Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201756
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
156 PM EDT SAT SEP 20 2014

.SYNOPSIS...
LOW PRESSURE OFF THE FLORIDA COAST THIS MORNING WILL LIFT NORTHEAST
THROUGH TONIGHT. HIGH PRESSURE WILL BRIEFLY BUILD OVER THE AREA ON
SUNDAY BEFORE A COLD FRONT DROPS THROUGH AND STALLS JUST SOUTH OF
THE AREA. A BROAD AREA OF HIGH PRESSURE WILL BUILD IN FROM THE NORTH
TUESDAY THROUGH FRIDAY WHILE A TROUGH OF LOW PRESSURE STRENGTHENS
OFF THE COAST.

&&

.NEAR TERM /THROUGH TONIGHT/...
EARLY THIS AFTERNOON...THE FORECAST REMAINS ON TRACK. THE DENSE
CLOUD SHIELD ASSOCIATED WITH THE WEAK SURFACE LOW TO THE SOUTH
CONTINUES TO COVER MUCH OF THE FORECAST AREA KEEPING TEMPERATURES
IN THE MID/UPPER 70S SO FAR. THIS HAS PREVENTED ANY SURFACE BASED
INSTABILITY AND AS A RESULT ALL THE PRECIPITATION HAS REMAINED
OFF THE COAST. UPDATED POPS TO MATCH CURRENT RADAR TRENDS AND
TIGHTEN THE GRADIENT ALONG THE COAST. OTHERWISE...NO NOTABLE
CHANGES.

TONIGHT...THE NON-TROPICAL COASTAL LOW SHOULD BEGIN TO SCOOT OFF ALONG
THE GULF STREAM...ACCELERATED BY A DIGGING UPPER TROUGH IN THE
GREAT LAKES. RAINS SHOULD GRADUALLY END ALONG THE COAST OVERNIGHT
WITH SURFACE WINDS BACKING TO A MORE NW COMPONENT LATE. THERE
COULD BE A BIT OF PATCHY FOG WELL INLAND BUT NOT EXPECTED TO BE
MUCH OF A CONCERN.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
THE UPPER LEVEL FLOW BECOMES BRIEFLY ZONAL ON SUNDAY WITH LOW-LEVEL
FLOW BECOMING SOUTHWESTERLY BY THE AFTERNOON. WNW FLOW AROUND 850 MB
WILL YIELD A DOWNSLOPE COMPONENT...WITH THE RESULTING SUBSIDENCE
GIVING US SUNNY SKIES AND SOME ADIABATIC WARMING. ASIDE FROM SOME
ISOLATED SHOWERS OVER THE COASTAL WATERS THAT WILL BE DIMINISHING AS
THE DAY WEARS ON...DRY CONDITIONS EXPECTED WITH HIGHS IN THE UPPER
80S.

ON MONDAY A SHORTWAVE TROUGH WILL DROP INTO NORTHERN GA/SC DURING
THE AFTERNOON...PUSHING A WEAK BACKDOOR COLD FRONT INTO THE AREA.
MOST OF THE GUIDANCE SHOWS A RELATIVELY MUTED VORT MAX...THOUGH THE
00Z ECMWF HAS A MUCH DEEPER TROUGH. FAIRLY DEEP WESTERLY FLOW AHEAD
OF THE FRONT WILL PRODUCE BROAD-SCALE SUBSIDENCE WITH HIGH TEMPS IN
THE UPPER 80S. THERE ARE SOME INDICATIONS THAT MOISTURE WILL POOL
AHEAD OF THE FRONT DURING THE AFTERNOON. AT LEAST ISOLATED SHOWERS
AND TSTMS EXPECTED...THOUGH IF ENOUGH INSTABILITY FORMS WE CANNOT
RULE OUT A FEW SEVERE TSTMS. FOR NOW WE MAINTAINED SLIGHT CHANCE
POPS MAINLY DURING THE AFTERNOON.

EXPANSIVE HIGH PRESSURE WILL BUILD IN FROM THE NORTH AND WEST MONDAY
NIGHT AND TUESDAY WITH THE FRONT STALLING ALONG THE SOUTHERN EDGE OF
THE FORECAST AREA. A COASTAL TROUGH AND SHARP MOISTURE GRADIENT WILL
LIKELY MAINTAIN SCATTERED SHOWERS AND TSTMS OVER THE COASTAL WATERS
AND IMMEDIATE COASTAL AREAS WHILE INLAND REMAINS DRY. COLD ADVECTION
WILL KEEP HIGH TEMPS IN THE LOWER 80S ON TUESDAY.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
THE EXTENDED PERIOD WILL BE DOMINATED BY A PERSISTENT WEDGE PATTERN
WITH COOL HIGH PRESSURE CONTINUING TO BUILD IN FROM THE NORTH WHILE
AN INVERTED TROUGH SITS JUST OFF OUR COAST. THERE ARE DIFFERENCES IN
THE MEDIUM RANGE GUIDANCE FOR LATE IN THE WEEK...WITH THE ECMWF MORE
BULLISH WITH A CLOSED UPPER LOW DEVELOPING ACROSS THE SOUTHEAST
STATES. WE CONTINUED TO LEAN TOWARD THE SLIGHTLY MORE OPTIMISTIC GFS
WHICH KEEPS THE MAIN TROUGH JUST OFFSHORE ALONG WITH THE BEST PRECIP
COVERAGE. HOWEVER WE DO SHOW INCREASING CLOUDS INLAND LATE IN THE
WEEK. TEMPS WILL BE A FEW DEGREES BELOW NORMAL DUE TO THE PERSISTENT
COLD ADVECTION...WITH HIGHS A FEW DEGREES EITHER SIDE OF 80 AND LOWS
IN THE 60S.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
VFR CONDITIONS ARE EXPECTED TO PREVAIL THROUGH 18Z SUNDAY. IT
APPEARS THAT VIRTUALLY ALL OF THE PRECIPITATION ASSOCIATED WITH
THE WEAK OFFSHORE SURFACE LOW WILL REMAIN OVER THE WATERS AND EAST
OF THE TERMINALS. OVERCAST SKIES AND VFR CEILINGS WILL REMAIN IN
PLACE THROUGH MUCH OF THE NIGHT BEFORE DRIER AIR WORKS IN AND
SKIES BEGIN TO CLEAR SUNDAY MORNING. WINDS WILL GRADUALLY BACK
FROM NORTHEAST TO BECOME OVERNIGHT AND IN THE MORNING.

EXTENDED AVIATION OUTLOOK...PRIMARILY VFR.

&&

.MARINE...
30-40 KT OF LOW LEVEL GEOSTROPHIC FLOW PROGGED BY THE MODELS INTO
TONIGHT. WITH THE LOW PRES PASSING OFFSHORE TONIGHT...ONGOING
PRES GRADIENT PINCHING SHOULD PERSIST UNTIL A WEAKENING TREND SETS
IN LATE TONIGHT/SUNDAY MORNING. A SOLID SMALL CRAFT ADVISORY IS
IN PLACE FOR ALL WATERS INCLUDING THE CHARLESTON HARBOR INTO THIS
EVENING. WE MAY BE A BIT TOO QUICK TO BRING WINDS DOWN OFF THE
UPPER SOUTH CAROLINA COAST LATER TONIGHT BUT LATER SHIFTS CAN
ANALYZE THE TRACK/TIMING OF THE EJECTING LOW. SEAS WERE ALREADY
5-7 FT NEAR SHORE AND 6-8 FT OFFSHORE. WE INCREASED SEAS TO 9 FT
TODAY OVER THE OUTER GEORGIA WATERS. ANOTHER ISSUE IS THE HEAVY
RAINS OVER THE WATERS THROUGH THIS EVENING. VSBYS WILL BE REDUCED
TO LESS THAN 3 NM FREQUENTLY IN TORRENTIAL DOWNPOURS. STRONGER
CONVECTIVE CELLS MOVING ALONG A GOOD CLIP COULD EASILY PRODUCE 35
KT GUSTS IN THIS PATTERN AS WELL.

FAIRLY WEAK WINDS EXPECTED SUNDAY INTO MONDAY DUE TO A WEAK GRADIENT
OVER THE AREA. LATE MONDAY INTO TUESDAY...AFTER A BACKDOOR COLD
FRONT DROPS THROUGH...STRONG HIGH PRESSURE WILL BUILD IN FROM THE
NORTH. AN ENHANCED NORTHEAST GRADIENT WILL SETUP FOR MID TO LATE
WEEK WITH WINDS/SEAS LIKELY REACHING SMALL CRAFT ADVISORY CRITERIA
MUCH OF THE TIME.

&&

.TIDES/COASTAL FLOODING...
EXTRATROPICAL SURGE GUIDANCE SUGGESTS THAT THE TIDE WILL EXCEED 7
FT MLLW ON CHARLESTON HARBOR THIS EVENING...REASONABLE GIVEN
THE CONTINUED ELEVATED PARALLEL/ONSHORE FLOW REGIME...AND ANOTHER
COASTAL FLOOD ADVISORY COULD BE NEEDED. WHILE THE PROBABILITY FOR
SHALLOW SALT WATER FLOODING WILL DIMINISH SUNDAY INTO EARLY NEXT
WEEK...TIDES WILL REMAIN ELEVATED AND WILL PEAK CLOSE TO ADVISORY
LEVELS DURING EACH HIGH TIDE CYCLE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 PM EDT THIS EVENING FOR AMZ330.
     SMALL CRAFT ADVISORY UNTIL 5 AM EDT SUNDAY FOR AMZ350-352-354.
     SMALL CRAFT ADVISORY UNTIL 8 AM EDT SUNDAY FOR AMZ374.

&&

$$

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




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