Area Forecast Discussion
Issued by NWS Charleston, SC

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103
FXUS62 KCHS 221124
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
724 AM EDT Tue Aug 22 2017

.SYNOPSIS...
High pressure will extend across the area through Wednesday
with a weak low developing inland. A cold front is expected to
pass over the region on Thursday. A wave of low pressure could
form along the front offshore Friday, as high pressure builds
from the north next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Calm winds, the wet grounds and near 100% RH will allow for fog
and low stratus in some areas through mid morning. Overnight
convection that developed in the NBC-HXD areas sent out an
outflow that allowed for convection to spread further N-NW, and
this will impact parts of SC as the day begins. PoP`s are no
more than 20-30%. Temps begin the day in the lower and middle
70s, upper 70s along the coast.

Today: A piece of short wave energy remains in place locally within
a weakness of a fairly prominent ridge aloft that begins to
shift a little south as a long wave trough shifts through the OH
valley to the Appalachians. Concurrently at the surface we find
the formation of a weak Piedmont trough with the western part of
sub-tropical ridging across the local area. Considerable deep
moisture from both the Gulf and Atlantic remains anchored in
place within a a deep S-SW flow throughout the vertical, with
Pwat`s well in excess of 2" or greater than 130% of normal. Lift
provided by both the sea breeze, nearby inland trough and the short
wave aloft, plus at least modest instability will act to enhance
convection.

Isolated activity will prevail this morning, mainly far inland
near the decaying outflow and over the coastal corridor
where the best convergence and isentropic ascent occur in
tandem with higher MLCAPE around 1000 J/kg. Then as the sea
breeze pushes inland this afternoon rain chances will ramp up at
least into the high-end scattered category, but once radar
trends are better defined we expect likely PoP`s or greater to
occur for parts of the region. There isn`t any severe risk, but
heavy rains will definitely be a concern given weak storm motion
and the abundance of moisture. Some places can pick up a quick
1-2 inches in an hour or less. The flash flooding risk is low,
but given many places getting rain yesterday it won`t take much
to cause at least minor to moderate flooding. Lightning will
also be prevalent in all storms.

Considerable cloud cover and the expected rains will keep temps
from getting any warmer than upper 80s or around 90F, about
where they should be for August 22. We attempted to alter the
hourly temp curve to account for rain-cooled conditions this
afternoon, so expect many places to drop into the 70s where the
steadier convective rains develop.

Tonight: With a stronger short wave trough dropping through the
broad long wave trough upstairs, it shunts the ridge to the
south, while at the surface the periphery of the Atlantic ridge
holds on. Convection will have faded quickly inland this
evening, leaving us with several hours of rain-free conditions.
But there is still some isentropic ascent on the 300K surfaces
with with some convergence within the S-SW planetary boundary
layer flow. This can cause isolated or scattered convection
from the Atlantic to move onshore after midnight, mainly
impacting the coastal counties with 20-30 PoP. There is likely
too much cloud cover for any serious fog concerns, and little to
no indications looking at soundings, MOS guidance and/or Hi-Res
model visibility forecasts. Temps generally down into the mid
and upper 70s for the coastal corridor, 73-74F far inland.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
Wednesday, the center of the H5 ridge should push south near the
panhandle of FL as a L/W trough swings over the Appalachians during
the afternoon. Forecast soundings indicate that the inversion will
lift to around H75 and inhibition should fall below 10 J/kg. Flow
should shift from the S to SSW during the daylight hours Wednesday.
A slow moving sea breeze will likely serve as a focus for moisture
convergence, supporting at least sct thunderstorms over the forecast
area during the day. High temperatures are forecast to range from
near 90 over the beaches to the mid 90s within the mid Savannah
River Valley.

Thursday: Guidance has been consistent with the timing of a cold
front across the CWA during the daylight hours Thursday. The axis of
the H5 trough is expected to slide over the region during the
afternoon. The arrival of the trough over the slowing front will
likely support a frontal wave to develop, the GFS and ECMWF indicate
the wave developing over the Coastal Plain of SC/GA. Given the
passage of the mid level trough and sfc front, PW near 2 inches, LFC
less than 4 kft, and widespread weak to moderate instability, I will
maintain likely PoPs. H85 CAA is not expected until Thursday night,
highs should range around 90 degrees for most areas Thursday
afternoon, possibly upper 80s north of the Edisto River Basin.

Friday: Building high pressure centered to the north will result in
sfc winds to shift from the NE. Guidance indicates that the cold
front will likely stall near the coast, possibly kinked west across
SE GA. Given lingering moisture and instability, coverage of showers
and thunderstorms should expand during the daylight hours on Friday,
greatest over SE GA and adjacent waters. High temperatures should
range in the mid to upper 80s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
High pressure sourced from Canada will remain centered well
north of the region through this week. Confidence in the day 5
through 7 forecast remains very low. Medium range guidance
indicates that the sfc high center will shift over New England,
ridging SW across the Carolinas into GA. In addition, both the
GFS and ECMWF show a coastal low developing off the GA/SC coast
by late this week, with run to run trends shifting east and
south. In addition, guidance suggests that low pressure
associated with the remnants of Harvey will take a curved track
from south TX Friday night to MS valley by Tuesday. With a low
to the east and west and ridging over the Carolinas, it appears
at least cloudy and possibly a wet period. Perhaps one area
where confidence is medium, the complex pressure pattern should
support prolonged NE winds across the CWA. NE flow should yield
afternoon dewpoints in the upper 60s inland by this weekend. I
will favor the marine and near shore zones for Chc to Schc PoPs
through much of the day 4 through 6 period. By Tuesday,
strengthening llvl flow and deeper moisture may provide greater
coverage of convection, I will forecast chc PoPs across the CWA.
Daytime temps should see limited warming given the cloud cover
and NE winds, mid 80s should be common through the medium range.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
Looks like another good shot at SHRA/TSRA with flight restrictions
along the sea breeze, aided by energy aloft during the afternoon
from about 17-22Z. Temporary flight restrictions are forecast,
mainly into the MVFR range. But a period of IFR conditions are
possible especially since rain could be heavy at times. Current
indications are that VFR conditions will return this evening and
persist through 12Z Wednesday. However, isolated to scattered
convection could reform overnight, with the latest thinking that
this activity stays east of the airfields. Only low end chances
of fog/stratus, unless higher level clouds diminish enough, then
chances would increase.

Extended Aviation Outlook: Thunderstorms may result in short
periods of flight restrictions, greatest potential during the
afternoon and early evening. Numerous thunderstorms should
develop along and ahead of a cold front during the daylight
hours Thursday.

&&

.MARINE...
Today and tonight: The coastal waters will lie under the
influence of high pressure extending onshore from the Atlantic,
while a lee side trough is found far inland. Sea breeze
circulations will cause southerly winds as high as 10 or 15 kt
this afternoon and evening (unless altered by convection), then
veering around to the SW with land breeze influences overnight.
Seas will average 2-4 ft, highest across the outer GA waters.

Waterspouts: A solid convergence line has formed from near
Charleston Harbor to about 20 nm off the coast from Edisto to
off Sapelo. Our locally derived Waterspout Index and SPC Non-
Supercell Tornado Parameter supports possible waterspouts. A
Marine Weather Statement has been issued through 11 am.

Wednesday through Thursday: Weak high pressure on Wednesday will
give way to a cold front on Thursday. Through this period, winds
will generally range from SE to SW with speeds 5 to 15 knots, with
some gusts to near 20 knots. Seas will range 2 to 4 feet.

Friday through Sunday: The pressure pattern is expected to feature
low pressure off the Carolina/Georgia coast with another low
pressure system expected over E TX/LA by late this weekend. Between
the two lows, high pressure is expected to ridge SW from a New
England centered high. Gusty NE winds maybe common across the marine
zones by this weekend, with gusts between 20 to 25 kts. Wave heights
may increase to 3 to 6 feet within 20 NM, with 8 to 9 feet beyond 30
NM.

Rip Currents...There remains around a 2 ft swell every 10
seconds that reaches the surf zone today, and that along with
onshore winds developing and lingering astronomical influences
will support a Moderate Risk of rip current at area beaches.

&&

.TIDES/COASTAL FLOODING...
Tide levels could flirt with 7.0 ft MLLW in Charleston Harbor
with the high tide this evening, and if so a Coastal Flood
Advisory could be required for Charleston, coastal Colleton and
Beaufort counties. This information will be included in the
Hazardous Weather Outflow. Conditions will likely stay below
advisory levels for Jasper County south to McIntosh County.

&&

.EQUIPMENT...
The KCLX radar continues to be down due to lighting strikes.
Parts are on order but the radar is not expected to return to
service until Tuesday afternoon at the earliest.

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

&&

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

&&

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



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