667
FXUS62 KCHS 151738
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
138 PM EDT Sun Sep 15 2024

.SYNOPSIS...
High pressure will remain over the region this week. Developing
low pressure offshore of the Southeast U.S. coast is expected
to move into coastal North Carolina Monday night into Tuesday
before moving into the Mid-Atlantic Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Rest of This Afternoon: At the surface the region will remain
placed between a developing low pressure offshore and high
pressure inland. The forecast for the remainder of the near term
period will be heavily influenced by the development and
eventual track of the low pressure system, which unfortunately
is still rather uncertain. The National Hurricane Center
continues to monitor this low pressure for possible subtropical
or tropical development. However, regardless of
tropical/subtropical characteristics, the local forecast area
will see gusty northeast winds along the coast, dangerous rip
currents, high surf, and coastal flooding.

As far as rain chances go, hi-res model guidance favors
clusters of rain showers rotating around the offshore low and
impacting the coastal waters and pushing onshore. In most
places, rainfall amounts should be a tenth of an inch or less.
The great totals are anticipated along the immediate coast,
especially the southeast South Carolina coast where 0.50-1.00"
will be possible by the evening hours. Given the transitory
nature of the passing showers and the expected lack of deep
convection and intense rainfall rates, the overall flooding
threat is relatively low.

The forecast area will sit in the pinched gradient region
between the inland ridge of high pressure and the deepening
offshore low. This setup will support breezy northeasterly flow
throughout the day. Inland, frequent gusts anywhere from 20-25
mph can be expected. The strongest winds are expected along the
immediate coast where gusts up around 30 mph will be more
common. Additionally gusts at Lake Moultrie could approach 20 to
25 knots and a Lake Wind Advisory may be required this afternoon
through the overnight period.

The presence of cloud cover, occasional areas of showers, and
northeast flow will keep temperatures below normal. Low 80s are
expected in most areas, with the exception being Charleston and
Berkeley counties where highs could top out in the upper 70s.

Tonight: Occasional rounds of showers showers will continue to
be possible within the flow around the offshore low. The best
chances for rain will be limited to the Charleston Tri-County
region, especially eastern Berkeley and Charleston counties.
Winds will remain elevated and breezy at times as the low gains
latitude and possibly shifts inland overnight (depending on
eventual timing), though wind directions will gradually shift
from northeasterly to more northerly. Lows are forecast to be in
the upper 60s for most areas.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Monday and Tuesday: The broad cut off low left over from the
remnants of Hurricane Francine will meander south into the lower
Mississippi Valley while slowly filling with time while low
pressure off the Southeast U.S. coast steadily organizes. Even
now, there remains much uncertainty with the track, intensity
and character of the cyclone since it is still in its early
stages of organizing. Phase evolution diagrams still support the
system becoming a somewhat atypical subtropical/hybrid-type
system, possibly remaining attached to the frontal structures
that extend to the northeast and southwest of the center of
circulation while still developing a bit of warm core aloft.
Whether this system remains baroclinic or evolves into a
subtropical/tropical cyclone, the impacts will be the same to
the local area.

The forecast through Tuesday continues to lean heavily on the
latest internally coordinated WPC surface progs which take
cyclone inland across Southeast North Carolina Monday night and
across the North Carolina and Virginia Piedmont Tuesday. If this
track verifies, expect the bulk of the rain associated with the
cyclone to fall to the north and east of the center, keeping
all of Southeast South Carolina and Southeast Georgia well
outside of the primary heavy rain envelope. Guidance continues
to trend drier with each run with a number of models showing a
tight moisture gradient between the wedge and the cyclone
itself. Significant rainfall is becoming less likely locally as
a result, especially with model signals showing that the wedge
will likely become reinforced as subsidence around the cyclone
strengthens, mainly west and south of where the low tracks. It
is worth noting that the main clustering of the 15/00z GEFS
ensemble tracks has continued a more southern and westward trend
(more middle and upper South Carolina coast versus North
Carolina), a trend started with the 14/12z run. For this reason,
it is worth emphasizing again that users should expect
additional changes, some possibly significant, in later
forecasts as the evolution of this system becomes clearer. Highs
Monday will range from the upper 70s/near 80 north the mid 80s
elsewhere with mid 80s prevailing away from the beaches Tuesday.
Lows both Monday and Tuesday nights will drop into the mid-
upper 60s inland to the lower 70s at the beaches.

Wednesday: The forecast for Wednesday will be heavily
influenced by how quickly the cyclone over North Carolina gains
latitude. If the system hangs back/is slower like the
operational ECMWF suggests, then rain chances will remain very
low. If it is more progressive like much of the model data
supports, then a bit more recovery will occur with rain coverage
increasing as a ribbon of high PWATs will combine with passing
shortwave energy rounding the base of the upper low. Pops were
held 20-30% for now. Temperatures Wednesday afternoon will
return to more seasonable levels with highs warming into the
mid-upper 80s away from the immediate coast.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
Broad high pressure will extend across the Southeast U.S. as
the broad, longwave trough aloft nudges east with time.
Temperatures will remain near or possibly a tad above seasonable
normals. Isolated showers/tstms will be possible each
afternoon, but most areas will remain rain-free. Yet another
early season wedge could build south into the area Friday into
Saturday.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
18Z TAFs: KCHS/KJZI/KSAV will all initialize with MVFR ceilings,
with conditions improving to VFR around 20Z at KCHS/KJZI and
21Z at KSAV. Brief showers could impact the terminals through
this afternoon, however impacts are expected to be minimal so
have left only mention of VCSH in the TAFs. Gusty NE winds will
continue this afternoon, gusting to 25 knots at times. Gusty
conditions will continue through the overnight period at
KCHS/KJZI while KSAV should see gusts diminish a few hours after
dark. All terminals should see a return to MVFR ceilings
overnight, persisting through the remainder of the 18Z TAF
period.

Extended Aviation Outlook: Gusty winds and lingering flight
restrictions from low clouds and/or showers will linger into
Monday, mainly at KCHS And KJZI. Otherwise, there are no
concerns from Tuesday through the end of the week.

&&

.MARINE...
Today through Tonight: Conditions across the local waters will
continue to deteriorate as the area remains situated in the
tight gradient between the inland high and the deepening
offshore low. Winds will steadily strengthen today into the
evening, with the strongest winds anticipated across the South
Carolina waters. Confidence has increased in the potential for
frequent gale force gusts in the Charleston County waters, and a
Gale Warning has been issued there that runs through the
period. Elsewhere, high end Small Craft Advisory conditions are
expected with gusts up to around 30 knots. Seas will be highest
in the Charleston County waters and the outer Georgia waters,
where seas up to around 10 feet will be possible late today and
through tonight. Elsewhere, seas will mostly be in the 5-8 ft
range. A Small Craft Advisory is also in effect for Charleston
Harbor, where frequent gusts to around 30 knots are expected.

Monday through Thursday: Conditions will begin to improve
across the waters Monday as low pressure gains latitude and
moves inland across coastal North Carolina. The risk for gales
will continue into the late morning hours for the South Santee-
Edisto Beach nearshore leg so the Gale Warning runs until then.
Elsewhere, Small Craft Advisories will continue into Monday
night before both winds and seas drop below advisory thresholds
(25 kt and seas 6 ft). The only exception will be the Georgia
offshore waters form 20-60 NM out where it will take until well
in the day Tuesday before seas drop below 6 ft. North to
northwest winds will quickly turn back onshore as the cyclone`s
circulation moves well to the north and high pressure can become
established across the waters.

Rip Currents: The combination of strong northeast winds and
increasing swell from a deepening low just offshore will result
in a High Risk of rip currents at all beaches today. Gusty
northerly winds combined with still elevated seas and lingering
tidal influences will support a high risk rip currents through
the day Monday.

High Surf: Breaking waves are expected to peak around 4-6 ft
today through night at southeast South Carolina beaches. A High
Surf Advisory remains in effect there. Georgia beaches should
see breakers shy of 5 feet, peaking around 3-4 ft, which does
not meet the criteria for a High Surf Advisory. Due to a
combination of elevated water levels and powerful breaking
waves, significant beach erosion is likely, especially along
east and northeast facing beaches.

&&

.TIDES/COASTAL FLOODING...
Charleston and Coastal Colleton Counties: Strong northeast
winds will continue to build tidal departures today before winds
turn more northerly late tonight into Monday. Tides in the
Charleston Harbor are expected to peak just sky minor flood
levels (7.0 ft MLLW) with this morning`s high tide with levels
reaching well into the moderate range (7.5 ft MLLW) with the
evening high tide cycle. If winds end up being a bit stronger
than expected this afternoon, then there is a chance that levels
could reach major flood levels (8.0 ft MLLW), but this is still
considered a low probability scenario at this time. Coastal
Flood Advisories will be needed today with an outside chance
that a Coastal Flood Warning could be issued. The forecast
becomes a bit more tricky and uncertain Monday as predicted tide
levels peak as winds turn more northerly with time. Offshore
winds tend to lower tidal departures, but these winds will be
fighting the influences of lowering surface pressure, a possible
slowing of the Gulf Stream and considerable tidal loading from
a fairly long duration of gusty northeast winds. For now, levels
were kept in the moderate flood level for the Monday evening
high tide, but further adjustments are likely including a
possible nudge to near major flooding. Later this week, the risk
for at least minor flooding will continue as elevated tides
persist.

Beaufort County, SC, south to McIntosh County, GA: Tidal
departures will continue to steadily build today. Tides at Fort
Pulaski should fall short of minor flood levels (9.5 ft MLLW)
with this morning`s high tide, but are on track to reach minor
flood for the evening high tide cycle. A Coastal Flood Advisory
could be needed from Beaufort County south later this afternoon.
Similar to tides farther to the north, the forecast becomes
less clear on Monday as highest predicted tide of this King Tide
cycle occurs. Water levels will be fighting northerly winds,
while influences from gusty northeast winds, considerable tidal
loading and a possible slowing of the Gulf Stream could
potentially diminish the offshore influence. For now, levels
were kept just below minor flood levels Monday, but later
forecasts may need to nudge this back closer to 9.5 ft MLLW.
Waters levels could be close to minor flood levels later this
week as onshore winds return, but should largely remain below.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...High Risk for Rip Currents until 8 PM EDT this evening for GAZ117-
     119-139-141.
SC...High Risk for Rip Currents until 8 PM EDT this evening for
     SCZ048>051.
     High Surf Advisory until 8 AM EDT Monday for SCZ048>051.
MARINE...Small Craft Advisory until 1 AM EDT Monday for AMZ330.
     Gale Warning until 11 AM EDT Monday for AMZ350.
     Small Craft Advisory until 11 PM EDT Monday for AMZ352.
     Small Craft Advisory until 5 PM EDT Monday for AMZ354.
     Small Craft Advisory until 1 PM EDT Tuesday for AMZ374.

&&

$$

NEAR TERM...BSH/CPM
SHORT TERM...
LONG TERM...
AVIATION...BSH/CPM
MARINE...BSH