Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 250502
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
102 AM EDT Fri Sep 25 2020

.SYNOPSIS...
Rain chances will increase on Friday as remnants from Beta and
an upper-level trough track just north of the region. A cold
front will approach the region early next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Introduced some slight chance pops in the Beaufort-Charleston
corridor where KCLX shows isolated showers developing in the
broad, warm air advection regime. Mesonet observations show
some of these showers are dropping a quick 0.01-0.02" as they
pass through. Otherwise, nudged overnight lows up a degree or
two in some spots where temperatures are likely not to drop too
much more prior to daybreak.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH SUNDAY/...
Friday through Sunday: High pressure builds in the Atlantic, as the
remnants of Beta to our west weakens as it tracks toward the
Northeast. Its associated upper-level trough gradually lifts as it
shifts to the east; staying north of our forecast area. Guidance
shows upper-level forcing concentrated to our north as well;
however, weak DPVA and upper divergence could brush our forecast
area. In addition, the dry air mass that has been in place the past
few days will be disrupted by a line of deep tropical moisture that
trails Beta to the south. Because the bulk of strong forcing for
ascent remains over the Mid-Atlantic states, the greatest chances
for unsettled weather Friday is over the SC/GA inland counties.
Relatively rapid destabilization of the atmosphere will support
scattered showers and thunderstorms. A few thunderstorms could
become organized/severe with with ample shear and moderate
instability (SBCAPE up to 3000 J/kg further SE) with the target area
being southeast GA. An isolated tornado cannot be ruled out;
however, damaging winds are the most likely hazard. In addition, WPC
has placed our forecast area in a marginal risk on Friday for
excessive rainfall considering PWATs increase to 2.25".

This weekend, ridging over the Atlantic erodes and a weak flow
pattern sets up. Centered between anticyclonic flow offshore and
weak cyclonic flow over western GA, low-level convergence will
promote showers and thunderstorms to continue. However, the severe
potential is lower this weekend with less instability and drier air
pushing in as a cold front approaches. Wet ground soil, weak surface
winds, and some slight clearing are all favorable conditions for
radiational fog early Saturday/Sunday morning in the inland GA
counties. The potential for fog depends more so on the likelihood of
cloud coverage; greater clearing would increase fog coverage,
allowing it to creep toward the coast. Temperatures will feel more
summer-like with highs in the mid to upper 80s and lows in the upper
60s/mid 70s through the weekend.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Overall, there is still some model inconsistency on how much deep
moisture will remain situated over the area as a cold front
approaches from the northwest. The first cold front also looks to be
a bit slower approaching the area on Monday then swiftly moving
through by early Tuesday morning. After the front passes through,
the GFS has drier air while the ECMWF now keeps things pretty moist,
with PWATs above 2.0 inches. Therefore, have maintained chance POPs
on Monday. Tuesday looks to be mostly dry before a second cold front
edges closer to the region. Models are still showing a deep trough
forming and trekking across much of the central to eastern US.
Though, less moisture looks to be associated with the cold front as
it moves through the area on Wednesday. Temperatures will continue
to decrease through the week and by Wednesday highs will be in the
upper 70s to low 80s. Lows are expected to dip back into the upper
50s to low 60s.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
VFR for much of the period. Some brief showers will be around
KCHS at the start of the 06z TAF period, but will have little/no
impact and should be north of the terminal by 07z.

The primary concern is the risk for tstm impacts at the
terminals today. Guidance similar in show a line of
showers/tstms moving east across the region late this afternoon
into early this evening as low pressure moves well to the north.
The best impact probabilities look to remain at KCHS where
impacts are expected roughly in the 22-02z window. Will carry
prevailing MVFR vsbys with low-end VFR cigs at this time with a
TEMPO for lower vsbys right at alternate minimums. This be
refined as the day progresses. For KSAV, it appears the best
tstms probabilities will remain just to the north and west of
the terminal. Confidence is not high enough to introduce tstms
at this time. Will carry VCSH 20-02z for now. The need for tstms
will be reevaluated for the 12z TAF cycle.

Extended Aviation Outlook: Over the weekend, flight restrictions
are possible in showers/thunderstorms as well as in the late
overnight/early mornings if patchy ground fog develops.
Prevailing VFR conditions should return by early next week.

&&

.MARINE...
Tonight: Southeast to south winds will persist tonight. There
will be an uptick in speeds after midnight, but conditions will
remain below Small Craft Advisory criteria. Seas will average
2-4 ft.

Friday through Sunday: Southeast winds will prevail
through late week as high pressure builds over the Atlantic. Winds
should remain below 15 kts through the weekend with seas 1 to 3 ft
in the nearshore waters and 3 to 4 ft in the outer Georgia waters.
Scattered showers and thunderstorms are possible this weekend caused
by the remnants Beta.

Rip Currents: As the dominant swell period continues to decrease,
the trend for elevated rip currents should continue to decrease as
well. A moderate risk remains in effect for the beaches through
Friday and Saturday`s forecast drops to a low risk.

&&

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

&&

$$

NEAR TERM...NED
SHORT TERM...BRM
LONG TERM...
AVIATION...NED
MARINE...BRM/ETM


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