Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 181957
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
357 PM EDT THU SEP 18 2014

.SYNOPSIS...
HIGH PRESSURE WILL BUILD TO THE NORTH OF THE REGION THROUGH FRIDAY
AS A WEAK AREA OF LOW PRESSURE DEVELOPS OFF THE SOUTHEAST COAST AND
LIFTS NORTHEASTWARD THROUGH THE WEEKEND. A COLD FRONT WILL APPROACH
FROM THE WEST EARLY NEXT WEEK...WITH MORE HIGH PRESSURE EXPECTED
THEREAFTER.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
SFC HIGH PRESSURE WILL CONTINUE TO BUILD FROM THE NORTH THROUGH
TONIGHT...RESULTING IN SLIGHTLY DRYER AND COOLER CONDITIONS.
HOWEVER...A BROAD MID LVL TROUGH OF LOW PRESSURE REMAINS OVER MUCH
OF THE EASTERN CONUS WITH ITS AXIS EXTENDING SOUTHWARD OFF THE
SOUTHEAST COAST. THE POSITION OF THE TROUGH AXIS ALONG WITH OFFSHORE
STEERING FLOW SHOULD LIMIT OVERALL CONVECTION TO LOCATIONS NEAR
THE COAST WHILE WEAK H5 SHORTWAVES CONTINUE TO ROUND THE BASE OF THE
MID LVL TROUGH CENTERED TO OUR NORTH/NORTHEAST. HAVE MAINTAINED THE
CURRENT FORECAST WITH SLIGHT CHANCE TO CHANCE OF SHOWERS AND/OR
THUNDERSTORMS THIS EVENING...HIGHEST CHANCES CLOSER TO THE COAST
WHERE ONSHORE FLOW RESULTS IN ENHANCED MOISTURE CONVERGENCE NEAR A
SOMEWHAT PINNED SEABREEZE. CONVECTIVE PARAMETERS ARE NOT AS
IMPRESSIVE INLAND...MAINLY DUE TO LESS MOISTURE LEVELS AND DELAYED
SFC HEATING FROM EARLIER CLOUD COVER. THUS BELIEVE MOST AREAS WILL
REMAIN RAIN-FREE INLAND BY SUNSET...WHILE PRECIP GRADUALLY
DIMINISHES ALONG THE COAST A FEW HOURS AFTER SUNSET. WE STILL CAN
NOT RULE THE POSSIBILITY OF A STRONGER STORM OR TWO WITH DAMAGING
WINDS AND/OR HEAVY RAINFALL...MAINLY BEFORE SUNSET ALONG THE COAST.

ONCE PRECIP ACTIVITY DIMINISHES EARLY TONIGHT...THE MAIN CONCERN
WILL ONCE AGAIN SWITCH OVER TO THE POSSIBILITY OF FOG LATE. BEST
CHANCES OF FOG WILL OCCUR MAINLY IN LOCATIONS THAT EXPERIENCE
RAINFALL THIS EVENING AND EARLY TONIGHT. HAVE INTRODUCED PATCHY FOG
OVER ALL AREAS SHORTLY AFTER MIDNIGHT INTO DAYBREAK FRIDAY MORNING.
OVERNIGHT LOW TEMPS SHOULD RANGE IN THE MID 60S INLAND TO UPPER
60S/LOWER 70S CLOSER TO THE COAST.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
FRIDAY AND FRIDAY NIGHT...ALOFT THE REGION WILL BE UNDER THE
INFLUENCE OF A BAGGY TROUGH THAT WILL SLOWLY ATTEMPT TO CLOSE OFF
INTO A BROAD UPPER LOW ACROSS THE SOUTHEASTERN CONUS LATE IN THE
OVERNIGHT. AT THE SURFACE...SEVERAL FEATURES WILL BE IN PLAY
INCLUDING HIGH PRESSURE SHIFTING EASTWARD ACROSS NEW ENGLAND...AND A
SUBTLE SURFACE TROUGH ALONG THE SOUTHEAST COAST. THE LARGER SCALE
STEERING FLOW WILL BECOME UNIFORMLY EASTERLY THROUGH THE PERIOD
WHICH WILL HELP TO INCREASE DEEPER MOISTURE ACROSS THE FORECAST
AREA. THIS ATLANTIC MOISTURE FEED WILL RESULT IN PWATS INCREASING TO
AROUND 1.75 INCHES LATE. MODELS SHOW THE SUBTLE SURFACE TROUGH
PUSHING BACK TO THE WEST WITH TIME AND BRINGING WITH IT ENOUGH
CONVERGENCE TO GENERATE GOOD COVERAGE OF SHOWERS AND ISOLATED
THUNDERSTORMS. THE CONVERGENCE ZONE WILL PRIMARILY BE FOCUSED ALONG
THE COAST SO THE HIGHEST POPS ARE OVER THE WATERS AND ALONG THE
IMMEDIATE COASTLINE...BUT HAVE GENERALLY BEEN HELD IN THE 30-40
PERCENT RANGE. OVERNIGHT...POPS INCREASE INTO THE LIKELY RANGE
ACROSS THE COASTAL WATERS WHERE THE MOISTURE/CONVERGENCE ALIGN THE
BEST. TEMPERATURES WILL BE TEMPERED BY PLENTIFUL CLOUD COVER THROUGH
THE PERIOD. HIGHS WILL BE KEPT IN THE LOW TO MID 80S...WHICH IS
ACTUALLY BELOW NORMAL FOR THIS TIME OF YEAR. LOWS WILL BE IN THE MID
TO UPPER 60S.

SATURDAY THROUGH SUNDAY...THE BROAD UPPER LOW WILL BEGIN TO OPEN
BACK UP AND LIFT TO THE NORTHEAST AS IT BEGINS TO FEEL THE HEIGHT
FALLS TO THE NORTH FROM A VIGOROUS TROUGH DIVING INTO THE GREAT
LAKES REGION. THE MAIN FEATURE OF INTEREST THROUGH THE WEEKEND WILL
BE A WEAK AREA OF LOW PRESSURE PROGGED TO DEVELOP OFF THE EAST COAST
OF FLORIDA ON SATURDAY. THE MODELS DISAGREE A BIT ON THE TIMING OF
THE SURFACE LOW DEVELOPMENT...BUT THEY DO AGREE THAT IT WILL BE WELL
OFF THE GEORGIA/SOUTH CAROLINA COAST AND WILL STEADILY LIFT
NORTHEASTWARD THROUGH SUNDAY. THE NHC CURRENTLY GIVES THIS SYSTEM A
LOW CHANCE OF TROPICAL DEVELOPMENT. FROM A PRECIPITATION
STANDPOINT...ANY RAINFALL ASSOCIATED WITH THIS FEATURE SHOULD
PRIMARILY BE OVER THE COASTAL WATERS AND RIGHT ALONG THE COAST.
THERE WILL BE A RELATIVELY TIGHT MOISTURE GRADIENT ON THE WESTERN
PERIPHERY OF THE LOW AND ANY LOW LEVEL CONVERGENCE WILL FAVOR AREAS
FURTHER EAST. THE OTHER IMPACT WILL BE A TIGHTENING PRESSURE
GRADIENT WHICH WILL DRIVE A FRESHENING NORTHEAST FLOW...ESPECIALLY
ON SATURDAY. EXPECT BREEZY CONDITIONS AT THE BEACHES ON SATURDAY.
TEMPERATURES WILL CONTINUE TO BE BELOW NORMAL...WITH HIGHS TOPPING
OUT IN THE LOW 80S AND LOWS FALLING INTO THE MID/UPPER 60S.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
A STRONG SHORTWAVE TROUGH WILL SWEEP ACROSS THE NEW ENGLAND AND
MID ATLANTIC STATES ON MONDAY...HELPING TO DRIVE A COLD FRONT
THROUGH THE FORECAST AREA. MEDIUM RANGE MODELS ARE IN DECENT
AGREEMENT THAT THE FRONT WILL BE ASSOCIATED WITH LITTLE SIGNIFICANT
PRECIPITATION...AND WILL THUS CONTINUE TO CAP RAIN CHANCES IN THE
SLIGHT CHANCE RANGE DURING FRONTAL PASSAGE. EXPANSIVE SURFACE HIGH
PRESSURE FOLLOWING IN THE WAKE OF THE COLD FRONT WILL HAVE ORIGINS
FROM THE NORTHERN PLAINS STATES...AND WILL BE ACCOMPANIED BY A
MORE NOTABLE CHANGE IN AIR MASS. AHEAD OF THE FRONT...TEMPERATURES
ON MONDAY WILL ACTUALLY WARM SEVERAL DEGREES WITHIN FRONTAL
COMPRESSION AND DOWNSLOPE FLOW. CONDITIONS WILL THEN QUICKLY
COOL...WITH LOW TEMPERATURES MONDAY NIGHT FALLING INTO THE LOW TO
MID 60S. EXPECT HIGH TEMPERATURES TUESDAY THROUGH MID WEEK TO ONLY
PEAK IN THE LOW 80S. NIGHTTIME LOWS WILL RANGE FROM THE LOW TO
MID 60S. WITH DRY HIGH PRESSURE POSSIBLY LINGERING THROUGH THE
EXTENDED PERIOD...HAVE GENERALLY KEPT RAIN CHANCES IN THE SLIGHT
CHANCE RANGE...HIGHEST ALONG THE COAST AND OFFSHORE.

&&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
VFR CONDITIONS ARE EXPECTED TO PREVAIL AT BOTH CHS AND SAV
TERMINALS THIS AFTERNOON AND EVENING. HOWEVER...THERE IS A LOW
CHANCE OF MVFR CIGS SHOULD A SHOWER IMPACT EITHER TERMINAL. HAVE
MAINTAINED VCSH AT THIS TIME AS MOST ACTIVITY IS LIKELY TO REMAIN
ALONG THE IMMEDIATE COAST. MVFR OR LOWER CONDITIONS WILL ALSO BE
POSSIBLE NEAR DAYBREAK FRIDAY MORNING DUE TO FOG AND/OR LOW
STRATUS...HOWEVER CONFIDENCE REMAINS TOO LOW TO INCLUDE IN THE
TAFS AT THIS TIME. CHANCES OF FLIGHT RESTRICTIONS WILL LIKELY
INCREASE LATE TONIGHT SHOULD EITHER TERMINAL SEE RAINFALL THIS
AFTERNOON/EVENING. VFR CONDITIONS SHOULD THEN PREVAIL BY 14Z
FRIDAY AT BOTH TERMINALS.

EXTENDED AVIATION OUTLOOK...LOW CLOUDS/FOG WILL BE POSSIBLE WITH
PERIODS OF MVFR OR LOWER CIGS EACH MORNING THROUGH AT LEAST
SATURDAY. OTHERWISE...MAINLY VFR OUTSIDE OF ANY ISOLATED TO
SCATTERED SHOWERS EACH DAY. IMPROVING CONDITIONS WITH MAINLY VFR
SUNDAY AND MONDAY.

&&

.MARINE...
THIS EVENING AND TONIGHT...CONDITIONS WILL REMAIN WELL BELOW SMALL
CRAFT ADVISORY THRESHOLDS WITH WIND SPEEDS AT OR BELOW 10 KTS. WE
COULD SEE A GRADUAL INCREASE IN WIND SPEEDS OVERNIGHT AS HIGH
PRESSURE CONTINUES TO BUILD TO OUR NORTH...RESULTING IN A SLIGHTLY
ENHANCED PRESSURE GRADIENT OVER THE WATERS. SEAS WILL RANGE BETWEEN
2-3 FT...MAINLY DUE TO LONG PERIOD SWELL. CHANCES OF SHOWERS AND/OR
THUNDERSTORMS WILL BE POSSIBLE OVER THE WATERS TONIGHT AS H5
SHORTWAVES ROUND THE BASE OF A MID LVL TROUGH OF LOW PRESSURE
CENTERED TO OUR NORTH/NORTHEAST.

FRIDAY THROUGH TUESDAY...WITH HIGH PRESSURE POSITIONED TO THE NORTH
OF THE AREA AND A WEAK AREA OF LOW PRESSURE DEVELOPING TO THE EAST A
TIGHTENING PRESSURE GRADIENT WILL RESULT IN STRENGTHENING
EAST/NORTHEAST FLOW THROUGH AT LEAST SATURDAY NIGHT. IN FACT...AS
EARLY AS FRIDAY NIGHT CONDITIONS COULD BECOME SUPPORTIVE OF SMALL
CRAFT ADVISORIES THANKS TO THE INCREASING WINDS AND BUILDING SEAS.
THE STRONGEST PERIOD FOR WINDS/SEAS WILL LIKELY BE SATURDAY BEFORE
SUBSIDING SATURDAY NIGHT THROUGH SUNDAY AS THE AFOREMENTIONED AREA
OF LOW PRESSURE MOVES NORTHEASTWARD AND AWAY FROM THE AREA.
THEREAFTER...CONDITIONS WILL BECOME MUCH QUIETER EARLY NEXT WEEK. A
WEAK COLD FRONT WILL MOVE THROUGH ON MONDAY FOLLOWED BY HIGH
PRESSURE INTO TUESDAY.

RIP CURRENTS...THE TIGHTENING PRESSURE GRADIENT AND RESULTING
INCREASE IN EAST/NORTHEAST WINDS WILL RESULT IN AN ELEVATED RISK FOR
RIP CURRENTS AT AREAS BEACHES FRIDAY AND SATURDAY.

&&

.TIDES/COASTAL FLOODING...
WITH A TIGHTENING PRESSURE GRADIENT EXPECTED TO SUPPORT INCREASING
EAST/NORTHEAST FLOW FRIDAY AND SATURDAY...INCREASED TIDAL DEPARTURES
ARE ANTICIPATED. IN FACT...DEPARTURES MAY BE HIGH ENOUGH TO SUPPORT
TIDE LEVELS APPROACHING SHALLOW COASTAL FLOODING THRESHOLDS WITH THE
EVENING HIGH TIDES THIS WEEKEND. SATURDAY WILL LIKELY BE CLOSEST AS
THAT IS WHEN THE STRONGEST WINDS ARE EXPECTED. TO REACH COASTAL
FLOOD ADVISORY THRESHOLDS...DEPARTURES WOULD HAVE TO ABOUT 1.2 FEET
WHICH IS PRETTY SIGNIFICANT. DUE TO THE MARGINAL NATURE OF THE
POTENTIAL...NO MENTION WILL BE ADDED TO THE HWO AT THIS TIME.

&&

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

&&

$$

NEAR TERM...DPB
SHORT TERM...BSH
LONG TERM...WMS
AVIATION...BSH/DPB
MARINE...BSH/DPB
TIDES/COASTAL FLOODING...BSH






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