce historic rainfall
amounts that will result in areas of catastrophic flooding across
southeast Georgia and southeast South Carolina. A Flood Watch
remains in effect for all of southeast Georgia and southeast South
Carolina through Friday morning and Flood Advisories/Flash Flood
Warnings will continue to be issued as needed.

The center of Debby is forecast to slowly move off the Georgia coast
later today, potentially strengthening and bringing additional
intense rainfall to areas within the main core of the system. The
combination of deep tropical moisture and a slowly moving tropical
system will set the table for a prolonged period of intense
rainfall. Radar estimates show most of the area has received 5-10
inches of rain so far, with some locations receiving near 15 inches.
The forecast advertises additional rainfall amounts of 10-20 inches
over southeast South Carolina and 5-10 inches over southeast
Georgia. Even more concerning is that the swath of heaviest
rain is likely to fall along the coastal corridor, including
heavily urbanized areas across Savannah, Hilton Head, Beaufort,
and Charleston. The entire forecast area remains in a rare High
Risk in the Excessive Rainfall Outlook today from the Weather
Prediction Center, and the northern part of the forecast area
through Thursday...further highlighting the potential for
widespread damaging flash flooding.

With these expected rainfall totals, impacts will be widespread and
severe, likely including numerous flooded homes and structures,
damage to roadways including washouts, and unprecedented flooding
along and near smaller creeks and streams. Additionally, there will
likely be an extended period of major flooding (possibly record
setting) along larger main stem rivers that continues well after
Debby moves away from the region. The most intense rainfall will
likely come in bands followed by periods of time of considerably
lighter rainfall or even no rain at all.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Tropical Storm Warning for GAZ099>101-114>119-137>141.
     Flood Watch through Friday morning for GAZ087-088-099>101-
     114>119-137>141.
     High Risk for Rip Currents through this evening for GAZ117-119-139-
     141.
     Storm Surge Warning for GAZ117-119-139-141.
SC...Tropical Storm Warning for SCZ045-047>052.
     Flood Watch through Friday morning for SCZ040-042>045-047>052.
     High Risk for Rip Currents through this evening for SCZ048>051.
     Storm Surge Warning for SCZ048>052.
MARINE...Tropical Storm Warning for AMZ330-350-352-354-374.
&&

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...

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