Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 191956
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
356 PM EDT Sat Aug 19 2017

.SYNOPSIS...
A weak stationary front will gradually dissipate across the
region through early next week. A stronger cold front is
expected to arrive in the middle to latter part of next week,
likely followed by a slightly cooler and drier airmass.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Until Sunset: Radar shows some convection developing along the
sea breeze. However, this convection is limited to our extreme
northern and southern portions of the forecast area. The
subsidence aloft has held together fairly well and this is
verified by satellite water vapor imagery, which shows drier
air over our area. Global and hi-res models either show or hint
at isolated convection for the next few hours. This seems
reasonable given SPC mesoscale analysis shows MLCAPES ~2,000
J/kg, mainly limited to the immediate coast. As a result, we`re
keeping at least slight chance POPs in the forecast during this
time frame. Though, most areas can expect to stay dry.

Tonight: Convection will quickly dissipate this evening and the
bulk of the night will be rainfree. However, a light southerly
flow from off the ocean and the resulting convergence could
allow for widely scattered late night showers and thunderstorms
to move onshore from the Atlantic around daybreak. We have
slight chance POPs in the forecast for the coastal counties to
cover this potential. Temperatures will again be above normal,
but a little "cooler" than the past few nights.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Sunday and Sunday night: The large mid and upper level ridge
centered over the subtropical Atlantic will expand and strengthen
across the forecast area. As this occurs aloft, surface high
pressure to the east will become more influential and help to drive
a persistent onshore flow. Overall convection chances look minimal
and primarily limited to the immediate coast where low level
convergence will be strongest. The severe threat will be quite low
thanks to warm profiles and weak instability. The onshore flow
should keep temperatures closer to the coast in the low 90s, though
mid 90s will be possible well inland. Heat index values will top out
in the 100-105 range. Overnight, a well defined disturbance aloft is
progged to approach the Southeast coast embedded within the upper
ridge. Model solutions are in good agreement that an increase in
shower and thunderstorm activity over the coastal waters will begin
to push onshore late and overall rain chances will increase. PoP`s
along the coast increase to around 40 percent, with 20-30 percent
chances further inland. Temperatures will remain quite warm, with
lows in the mid to upper 70s.

Monday and Monday night: The forecast for eclipse day is certainly
tricky. Viewing conditions across the forecast area will depend
heavily on how morning shower and thunderstorm activity
evolves. There is good agreement across forecast models that
rain chances are increasing for this time period as the
aforementioned mid/upper level disturbance within the ridge
settles along the Southeast coast. It appears there should be at
least scattered showers and thunderstorms for much of the
morning, with perhaps even numerous coverage over the coastal
waters. Current thinking is that this morning activity will
begin to dissipate some time in the late morning/early afternoon
period with the focus then becoming inland areas later in the
day. The best scenario for viewing the eclipse will be if the
morning activity can dissipate in time for some possible partial
clearing by late afternoon. However, models such as the NAM
would suggest that showers and thunderstorms will continue to
develop through the early afternoon with plentiful obscuring
cloud cover. Forecast confidence is low at this time.
Temperatures tricky and will also be dependent on how convection
plays out. Overnight, another round of isolated to scattered
showers/storms will be possible mainly over the coastal waters.

Tuesday: The embedded mid/upper level disturbance will shift further
inland and rain chances will lower back into the 20-30 percent
range. The focus is again going to be along the land/sea interface
where sufficient convergence will be confined. Temperatures will
continue to be above normal with low 90s expected in most areas.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Wednesday through Saturday: Broad high pressure will gradually
weaken across the region on Wednesday as a cold front slides across
the southern Appalachians by the afternoon. A cold front should
push across the forecast area on Thursday, likely supporting a band
of thunderstorms. High pressure sourced from Canada will remain
centered well north of the region through the rest of the week.
However, the forecast area should see slightly cooler and drier
conditions Friday night into the weekend. In fact, guidance supports
afternoon dewpoints in the upper 60s inland by Saturday afternoon.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
TSRA have developed along portions of the sea breeze, so we
have VCTS in the TAFs for a few more hours. Brief flight
restrictions could occur, mainly at CHS. Convection will
quickly dissipate this evening with VFR expected overnight.

Extended Aviation Outlook: Thunderstorms may result in short periods
of flight restrictions, greatest potential during the afternoon and
early evening.

&&

.MARINE...
Tonight: Once sea breeze influences fade this evening, the
pressure gradient slackens as the cold front attempts to nudge a
little closer. S to SW winds will hold at or below 15 kt with
seas 2-3 ft.

Sunday through Thursday: Subtropical high pressure over the Atlantic
will drive onshore flow Sunday and Monday, taking on a more
southerly component through Wednesday. By Thursday, a cold front
will approach the region and winds will prevail out of the
southwest. Wind speeds will stay at or below 15 knots through the
period, primarily highest along the land/sea interface each
afternoon. Seas will average 2-3 feet through the period, up to 4
feet at times in the outer waters.

&&

.TIDES/COASTAL FLOODING...
Astronomical influences will lead to a round of elevated tides
into early next week. Only small tidal departures are necessary
and we could approach shallow coastal flooding levels with the
evening high tides through Tuesday along parts of the SC coast.

&&

.CLIMATE...
Record High minimums for 19 August...
KCHS 79/2010
KCXM 81/2009
KSAV 79/1878

&&

.EQUIPMENT...
The temperature and dew point sensors at the Downtown Charleston
observation site (KCXM) could periodically fail. Technicians
plan on fixing the problem.

&&

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

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...
CLIMATE...
EQUIPMENT...



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