Flash Flood Warning 22, NWS Columbia, SC

Area Forecast Discussion
National Weather Service Columbia SC
525 AM EDT Tue Aug 6 2024

.SYNOPSIS...
Tropical Storm Debby will continue slowly crossing the region
through Thursday night then begin moving away from the area.
Confidence remains high for potential heavy rainfall and
flooding over the next several days. Drier air should finally
move into the region by the weekend leading to lowering rain
chances.

&&

.NEAR TERM /THROUGH TONIGHT/...
Key Message(s):

- Heavy rain and flooding potential remain the highest concern
- Flood Watch remains in effect for portions of the SC Midlands
  and CSRA.
- Highest rainfall amounts through tonight will again be over
  along and south of I-20.

At 2 AM EDT, Tropical Storm Debby was about 25 miles W of
Brunswick, GA with max sustained winds of 45 mph, moving toward
the NE at 7 mph. The slow northeastward movement is expected to
continue through tonight with the storm center being offshore of
Savannah, GA.

Tropical Storm Debby continues to slowly move through
southeastern GA and will move offshore into the Atlantic later
today. As Debby moves offshore it will begin to slowly
strengthen while it slowly moves northeastward however the main
focus for the forecast area remains solidly on rainfall and
flooding. This will keep a moisture plume across the area
through the period with heavy rain expected at times, mainly
along and south of I-20. Currently on satellite imagery are two
convective bands pushing onshore with one in the Charleston area
and the other near Myrtle Beach. As these bands move further
onshore this morning expect rain intensity to increase over the
eastern Midlands and southern CSRA with locally heavy rain
possible. Through the afternoon expect the rain intensity to
continue slowly increasing as the moisture plume remains focused
on the SC Coast while Debby makes little progress
northeastward. The scenario will show little change tonight as
Debby continues inching northeastward while the moisture
continues streaming into the area. Rainfall totals today and
tonight will again have a tight gradient from west to east with
and inch or less in the far western Midlands and northern CSRA
to 1-3 inches in the central Midlands and CSRA and 4-6 inches in
the eastern Midlands. With the nearly saturated soils across
much of the central and eastern portions of the forecast area
and the expected rainfall WPC has kept the central Midlands and
CSRA in a moderate risk of excessive rainfall with the eastern
portions in a high risk. As such the flood watch remains in
effect and had been expanded to include Chesterfield County.

With the convective bands pushing onshore there is also the
potential for severe thunderstorms and possible tornadoes. The
area is under a SPC marginal risk of severe weather with a
slight risk along and east of I-95 with the main threat being an
isolated tornado. Winds also remain in focus however are not
expected to be impactful. Sustained winds in the along and south
of I-20 will generally be around 15 mph with gusts around 25 mph
and a few in the far eastern Midlands up to 30 mph with lower
speeds in the western Midlands and northern CSRA.

With the rain and overcast skies blanketing the area today and
tonight high temperatures in the upper 70s with lows tonight in
the lower 70s.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Key Message(s):

- Potential for life threatening flash flooding continues
  Wednesday through Thursday
- Portions of the area are in a rare High Risk on both Wednesday
  and Thursday for excessive rainfall
- A Flood Watch is in effect through Thursday evening

Updates: There has been some subtle shifts in the track forecast for
Debby in the guidance over the couple of model suites; EC and some
of the hi-res members are now favoring a slightly further west track
but the GFS and HWRF solutions still remain the extreme outlier with
their westerly track. Generally, the forecast remains on track with
regardless through the short term period with only some slight
increases in precip along I-20.

Forecast and Impacts: Widespread heavy rainfall likely throughout
much of the short term period with life threatening flash flooding
possible in portions of the eastern half of our forecast area. The
combination of deep tropical moisture of PWAT's over 2.5" and
an extremely slow moving system will yield rainfall totals of
generally 3-5" from Wednesday morning through Thursday in the
central Midlands, and 4-6" with locally near 12" probable in the
eastern Midlands; the EC and some hi-res guidance develop an
extremely strong moisture convergence boundary directly over
the Midlands early Thursday with PWAT's climbing to over 3".
While this will likely not come to fruition exactly, this
highlights the high end potential generally between I-95 and
I-20. From a timing perspective, we may see a bit of lull early
Wednesday as Debby reaches its furthest point offshore, before
rain coverage and intensity ramps back up in the afternoon. It
appears that the heaviest precip will likely fall early Thursday
into Thursday afternoon. WPC has continued their high risk
across the eastern half of the area on both Wednesday and
Thursday, with moderate risk extending further west into the
Midlands. It be should be noted the rarity of such a situation
with portions of our in area in Moderate and High Risks for
three consecutive days. While life threatening flash flooding
across the I-20, and especially the I-95 corridor is the primary
threat, will we have low end tornado threat throughout much of
Thursday across the eastern and half of the forecast area.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
Key Message(s):

- Debby will lift north on Friday with steadily improving
  conditions, but river flooding will remain an issue in the
  eastern Midlands.

As is typical behind an exiting tropical system, drier air with
PWAT's below average will wrap in behind Debby by Friday and
especially into Saturday. The exact timing is still somewhat
unclear as to when exactly Debby will accelerate north, but
even the slowest model solutions pull it north by Friday
afternoon. Otherwise, fairly calm and near average weather
expected as we move into next week.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
Restrictions expected to dominate the period.

Tropical Storm Debby continues to slowly move through
southeastern GA and will move into the Atlantic today. This
will keep a plume of moisture focused on the area with
rain...heavy at times...over the terminals. This will keep cigs
mainly IFR with MVFR vsbys through the period. Winds through the
period will be northeasterly to easterly with sustained speeds
around 15 knots and gusts around 25 knots.

EXTENDED AVIATION OUTLOOK...Low ceilings, reduced visibility and
rain, heavy at times, are expected to continue through at least
Thursday. Gusty winds may be possible through Wednesday night
as Debby moves through the region.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
SC...Flood Watch through Thursday evening for SCZ022-027>031-035-
     037-038-041-135>137.
GA...Flood Watch through Thursday evening for GAZ063>065-077.

&&