Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 180519
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
119 AM EDT FRI APR 18 2014

.SYNOPSIS...
HIGH PRESSURE TO THE NORTH WILL SLOWLY WEAKEN THROUGH FRIDAY.
MEANWHILE A LOW PRESSURE SYSTEM WILL MOVE OUT OF THE GULF OF MEXICO
TO A POSITION OFF THE SOUTH CAROLINA COAST FRIDAY NIGHT AND SATURDAY
BEFORE SLOWLY MOVING EAST NORTHEAST AWAY FROM THE AREA. HIGH
PRESSURE WILL BUILD BACK IN EARLY NEXT WEEK. A WEAK COLD FRONT MAY
APPROACH THE AREA FROM THE NORTH BY MID WEEK.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
FORECAST ON TRACK OVERNIGHT WITH ONLY MINOR TWEAKS ON THE MIDNIGHT
UPDATE PACKAGE. HIGH CLOUDS WERE THICKENING AND SPREADING NE OVER
COASTAL CAROLINA PER LATEST SATELLITE IMAGERY...IN LINE WITH GENERAL
SKY COVER TRENDS.

HIGH PRESSURE WILL WEAKEN OVER THE AREA TONIGHT AS LOW PRESSURE
BEGINS TO TAKE SHAPE OVER THE FAR SOUTHERN GULF OF MEXICO. VERY
WEAK ISENTROPIC LIFT HAS BEEN SUPPORTING A DECK OF CLOUDS AROUND
6 KFT ACROSS MUCH OF SOUTHEAST GEORGIA TODAY WHILE MOSTLY SUNNY
SKIES HAVE PREVAILED ACROSS MOST OF SOUTHEAST SOUTH CAROLINA.
ISENTROPIC ASSENT SLOWLY INCREASE OVERNIGHT AND SPREAD NORTH...
BUT CONDENSATION PRESSURE DEFICITS ARE PROGGED TO REMAIN ABOVE 50
BM UNTIL LATE. THEREFORE...MOST OF THE ASSENT WILL GO TOWARDS
CLOUD GENERATION VERSUS LIGHT RAIN. EXPECT MOSTLY CLOUDY SKIES TO
DOMINATE ALL AREAS BY SUNRISE FRIDAY...ALTHOUGH THE MODELS MAY BE
A LITTLE TOO AGGRESSIVE IN MOVING LOW CLOUDS INTO AREAS NORTH OF
HAMPTON-BEAUFORT LINE BY LATE EVENING. ADJUST SKY COVER TO MATCH
EXPECTED TRENDS. CUT BACK ON GRIDDED POPS JUST A BIT ACROSS THE
NORTH WHERE MEASURABLE RAIN IS NOT EXPECTED TO REACH UNTIL LATE
FRIDAY MORNING...BUT DID MAINTAIN 20 PERCENT POPS FOR THE GEORGIA
COASTAL COUNTIES.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH SUNDAY/...
A POWERFUL UPPER LOW WILL MOVE ALONG THE GULF COAST REGION ON FRIDAY
AS A SURFACE LOW PRESSURE SYSTEM MOVES INTO NORTHERN FLORIDA.
MEANWHILE WEAK HIGH PRESSURE WILL PERSIST NORTH OF THE AREA...
ALBEIT GRADUALLY RETREATING TO THE NNE. AS THE DEEP LAYERED LOW
PRESSURE MOVES CLOSER TO THE AREA...LOW-LEVEL FLOW WILL INCREASE OUT
OF THE SOUTHEAST WHILE MID-LEVELS BECOME SOUTHERLY. THE COLUMN WILL
RAPIDLY MOISTEN DURING THE DAY FRIDAY. PWATS ARE EXPECTED TO RANGE
FROM 1.6 TO 1.8 INCHES BY FRIDAY AFTERNOON. A 100 KT JET SHIFTING
INTO THE EASTERN GULF WILL SPREAD UPPER DIVERGENCE ACROSS THE
FORECAST AREA DURING THE AFTERNOON. MODERATE TO STRONG ISENTROPIC
LIFT WILL SPREAD ACROSS SOUTHEAST GA DURING THE MORNING AND THEN
INTO SOUTHERN SC DURING THE AFTERNOON. WE HAVE LIKELY TO CATEGORICAL
POPS FOR FRIDAY...HIGHEST TO THE SOUTH.

WE HAVE BEEN LEANING TOWARD THE GFS AND ECMWF WITH THIS SYSTEM
THOUGH THE ECMWF REMAINS A BIT SLOWER WITH THE LOW TRACK. THE
STRONGEST QC FORCING LOOKS TO PUSH NORTH THROUGH THE AREA FRIDAY
AFTERNOON AND EVENING BEFORE SHIFTING OUT. THIS WILL BE ACCOMPANIED
BY AMPLE FORCING AND A JUICY AIRMASS IN PLACE. THERE WILL BE A
THREAT FOR HEAVY RAINFALL WITH A WIDESPREAD 2-3 INCHES FORECAST BY
WPC. MORE DETAILS IN THE HYDROLOGY SECTION.

THE UPPER LOW WILL MOVE THROUGH ON SATURDAY...BECOMING NEARLY
VERTICALLY STACKED WITH THE SURFACE LOW AS THE SYSTEM MOVES SLOWLY
EAST INTO THE ATLANTIC. LIGHT PRECIP WILL LINGER THROUGH THE DAY DUE
TO ISENTROPIC ASCENT CAUSED BY MOISTURE WRAPPING AROUND THE WESTERN
SIDE OF THE LOW.

IMPROVING CONDITIONS EXPECTED SATURDAY NIGHT AND SUNDAY WITH SOME
POSSIBLE LATE DAY CLEARING.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
OVERALL THE LONG TERM PERIOD LOOKS QUIET FOR THE FORECAST AREA.
RIDGING ALOFT WILL MOVE IN FOR EARLY IN THE WEEK BEHIND THE
DEPARTING UPPER LOW. A WEAK TROUGH MAY MOVE THROUGH BY THE MID WEEK
TIME PERIOD. WEAK HIGH PRESSURE WILL SETTLE OVER THE REGION THROUGH
TUESDAY...WITH A WEAK COLD FRONT ATTEMPTING TO CROSS BY WEDNESDAY.
THERE MAY BE A FEW SHOWERS WITH THE FRONT...BUT POPS HAVE BEEN
LIMITED TO THE SLIGHT CHANCE RANGE. TEMPERATURES FEATURE A WARMING
TREND...REACHING AROUND THE 80 DEGREE MARK BY TUESDAY AND CONTINUING
THROUGH THE END OF THE FORECAST PERIOD.

&&

.AVIATION /05Z FRIDAY THROUGH TUESDAY/...
LOW CLOUDS WILL EXPAND AND SLOWLY MOVE NORTH LATE TONIGHT INTO
FRIDAY AS LOW PRESSURE APPROACHES FROM THE SOUTH. SUSPECT MODEL
GUIDANCE IS TOO QUICK IN BRINGING MVFR CIGS INTO BOTH TERMINALS
OVERNIGHT...ESPECIALLY AT KCHS. MAINTAINED A SLIGHTLY SLOWER
SOLUTION WITH MVFR CIGS REACHING KSAV PRIOR TO DAYBREAK. KEPT KCHS JUST
ABOVE MVFR FOR NOW. AN EXPANSIVE AREA OF RAIN WILL MOVE SOUTH-
NORTH DURING THE DAY FRIDAY. EXPECT PREVAILING RAINS TO REACH KSAV
BY MID AFTERNOON...ROUGHLY 18Z-19Z WITH PREVAILING RAINFALL NOT
REACHING KCHS UNTIL VERY LATE IN THE DAY. MVFR/IFR CONDITIONS
EXPECTED IN THE 00Z-06Z TIME FRAME SATURDAY...BOTH CIGS AND VSBYS
IN RAIN.

EXTENDED AVIATION OUTLOOK...CHANCES OF SUB-MVFR CONDITIONS ARE
INCREASING...ESPECIALLY BEGINNING FRIDAY NIGHT. THESE CONDITIONS
WILL LIKELY PERSIST THROUGH SATURDAY...IMPROVING SUNDAY AND INTO
EARLY NEXT WEEK.

&&

.MARINE...
TONIGHT...NASTY CONDITIONS WILL PERSIST OVER THE WATERS TONIGHT
WITH WINDS NE 20-25 KT WITH GUSTS TO 30 KT. SEAS WILL AVERAGE 5-7
FT NEARSHORE WATERS AND 7-9 FT OFFSHORE WATERS. OPTED TO EXTEND
FLAGS IN THE CHARLESTON HARBOR THROUGH THE NIGHT WITH GUSTS TO 25
KT LIKELY TO PERSIST.

FAIRLY STRONG NORTHEAST FLOW WILL CONTINUE FRIDAY INTO FRIDAY
EVENING BEFORE TEMPORARILY DIMINISHING AS THE SURFACE LOW MOVES
FAIRLY CLOSE TO THE WATERS AND THE STRONGEST GRADIENT SHIFTS NORTH
AND WEST. THEN A STRONG NORTHEAST GRADIENT REDEVELOPS SATURDAY NIGHT
INTO SUNDAY BEHIND THE LOW AS HIGH PRESSURE TRIES TO REBUILD TO THE
WEST. SOLID SMALL CRAFT CONDITIONS EXPECTED FOR MUCH OF THE PERIOD
FRIDAY THROUGH SUNDAY NIGHT WITH THE POSSIBILITY OF GALE CONDITIONS
OVER PORTIONS OF THE WATERS SATURDAY NIGHT AND SUNDAY. WILL MAINTAIN
ONGOING SMALL CRAFT ADVISORIES BUT DISCUSS GALE POTENTIAL IN THE
HAZARDOUS WEATHER OUTLOOK.

RIP CURRENTS...CONSIDERABLE UNCERTAINTY WITH REGARD TO SWELL ON
FRIDAY BUT WE MAINTAINED SIMILAR CONDITIONS TO TODAY WITH 3 FT 10
SEC SWELL. THIS MAINTAINS A MODERATE RISK FOR CHARLESTON AND HIGH
RISK FARTHER SOUTH. AN ENHANCED RISK FOR RIP CURRENTS WILL PERSIST
THROUGH THE WEEKEND AND PERHAPS INTO EARLY NEXT WEEK.

&&

.HYDROLOGY...
PLENTY OF MOISTURE AND FORCING FOR WIDESPREAD MODERATE TO HEAVY
RAINFALL FRIDAY AFTERNOON THROUGH FRIDAY NIGHT. LATEST WPC GUIDANCE
SHOWS 2-3 INCHES DURING THIS PERIOD WITH ISOLATED POCKETS OF 3-4
INCHES. THE LATEST FLASH FLOOD GUIDANCE SHOWS 6 HR VALUES GREATER
THAN 4 INCHES. SINCE WE HAVE NOT SEEN ANY RECENT TORRENTIAL
RAINFALL EVENTS MOST AREAS SHOULD EASILY BE ABLE TO HANDLE THE
EXPECTED RAINFALL. THE ONLY CONCERN WILL BE COASTAL AREAS DUE TO
THE TIDES RUNNING ABOVE NORMAL. THE FRIDAY LATE EVENING HIGH TIDE
COULD BE AT OR NEAR COASTAL FLOOD ADVISORY CRITERIA IN CHARLESTON
AND/OR SAVANNAH DUE TO THE NORTHEAST WINDS. THUS ANY HEAVY
RAINFALL ON TOP OF THE TIDE COULD RESULT IN SIGNIFICANT FLOODING
ISSUES IN URBAN COASTAL LOCATIONS. FOR NOW WE WILL DISCUSS IN THE
HAZARDOUS WEATHER OUTLOOK.

MAIN STEM RIVERS AND ASSOCIATED TRIBUTARIES WILL LIKELY SEE RISES
WITH THE POTENTIAL FOR MINOR FLOODING DEPENDING ON OBSERVED RAINFALL
AMOUNTS.

&&

.TIDES/COASTAL FLOODING...
TIDES WILL CONTINUE TO RUN HIGH THROUGH THE WEEKEND AND COASTAL
FLOOD ADVISORIES COULD BE NEEDED DURING THE NIGHTTIME HIGH TIDE
CYCLES.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...HIGH RISK FOR RIP CURRENTS THROUGH THIS EVENING FOR GAZ117-119-
     139-141.
SC...HIGH RISK FOR RIP CURRENTS THROUGH THIS EVENING FOR SCZ048-049-
     051.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 AM EDT MONDAY FOR AMZ350-352.
     SMALL CRAFT ADVISORY UNTIL 6 AM EDT MONDAY FOR AMZ330-354.
     SMALL CRAFT ADVISORY UNTIL 5 AM EDT TUESDAY FOR AMZ374.

&&

$$

SYNOPSIS...
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
HYDROLOGY...
TIDES/COASTAL FLOODING...






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