Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 280850
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
450 AM EDT FRI AUG 28 2015

.SYNOPSIS...
A STATIONARY FRONT WILL DISSIPATE NEAR THE COAST TODAY. ATLANTIC
HIGH PRESSURE WILL REMAIN IN PLACE INTO EARLY NEXT WEEK. TROPICAL
CYCLONE ERIKA COULD IMPACT THE SOUTHEAST UNITED STATES BY THE
MIDDLE OF NEXT WEEK. REFER TO THE LATEST ADVISORY ON ERIKA ISSUED
BY THE NATIONAL HURRICANE CENTER.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A DECAYING STATIONARY FRONT WILL EXTEND FROM THE SOUTH CAROLINA
COASTAL WATERS INTO THE I-95 CORRIDOR OF SOUTHEAST GEORGIA TODAY
AS HIGH PRESSURE TRIES TO NUDGE IN FROM THE NORTH. MODELS ARE
SIMILAR IN SHOWING THE BETTER CHANCES FOR SHOWERS/TSTMS FOCUSING
ROUGHLY ALONG/EAST OF A GLENNVILLE-SAVANNAH-HILTON HEAD LINE TODAY
WHERE THE SOUTHWEST-NORTHEAST ORIENTED RESULTANT SEA BREEZE WILL
INTERSECT THE DECAYING STATIONARY FRONT. THIS ENHANCEMENT TO THE
LOW-LEVEL CONVERGENCE FIELD COMBINED WITH MODEST INSTABILITY AND
WEAK UVM INDUCED BY PASSING IMPULSES OF MID-LEVEL VORTICITY
EJECTING NORTHEAST AHEAD AHEAD OF A PRONOUNCED NORTH-SOUTH SHEAR
AXIS OVER THE NORTHERN GULF/SOUTHERN MISSISSIPPI VALLEY WILL
SUPPORT ISOLATED TO SCATTERED SHOWERS/TSTMS.

20-40 PERCENT POPS LOOK REASONABLE TODAY GIVEN THE SYNOPTIC
SETUP. IT IS VERY POSSIBLE COMMUNITIES FROM ALLENDALE TO THE
CHARLESTON METRO AREA AND POINTS NORTH WILL SEE LITTLE TO NO
CONVECTION AS THE ATMOSPHERE LOOKS TO BECOME INCREASING CAPPED IN
RESPONSE TO LOWERING 700 HPA THETA-E ASSOCIATED WITH THE SOUTHERN
FRINGE OF A SHORTWAVE RIDGE POSITIONED OVER CENTRAL/EASTERN NORTH
CAROLINA AND THE VIRGINIA TIDEWATER. HIGHS WILL RANGE FROM UPPER
80S TO AROUND 90 INLAND TO THE MID 80S AT THE BEACHES.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/...
TONIGHT...CONVECTION WILL DISSIPATE QUICKLY THIS EVENING. ISOLATED
TO PERHAPS SCATTERED SHOWERS/TSTMS COULD RE-FIRE OVER THE ATLANTIC
AS LOW-LEVEL CONVERGENCE INCREASES ALONG THE WESTERN WALL OF THE
GULF STREAM. WITH AND EAST/NORTHEAST LOW-LEVEL FLOW IN
PLACE...SOME OF THIS ACTIVITY COULD MAKE A RUN FOR THE
BEACHES...ESPECIALLY FROM HILTON HEAD SOUTH TO DARIEN AND SAPELO
ISLAND GIVEN THE ORIENTATION OF THE COASTLINE. 20-30 PERCENT POPS
WILL BE MAINTAINED OVERNIGHT AT THE COAST TO ACCOUNT FOR THIS WITH
DRY CONDITIONS INLAND. LOWS WILL RANGE FROM THE LOWER 70S INLAND
WITH MID-UPPER 70S AT THE BEACHES.

SATURDAY INTO SATURDAY NIGHT...28/00Z GUIDANCE SUGGESTS THAT DRIER
AIR WILL PUSH INTO NORTHERN COUNTIES OF THE FORECAST AREA...WHILE
THE UPPER LOW CENTERED WEST OF THE REGION WILL CONTINUE TO PUSH
MOISTURE INTO SOUTHERN COUNTIES OF THE FORECAST AREA. THIS SCENARIO
COULD SUPPORT A FAIRLY SIGNIFICANT NORTH TO SOUTH MOISTURE GRADIENT.
ACCORDINGLY...MAXIMUM POPS ARE GRADUATED FROM SLIGHT CHANCE NORTH TO
HIGH CHANCE SOUTH...AND LOCALLY HIGHER POPS COULD EVENTUALLY BE
REQUIRED ACROSS THE SOUTH. PWATS AROUND 2 INCHES AND WEAK STEERING
FLOW COULD SUPPORT LOCALLY HEAVY RAIN ACROSS THE SOUTHERN 1/2 OF THE
FORECAST AREA.

SUNDAY INTO MONDAY...THE UPPER LOW SHOULD OPEN AND GRADUALLY EJECT
NORTHEAST AS AN OPEN WAVE...PERHAPS BREAKING DOWN INTO MULTIPLE
SHORTWAVE TROUGHS WHICH COULD CROSS THE REGION. THIS SCENARIO SHOULD
SUPPORT SCATTERED TO NUMEROUS SHOWERS/THUNDERSTORMS. SUNDAY...THE
ONGOING NORTH TO SOUTH MOISTURE GRADIENT COULD PERSIST...THIS POPS
ARE TAPERED FROM LIKELY ACROSS THE SOUTH TO CHANCE NORTH.
MONDAY...THE INCREASINGLY ILL-DEFINED MOISTURE GRADIENT SHIFTS TO A
WEST/NORTHWEST TO EAST/SOUTHEAST ORIENTATION...AND POPS ARE
DISTRIBUTED ACCORDINGLY WITH LIKELY POPS ACROSS CENTRAL/SOUTHERN
COASTAL COUNTIES AND ADJACENT COASTAL WATERS.

NEAR NORMAL TEMPERATURES ARE EXPECTED THROUGH THE PERIOD...ALTHOUGH
LOCALLY COOLER DAYTIME HIGH TEMPS IN THE LOWER/MIDDLE 80S CANNOT BE
RULED OUT WHERE PRECIPITATION PERSISTS.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
MANY FORECAST DETAILS DEPEND ON THE HIGHLY UNCERTAIN TRACK AND
STRENGTH OF TROPICAL STORM ERIKA NEXT WEEK. AS OF 5 AM EDT...THE
NATIONAL HURRICANE CENTER FORECAST SHIFTED A WEAKER SYSTEM FARTHER
WEST...RESULTING IN A LANDFALL OVER SOUTHERN FLORIDA AND NOTHING
STRONGER THAN A TROPICAL STORM TRACKING NORTH OVER LAND THROUGH
EARLY NEXT WEDNESDAY MORNING. ALSO OF NOTE...MOST INVEST TRACKS HAVE
ALSO SHIFTED FARTHER WEST...AND OF PARTICULAR INTEREST 28/00Z
OPERATIONAL RUNS OF THE GFS AND EUROPEAN DEPICT AN EVEN WEAKER
SYSTEM PROGRESSING FARTHER W/NW...PUSHING INTO THE GULF OF MEXICO
THEN PERHAPS COMING ASHORE ALONG THE NORTHEAST GULF COAST. THE
LATEST FORECAST STILL OFFERS THE POTENTIAL FOR SOME IMPACTS
INCLUDING HEAVY RAIN, GUSTY WINDS, MARINE HAZARDS, SHALLOW COASTAL
FLOODING AND EVEN A THREAT FOR TORNADOES AROUND THE MIDDLE OF NEXT
WEEK. THE GFS/EURO SCENARIO COULD BE ACCOMPANIED BY AN INFLUX OF
ENHANCED TROPICAL MOISTURE FROM THE SOUTH/SOUTHEAST BUT OTHERWISE
SUGGESTS NO TROPICAL CYCLONE IMPACTS IN OUR FORECAST AREA. ASIDE
FROM LOWERING WIND SPEEDS OVER LAND...SIGNIFICANT CHANGES WERE NOT
REQUIRED FOR ONGOING WEATHER/POPS GRIDS THROUGH THIS PERIOD.

PLEASE CONTINUE TO MONITOR THE LATEST FORECASTS FROM THE NATIONAL
HURRICANE CENTER AND OUR OFFICE CONCERNING ERIKA.

&&

.AVIATION /09Z FRIDAY THROUGH TUESDAY/...
STRATUS HAS BECOME MUCH MORE WIDESPREAD THAN WHAT WAS EXPECTED
EARLIER. GOES-EAST FOG PRODUCT SHOWS STRATUS RAPIDLY EXPANDING
WEST AND SOUTH FROM KCHS AND MAY APPROACH KSAV PRIOR TO SUNRISE.
HAVE AMENDED KCHS TO INCLUDE IFR CIGS THROUGH ABOUT 12Z...BUT RAP
SHOWS SOME DRY AIR FILTERING SOUTH...WHICH MAY ERODE THE STRATUS
BEFORE THEN. MAY VERY WELL NEED TO AMEND KSAV SOON TO INCLUDE IFR
CIGS IF SATELLITE TRENDS POINT THAT WAY. VFR SHOULD RETURN 13-15Z.
MAY NEED TO CONSIDER ADDING TSRA TO THE KSAV TAF WITH THE 12Z TAF
CYCLE.

EXTENDED AVIATION OUTLOOK...THIS WEEKEND INTO EARLY NEXT WEEK...
FLIGHT RESTRICTIONS COULD OCCUR WITH SHOWERS/THUNDERSTORMS AND
IN LATE NIGHT/EARLY MORNING STRATUS/FOG. OTHERWISE...MAINLY VFR.
TUESDAY AND BEYOND...CONDITIONS REMAIN HIGHLY DEPENDENT ON THE
UNCERTAIN TRACK AND INTENSITY OF TROPICAL CYCLONE ERIKA.

&&

.MARINE...
TODAY...A BIT OF AN EAST TO NORTHEAST WIND SURGE WILL WORK DOWN
THE COAST TODAY. THE STRENGTH OF SURGE IS UNCERTAIN WITH THE
LATEST NAM SHOWING WINDS INCREASING TO 15-20 KT FOR THE SOUTH
CAROLINA COASTAL WATERS. THIS IS HIGHER THAN ANY GUIDANCE AND IS
LIKELY DUE TO THE MODEL/S DEPICTION OF A STRENGTHENING PRESSURE
GRADIENT INDUCED BY DEVELOPING LOW PRESSURE WELL OFFSHORE. IT IS
UNCLEAR WHETHER THIS LOW WILL DEVELOP OR NOT...BUT THE NAM IS
REALLY THE ONLY MODEL SHOWING THIS SIGNAL. WILL THEREFORE DISCOUNT
THIS SOLUTION AND BLEND THE VARIOUS SHORT TERM MODELS TO CONSTRUCT
WINDS THROUGH THIS AFTERNOON. THIS YIELDS SPEEDS AROUND 15 KT
CHARLESTON COUNTY WATERS...10-15 KT BEAUFORT COUNTY WATERS WITH
AROUND 10 KT FOR THE GEORGIA WATERS. SEAS WILL AVERAGE 2-4 FT.

TONIGHT...THE LOW-LEVEL WIND FIELD WILL RELAX A BIT OVERNIGHT WITH
SPEEDS SETTLING IN AROUND 10 KT FOR MUCH OF THE NIGHT OUT OF THE
EAST OR NORTHEAST. SEAS WILL AVERAGE 2-4 FT.

SATURDAY THROUGH MONDAY...ON THE PERIPHERY OF ATLANTIC HIGH PRESSURE
E/NE WINDS DURING THE LATE NIGHT/MORNING HOURS AND E/SE WINDS DURING
THE AFTERNOON/EVENING HOURS WILL AVERAGE NO MORE THAN 10-15 KNOTS.
SEAS OF 2-4 FT SATURDAY WILL GRADUALLY BUILD MAINLY DUE TO
INCREASING LONG PERIOD SWELL FROM THE EAST/SOUTHEAST...AND BY LATE
SUNDAY THROUGH MONDAY 6-7 FT SEAS COULD PUSH INTO AMZ374...WITH 4-5
FT SEAS SPREADING INTO NEARSHORE WATERS.

MONDAY NIGHT THROUGH MIDWEEK...WINDS/SEAS WILL DEPEND ON THE
UNCERTAIN TRACK AND INTENSITY OF TROPICAL STORM ERIKA. BASED ON THE
LATEST FORECAST TRACK PUSHING A WEAKER SYSTEM INLAND OVER THE
FLORIDA PENINSULA...LOWERED WINDS/SEAS OVER COASTAL WATERS DURING
THIS PERIOD. HOWEVER...WINDS/SEAS COULD STILL BECOME HAZARDOUS FOR
SMALL CRAFT.

PLEASE STAY TUNED TO THE LATEST FORECASTS FROM THE NATIONAL
HURRICANE CENTER AND OUR OFFICE CONCERNING ERIKA.

RIP CURRENTS...SLOWLY BUILDING EAST/SOUTHEAST SWELL COMBINED WITH
INCREASING COASTAL WINDS WILL SUPPORT A MODERATE RISK FOR RIP
CURRENTS FOR ALL BEACHES TODAY. A PERSISTENT LONG PERIOD SWELL...
ONSHORE WINDS AND ASTRONOMICAL INFLUENCES WILL LIKELY SUPPORT AN
ELEVATED RISK OF RIP CURRENTS THIS WEEKEND. IF TROPICAL CYCLONE
ERIKA APPROACHES THE REGION NEXT WEEK...DANGEROUS RIP CURRENTS
COULD DEVELOP ON AREA BEACHES.

&&

.TIDES/COASTAL FLOODING...
THIS EVENING...EXPECT POSITIVE TIDAL ANOMALIES TO INCREASE TODAY
AS STRENGTHENING EAST TO NORTHEAST WINDS HELP TO PILE WATER INTO
THE SOUTH CAROLINA AND GEORGIA COAST. EARLY MORNING SURGE GUIDANCE
BRINGS TIDE LEVELS TO SHALLOW COASTAL FLOODING THRESHOLDS AT BOTH
THE CHARLESTON HARBOR AND FORT PULASKI TIDE GAGES...BUT THE
FORECASTED LEVELS LOOK A BIT TOO HIGH GIVEN PREVIOUS TIDAL TRENDS
AND EXPECTED WIND SPEEDS. EVEN AFTER COMPENSATING FOR THIS...TIDE
LEVELS AT THE CHARLESTON HARBOR ARE STILL EXPECTED TO PEAK 7.0-7.2
FT MLLW SO WILL GO AHEAD AND HOIST A COASTAL FLOOD ADVISORY FOR
ALL OF THE SOUTH CAROLINA COASTAL ZONES...VALID FROM 6PM-10PM
TONIGHT. LEVELS AT FORT PULASKI ARE EXPECTED TO ONLY PEAK 9.0-9.2
FT MLLW...WHICH IS AT OR JUST BELOW COASTAL FLOOD ADVISORY
CRITERIA FOR COASTAL GEORGIA. CONFIDENCE IS THEREFORE NOT HIGH
ENOUGH TO JUSTIFY THE ISSUANCE OF A COASTAL FLOOD ADVISORY FOR ANY
PORTION OF THE GEORGIA COAST WITH THIS FORECAST CYCLE. TIDE TRENDS
WILL BE MONITORED CAREFULLY THROUGHOUT THE DAY.

SATURDAY THROUGH MONDAY...ASTRONOMICAL INFLUENCES AND ONSHORE WINDS
WILL MAINTAIN ELEVATED HIGH TIDE LEVELS. THE EVENING HIGH TIDES ARE
FORECAST TO EXCEED SHALLOW COASTAL FLOODING LEVELS ON CHARLESTON
HARBOR AND SHOULD AT LEAST APPROACH COASTAL FLOOD ADVISORY
THRESHOLDS FARTHER SOUTH INCLUDING THE GEORGIA COAST. ON CHARLESTON
HARBOR...EVEN THE MORNING HIGH TIDES ARE FORECAST TO APPROACH 7 FEET
MEAN LOWER LOW WATER EACH DAY.

TUESDAY THROUGH WEDNESDAY/THURSDAY...ASTRONOMICAL INFLUENCES WILL
WANE. HOWEVER...IF THE LATEST FORECAST FROM NHC FOR ERIKA HOLDS...WE
WILL HAVE THE POTENTIAL FOR MORE SIGNIFICANT RISES IN THE TIDES
ALONG WITH SOME DEGREE OF COASTAL FLOODING.

WITH REGARDS TO POTENTIAL CONDITIONS NEXT WEEK...PLEASE STAY TUNED
TO THE LATEST FORECASTS FROM THE NATIONAL HURRICANE CENTER AND OUR
OFFICE CONCERNING ERIKA.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...COASTAL FLOOD ADVISORY FROM 6 PM TO 10 PM EDT THIS EVENING FOR
     SCZ048>051.
MARINE...NONE.

&&

$$

ST/SPR


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