Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 122217
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
617 PM EDT SAT JUL 12 2014

.SYNOPSIS...
A WEAK TROUGH OF LOW PRESSURE ALONG THE COAST WILL GRADUALLY DISSIPATE
OVERNIGHT. ATLANTIC HIGH PRESSURE WILL PREVAIL SUNDAY INTO MONDAY...THEN
A COLD FRONT WILL SLOWLY DROP SOUTHEAST AND STALL ACROSS THE AREA MID TO
LATE NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
THIS EVENING...SHOWER/TSTM ACTIVITY IS ON THE WANE THIS EVENING
AS MUCH OF THE AREA HAS PRETTY WELL BEEN WORKED OVER. SBCAPE HAS
DIMINISHED TO 1000-1500 J/KG WITH LIFTED INDICES DOWN TO -2 TO
-3C. ALTHOUGH THERE IS STILL ANOTHER 1-2 HOURS OF SOLID INSOLATION
LIKELY PRIOR TO SUNSET...EXPECT ONLY VERY MINOR REDEVELOPMENT OVER
THE NEXT FEW HOURS. ADJUSTED NEAR TERM POPS TO MATCH GOING TRENDS
WITH THE HIGHEST GRIDDED POPS...UP TO 30 PERCENT...CONFINED TO FAR
INTERIOR SOUTHEAST GEORGIA.

OVERNIGHT...WATER VAPOR IMAGERY SHOWS A PRONOUNCED RIBBON OF
TROPICAL MOISTURE EXTENDING FROM THE EASTERN GULF OF MEXICO INTO
SOUTHERN SOUTH CAROLINA. SOME DRIER AIR IS TRYING TO NUDGE IN FROM
THE NORTH...BUT ITS BEING HELD AT BAY BY A WEAK 700 HPA VORTICITY
CENTER SEEN SPINNING OVER MCINTOSH COUNTY GEORGIA. MODELS SHOW
THIS FEATURE PROPAGATING EVER SLOW SLIGHTLY TO THE NORTHEAST
OVERNIGHT WITHIN A SHARP MID-LEVEL SHEAR AXIS. THIS SHOULD KEEP
MOST OF THE AREA EMBEDDED IN THE RIBBON OF TROPICAL MOISTURE
THROUGH THE NIGHT...BEFORE THE VORTICITY CENTER EXITS OFF THE
COAST INTO THE OPEN ATLANTIC AFTER SUNRISE SUNDAY. WEAK UVM
ASSOCIATED THE VORTICITY MAXIMUM COUPLED WITH INCREASING SURFACE
MOISTURE CONVERGENCE OVER THE ATLANTIC AND COASTAL COUNTIES
INDUCED BY AN INLAND DECOUPLED BOUNDARY LAYER SHOULD RESULT IN
ANOTHER NOCTURNAL SURGE IN CONVECTION EARLY SUNDAY MORNING.
ADJUSTED POPS UP SLIGHTLY TO 30-40 PERCENT ACROSS THE EASTERN
HALF OF THE FORECAST AREA PER OUTPUT FROM THE LATEST RUNS OF THE
H3R...RAP AND 4KM-WRF WHICH ALL SHOW SCATTERED SHOWERS/TSTMS
MOVING ONSHORE LATE TONIGHT. HIGHER POPS AND QPF MAY EVENTUALLY BE
NEEDED...BUT ITS HARD TO PIN THAT DOWN THIS FAR OUT. LOWS WILL
RANGE FROM THE LOWER 70S WELL INLAND TO THE MID-UPPER 70S AT THE
COAST AND IN DOWNTOWN CHARLESTON.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
SUNDAY...WITH SOME WEAK SURFACE CONVERGENCE STILL EXPECTED JUST OFF
THE COAST...I DO EXPECT SCATTERED CONVECTION THAT WILL BE OVER THE
COASTAL WATERS TO AGAIN HAVE THE POTENTIAL OF BRUSHING COASTAL AREAS
DURING THE MORNING...SO THE HIGHEST POPS ARE FOR THE BEACHES. THE
POPS WILL DECREASE AS ONE GOES INLAND WITH A LITTLE DRIER AIR...BUT
THERE STILL COULD BE SOME ISOLATED AFTERNOON CONVECTION INLAND AS
THE SEA BREEZE WORKS ITS WAY WEST/NORTHWESTWARD DURING THE
AFTERNOON. WITH LESS CLOUD COVER EXPECTED...TEMPS WILL WARM BACK UP
INTO THE 90S EVERYWHERE AWAY FROM THE COAST.

WINDS WILL START OUT VERY LIGHT OVER LAND...BUT A STEADY SOUTHEAST
WIND WILL DEVELOP DURING THE AFTERNOON AS THE SURFACE RIDGE SLIDES
OUT INTO THE WESTERN ATLANTIC. WINDS WILL BE STRONGEST NEAR THE
COAST IN THE AFTERNOON AS USUAL WITH THE WEAK GRADIENT WIND ENHANCED
BY THE SEA BREEZE.

ANY CONVECTION THAT DEVELOPS OVER LAND IN THE AFTERNOON INTO THE
EARLY EVENING SHOULD DISSIPATE NOT LONG AFTER SUNSET DUE TO THE LACK
OF ANY FORCING.

MONDAY...THE BEGINNING OF THE WEEK WILL START VERY WARM AND MAINLY
DRY...ALTHOUGH AGAIN AN ISOLATED THUNDERSTORM CANNOT BE RULED OUT
DURING THE AFTERNOON AND EVENING HOURS...ESPECIALLY OVER INTERIOR
GEORGIA WHERE A SUBTLE VORT MAX AT MID LEVELS MAY SLIGHTLY ENHANCE
THE POTENTIAL FOR UPWARD VERTICAL MOTION. TEMPS WILL BE INTO THE MD
TO EVEN UPPER 90S IN A FEW PLACES AWAY FROM THE COAST AS 925 MB
TEMPS HEAT UP TO GREATER THAN 25C. THE HEAT WILL ALSO BE HELPED BY A
SLIGHT INCREASE IN THE PRESSURE GRADIENT DUE TO A DEVELOPING LEE
TROF IN THE AFTERNOON...MEANING A SLOWER ADVANCE OF THE SEA BREEZE.

TUESDAY...ALL THE MODELS AGREE THAT AN UNUSUALLY STRONG UPPER LEVEL
LOW WILL DIVE INTO THE GREAT LAKES REGION ON TUESDAY...LOWERING
HEIGHTS ACROSS ALMOST THE ENTIRE EASTERN HALF OF THE COUNTRY.
HOWEVER...THERE ARE SOME PRETTY IMPORTANT DETAILS THAT ARE
DIFFERENT...SUCH AS THE DEPTH AND SHAPE OF THE RESULTING UPPER TROF.
THEREFORE...I HAVE NOT MADE MUCH CHANGE TO THE PREVIOUS FORECAST WITH
INCREASING CHANCES FOR THUNDERSTORMS ACROSS THE AREA ALONG WITH A
SIGNIFICANT AMOUNT OF CLOUD COVER. IT IS POSSIBLE THAT OUR CURRENT
POPS ARE A BIT OVERDONE AND THAT MOST OF THE CONVECTION WONT GET
INTO THE AREA UNTIL LATE TUESDAY OR EVEN TUESDAY NIGHT.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
MUCH OF THE PERIOD WILL FEATURE ABOVE NORMAL RAIN CHANCES AND NEAR
TO SLIGHTLY BELOW NORMAL TEMPERATURES. AN UNSEASONABLY STRONG AND
DEEP LONGWAVE TROUGH WILL REMAIN ACROSS THE CENTRAL AND EASTERN
UNITED STATES TUESDAY NIGHT THROUGH THURSDAY BEFORE EVENTUALLY
FLATTENING OUT. A COLD FRONT WILL BE STALLED OVER THE AREA THROUGH
THURSDAY MORNING...POSSIBLY MOVING OFF THE COAST THURSDAY AFTERNOON.
THERE REMAIN CONSIDERABLE DIFFERENCES BETWEEN THE GFS AND ECMWF
REGARDING PRECIPITATION POTENTIAL THURSDAY AND BEYOND...THOUGH
TUESDAY NIGHT INTO WEDNESDAY WILL LIKELY FEATURE NUMEROUS SHOWERS
AND TSTMS.

&&

.AVIATION /22Z SATURDAY THROUGH THURSDAY/...
TO BE ISSUED.

EXTENDED AVIATION OUTLOOK...BRIEF FLIGHT RESTRICTIONS POSSIBLE
DUE MAINLY TO AFTERNOON SHOWERS/THUNDERSTORMS WITH THE GREATEST
COVERAGE TUESDAY INTO WEDNESDAY.

&&

.MARINE...
TONIGHT...THE LOCAL WATERS WILL IN A WEAK FLOW REGIME FOR MUCH OF
THE OVERNIGHT. EXPECT SOUTH TO SOUTHEAST FLOW 10 KT OR LESS WITH
SEAS 1-2 FT WITHIN 20 NM AND 2-3 FT BEYOND. ANOTHER ROUND OF
NOCTURNAL SHOWERS AND THUNDERSTORMS IS EXPECTED TO FLARE UP LATE
TONIGHT ACROSS THE WATERS. ALSO...WITHIN THE WEAK FLOW REGIME THE
SETUP WILL BE FAVORABLE FOR WATERSPOUT DEVELOPMENT AS SUNRISE
APPROACHES.

SUNDAY MORNING COULD AGAIN SEE CONDITIONS FAVORABLE FOR A FEW
WATERSPOUTS IN THE COASTAL WATERS AND WIND FIELDS WILL BE GENERALLY
LIGHT AND SCATTERED CONVECTION IS EXPECTED.

ATLANTIC HIGH PRESSURE WILL PREVAIL OVER THE WATERS SUNDAY THROUGH
FRIDAY. A COLD FRONT APPROACHING FROM THE NORTHWEST TUESDAY AND
WEDNESDAY WILL TIGHTEN THE PRESSURE GRADIENT...PUSHING WINDS INTO
THE 15-20 KT RANGE AT TIMES. BY THURSDAY THE FRONT WILL BE WEAKENING
OVER INLAND AREAS...ALLOWING WINDS TO DROP BACK TO MORE TYPICAL JULY
VALUES OF 10-15 KTS. THE DAYTIME SEA BREEZE WILL BE PRETTY STRONG
MONDAY AND TUESDAY.

&&

.TIDES/COASTAL FLOODING...
EXPANDED THE COASTAL FLOOD ADVISORY TO INCLUDE THE ENTIRE LOWER
SOUTH CAROLINA COAST AS SOME SHALLOW COASTAL FLOODING IS LIKELY TO
OCCUR FARTHER DOWN THE COAST DUE TO ONGOING TIDAL LOADING OVER THE
PAST FEW HIGH TIDE CYCLES. PEAK TIDE IN THE CHARLESTON HARBOR IS
STILL EXPECTED TO BE 7.0-7.2 FT MLLW.

AS FOR THE GEORGIA COAST...THE ONGOING TIDE GAGE OUTAGE AT FORT
PULASKI IS MAKING IT DIFFICULT TO ASSESS THE RISK FOR COASTAL FLOODING
ALONG THE UPPER GEORGIA COAST. EXTRATROPICAL STORM SURGE DATA IS
USELESS WITHOUT CURRENT TIDE DATA...BUT SINCE ONLY A MARGINAL
EVENT IS EXPECTED IN THE CHARLESTON HARBOR...THE CURRENT THINKING
IS THAT TIDES WILL LIKELY PEAK BELOW COASTAL FLOOD ADVISORY
THRESHOLDS FROM CHATHAM COUNTY SOUTH TO MCINTOSH COUNTY.

ASTRONOMICAL HIGH TIDES DUE TO THE FULL MOON/PERIGEE AND LIGHT TO
MODERATE ONSHORE WINDS COULD RESULT IN MINOR COASTAL FLOODING NEAR
THE TIMES OF HIGH TIDE DURING THE EVENING HOURS SUNDAY THROUGH
EARLY NEXT WEEK.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...COASTAL FLOOD ADVISORY UNTIL 11 PM EDT THIS EVENING FOR
     SCZ048>051.
MARINE...NONE.

&&

$$

ST





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