Coosaw, South Carolina Weather Forecast Discussion

844
FXUS62 KCHS 271831
AFDCHS
Area Forecast Discussion
National Weather Service Charleston SC
231 PM EDT Sat Sep 27 2025
.SYNOPSIS...
A weak front will remain stalled along the coast through the
weekend. Tropical Depression Nine is forecast to become a
tropical storm near the Bahamas this weekend then approach the
Southeast U.S. coast early next week as a tropical storm or
hurricane.
&&
.NEAR TERM /THROUGH TONIGHT/...
The mid-levels will consist of a trough along the spine of the
Appalachians this evening. It`ll slowly shift to the east
overnight, while weakening slightly. At the surface, a front is
located just to our west late this afternoon. Southwest flow
ahead of this front is ushering deep moisture into the region,
with a plume of PWATs ranging from 1-5-2" across our area this
evening. It`ll slowly shift to the east as time progresses. The
CAMs continue to have scattered to numerous showers across our
area late this afternoon, especially closer to the coast.
They`ll shift east into this evening while weakening. There is
some instability in place, generally MLCAPEs 1,000 J/kg,
greatest along the coast. Effective bulk shear is 30+ kt. So a
few thunderstorms could occur. But the risk of widespread strong
thunderstorms is low. The storms should move at a decent speed,
which would limit the overall rainfall potential. But a quick
~1" is possible in areas where there is training. A cold front
will be moving east through our area this evening, which will
push the showers offshore. The front will be located near our
coast this evening, then offshore overnight. Behind the front,
drier air will move in from the west. Low temperatures are
forecasted to range from the mid 60s far inland to the low 70s
along the coast.
&&
.SHORT TERM /SUNDAY THROUGH TUESDAY/...
The weak front/inverted trough will be stalled just off the
SC/GA coast Sunday morning, then may drift onshore later in the
day. Onshore moisture flux will increase during the afternoon
with PWATs pushing above 2" in the afternoon. Temps are expected
to climb into the mid/upper 80s inland, with lower 80s near the
coast. The combination of a weak sea breeze and the coastal
trough plus the development of moderate SBCAPE in the afternoon
should allow for scattered showers and tstms to develop near the
coast and move inland later in the day.
Additional moisture advection Sunday night, plus a large area
of mid-level vorticity well ahead of TD 9, will rotate onshore,
bringing an increase in shower and thunderstorm coverage. Some
heavy rainfall is possible, particularly around daybreak Monday
near the coast.
The forecast becomes much less certain Monday into Tuesday as
the details will depend heavily on the eventual track and
intensity of TD 9. The track forecast is particularly complex
because it will also be influenced by the eventual track and
strength of Hurricane Humberto farther to the east. There has
been a noticeable slower and eastward trend for the TD 9
forecast during the last 12-24 hours. The majority of both
deterministic and ensemble model members are now favoring a
sharp east turn around the time the system gets as far north as
the FL/GA border while still more than 100 miles offshore. Based
on this latest forecast, the local risk for damaging winds or
significant storm surge would be lower. However, a lot can still
change, and it`s worth noting that some ensemble members still
show a more northwest trajectory.
The threat for heavy rainfall across coastal SC and potentially
also in GA could exist even if the current forecast verifies
due to interaction between the developing TD 9 and inland
trough. Most guidance shows the greatest precipitation shield
north and northwest of the center Monday through Tuesday, with
the potential for 4-6" of rainfall over the SC quad-county area.
&&
.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
Heavy rainfall could continue to be a concern for the eastern
half of the forecast area Tuesday night into Wednesday if TC 9
follows the current forecast track. Once TC 9 is hopefully out
of the way, cooler high pressure will build from the north,
though we can`t rule out scattered showers late in the week if a
weak surface trough redevelops.
&&
.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
18Z TAFs: Scattered thunderstorm activity remains possible
through the first half of this evening, generally between 18-22z
for KSAV and 19-23Z for KCHS and KJZI. Storm chances diminish
between 00- 03Z tonight, expecting primarily dry conditions
thereafter.
VFR persists through 00Z this evening, except inside
thunderstorms where MVFR is possible. After midnight,
conditions look to deteriorate to MVFR region wide with the
possibility of IFR to LIFR by sunrise, particularly at KCHS and
KJZI. Visibility reductions to between 3-5SM possible.
Confidence in prolonged IFR to LIFR is low, favored MVFR through
12Z with scattered IFR cigs. MVFR will linger through the first
half of Sunday, with KSAV having the best chance at becoming
VFR between 15-18Z. MVFR looks to persist through the period at
KCHS and KJZI.
Calm to light Northeast overnight; generally less than 5kt
through 12z before increasing modestly to around 5kt by 18Z
Sunday. Gusts 10- 15kt possible for all terminals but trending
towards the higher end of that range for KCHS and KJZI by the
end of the period.
Extended Aviation Forecast: A prolonged period of gusty N to NE
winds possible Monday through Thursday due to interactions
between Tropical Cyclone Nine and inland high pressure. MVFR or
lower ceilings and occasionally vsby reductions expected for all
terminals starting as early as Sunday night and persisting for
much of the week.
&&
.MARINE...
Through Tonight: A cold front will be just inland this evening.
It`ll slowly move eastward through the coastal waters
overnight, becoming located just east of our waters by daybreak
Sunday. This will cause light E winds in the evening to shift
to the NNE overnight. Seas will average 2-3 ft.
NE winds 10-15 kt expected Sunday, then the gradient really
starts to tighten Sunday night into Monday as Tropical Cyclone
Nine starts moving north out of the Bahamas and interacts with
high pressure over the eastern US. The forecast for the
remainder of the week will depend highly on the eventual track
and strength of the tropical cyclone. Mariners should closely
monitor the tropics and the local marine forecast as dangerous
conditions are anticipated next week. In addition to the strong
winds, numerous heavy bands of showers and tstms will affect the
waters.
Rip Currents: Increasing swells and winds will yield a High
Risk for rip currents on Monday. An enhanced risk will continue
through at least Wednesday.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.
&&
$$
NEAR TERM...
SHORT TERM...JRL
LONG TERM...JRL
AVIATION...JRL/KS
MARINE...JRL

NWS CHS Office Area Forecast Discussion