Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 091331
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
931 AM EDT Sun Aug 9 2020

.SYNOPSIS...
The region will remain positioned between Atlantic high
pressure centered offshore, and an inland area of broad low
pressure.

&&

.NEAR TERM /THROUGH TONIGHT/...
The main pieces of the forecast puzzle remain in place, so we
didn`t make any meaningful changes with this update.

Weak upper level troughing will prevail across the southeast
through tonight with weak upper disturbances moving through the
area. Precipitable water levels are expected to remain near 2
inches. Surface heating will help CAPE values rise to 1500 to
2000 J/KG this afternoon and this combined with an inland moving
sea breeze will result in scattered afternoon and evening
showers and thunderstorms, most concentrated along and west of
the sea breeze. Storm motion will be dominated by outflow and
local enhanced convergence so training of storms or nearly
stationary storms could result in locally heavy rainfall. Also,
a few storms could briefly become strong with frequent
lightning. Convection should dissipate by late evening, but
cannot rule out a few showers and thunderstorms redeveloping
overnight. Highs today expected to be in the lower to mid 90s
away from the coast and in the upper 80s at the coast. Lows
tonight from the lower 70s well inland to the upper 70 near the
coast.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
The large scale pattern will continue to be nearly locked in place
through the period with a broad, deep layer trough of low pressure
well west of the region, and an Atlantic ridge of high pressure well
to the southeast. Models show a hint of a weak upper level short
wave to drift over the region Monday, but other than that, no real
discernible large scale features to help focus and force convection.
Therefore, the sea breeze will be the primary initial trigger for
convection, followed by convective outflow boundaries during the
afternoon and evening. Lingering outflow boundaries may spawn
isolated showers or thunderstorms through the night each night.
Winds will be light southerly less than 10 mph, becoming onshore
near the coast each afternoon in the form of a weak seabreeze.  With
abundant moisture and light, unidirectional deep layer flow, locally
heavy rainfall will again be possible. High temperatures close to
normal, reaching 90 to the lower 90s. Lows will continue warm, in
the lower to mid 70s most areas, around 80 close to the coast.
Afternoon heat indices expected to reach 100 to 105 each day as
surface dewpoint temperatures stay in the lower to mid 70s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
Very little change in the large scale pattern is shown by
global models through the period as a broad trough persists
inland and higher pressure well offshore over the Atlantic. This
will continue light southerly winds with afternoon sea breezes
each day. Although deep layer moisture will be sufficient with
PWs 1.5 to 2 inches, lack of any significant large scale forcing
will mean that the sea breeze will continue to be the primary
way convection will initiate. Model blended PoPs continue to be
in the likely range each afternoon, which is about 20 percent
above climo. Continue to feel a little uncomfortable with PoPs
in that range so far out. However, the higher blended PoPs
collaborate well with surrounding offices. Did adjust PoPs
downward by 5-10 percent, especially toward the end of the
period. High temperatures will continue near normal, while low
temperatures likely to continue a few degrees above normal.
Given light steering flow, locally heavy rainfall cannot be
ruled out each afternoon.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Expect showers and thunderstorms to develop this afternoon and
continue into the evening. Due to uncertainty regarding timing,
location, and duration of activity and any impacts to the
terminals, opted to include VCTS at both KCHS and KSAV starting
at the sea breeze passage, 19Z @ KCHS and 20Z @ KSAV and
continue it into the evening. It is quite possible that TRW will
need to be included in at least one of the terminals in later
forecasts/updates and it is possible that could occur prior to
the current inclusion of VCTS, but most likely during the
forecast VCTS time frame. Outside of showers and thunderstorms
at the terminals, expect VFR conditions to prevail. There is a
low chance for some light fog or patchy ground fog late tonight,
but this seems more likely to occur if significant rain falls
at the site later this afternoon or this evening. Should that
occur or become likely to occur, later forecast may need to
include it.

Extended Aviation Outlook: Scattered to numerous mainly
afternoon and early evening showers and thunderstorms could
result in brief flight restrictions at both sites each day.

&&

.MARINE...
Today and tonight: Extremely tranquil period outside of any
convection with winds greatest near the coast in association
with afternoon/evening sea breeze, but even there winds will
only be around 10 knots. Seas will be from 1 to 2 feet.

Monday through Thursday: No highlights are expected through the
period. The waters will remain between Atlantic high pressure
to the east and a broad area of lower pressure well inland. This
pattern will continue generally south-southwest winds of 15
knots or less and seas of 1 to 3 feet. Winds are expected to
veer to south-southeast each afternoon near the coast due to
the sea breeze circulation.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$
NEAR TERM...
SHORT TERM...RFM
LONG TERM...RFM
AVIATION...MTE/RFM
MARINE...MTE/RFM



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