Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 221500

Area Forecast Discussion
National Weather Service Charleston SC
1100 AM EDT Tue Aug 22 2017

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.


Late this morning: Conditions are quite similar to 24 hours ago
with shortwave energy embedded within the large anticyclone
aloft. The surface pattern is rather weakly forced, consisting
primarily of the western extension of subtropical high pressure
over the Atlantic. The 12z KCHS sounding reveals a moist airmass
with precipitable water values in excess of 2 inches, with weak
lapse rates and other severe weather indices. Much like
yesterday, with the onset of diurnal heating convective clouds
have quickly filled in across the forecast area. Showers and
storms have started developing across southeast Georgia, and the
expectation is that this activity will expand through the
morning and into the afternoon. The current forecast PoP scheme
looks quite reasonable so no significant changes were made.
There will almost certainly be areas that need likely PoP`s at
times, but confidence in exactly where and when is quite low.
The main thing to watch out for today is locally heavy rainfall.
Storm motion will be no more than 5-10 mph and storms will
likely just evolve on outflow boundaries due to the lack of
significant steering flow. Some locations could pick up a few
inches of rain directly under thunderstorms, and minor flooding
issues could occur especially in low lying and poor drainage
areas. Temperatures will be quite variable hour to hour,
especially this afternoon, as outflow boundaries move around and
locations experience rain cooled conditions with plentiful cloud
cover. Still, highs will likely top out around 90.

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.


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.


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.


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.


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.

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

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

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.


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.


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.





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