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Area Forecast Discussion

034
FXUS62 KCHS 200837
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
437 AM 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/...
PRE-DAWN...AN UNSETTLED MORNING ALONG AND EAST OF I-95 WITH RAINS
STEADIER RIGHT ALONG THE COAST AND OCCASIONALLY HEAVY AT TIMES EAST
OF U.S. 17 IN SOUTH CAROLINA AND EAST OF I-95 IN GEORGIA. THE MAIN
CONCERN TOWARD DAYBREAK IS A POTENTIAL COMBO OF HEAVY RAINS AROUND
THE TIME OF HIGH TIDE ALONG COASTAL GEORGIA AND SOUTH CAROLINA.
LOW TIDE WAS HELD IN BY STRONG ONSHORE FLOW OVERNIGHT AND TIDAL
ANOMALIES COULD BE GREATER THAN 1.3 FT AROUND 6 AM. HEAVY OR EVEN
STEADY MODERATE RAINS COULD CAUSE FLOODING PROBLEMS IN DOWNTOWN
CHARLESTON OR ANY COASTAL LOCATION SUSCEPTIBLE TO SIMILAR LOW
FLOODING THRESHOLDS. OTHERWISE...LOW CLOUDS WERE FILLING OVER
INLAND AREAS WEST OF THE SHARP EDGE OF THE HIGH CLOUD FIELD ALONG
I-95. LOW LEVEL WIND FIELDS INLAND A BIT TOO STRONG FOR BUILD-DOWN
FOG DEVELOPMENT. GUSTY ONSHORE WINDS HAVE BEEN GUSTING OVER 25 MPH AT
TIMES ON THE BEACHES AND BARRIER ISLANDS THIS MORNING...EVEN STRONGER
GUSTS ARE POSSIBLE IN SHOWERS AND ISOLATED TSTMS.

SURFACE CYCLOGENESIS UNDERWAY EARLY THIS MORNING OFF CAPE
CANAVERAL FLORIDA AS THE UPPER FLOW ACROSS SE GEORGIA AND SOUTHERN
SOUTH CAROLINA BACKS IN TANDEM WITH SUBSTANTIAL MID LEVEL PVA
POISED TO EJECT FROM CENTRAL FLORIDA TOWARD THE GULF STREAM
TODAY. THE FORECAST TODAY IS ONE OF GRADIENTS...SHARPER THAN
TYPICALLY SEEN THIS TIME YEAR BUT INDICATIVE OF THE SYNOPTIC
PATTERN AT HAND. A SWATH OF DEEPER MOISTURE IS ALREADY ENTRENCHED
ACROSS THE COASTAL ZONES AND ADJACENT WATERS THIS MORNING BUT
PWATS WERE HIGHEST ACROSS CENTRAL FLORIDA AND OFFSHORE. STRONG
COASTAL SPEED CONVERGENCE HAS PERIODIC RAINS RUSHING INTO THE
COASTAL ZONES IN BANDS BUT PERHAPS LATER TODAY WE SHOULD SEE MORE
RAINS GETTING INTO OR JUST WEST OF THE I-95 CORRIDOR. STEADIER AND
MORE SUBSTANTIAL RAINS SHOULD BE WITHIN THE FIRST 20 MILES OF
COAST LINE TODAY. AREAS WEST OF I-95 COULD SEE BREAKS OF SUNSHINE
AT TIMES TODAY AND WE HAVE WARMEST TEMPS THERE TODAY...MID 80S.
COASTAL AREAS SHOULD BE MIRED IN WIDESPREAD LAYERED CLOUDS AND
PERIODIC RAINS. WE LOWERED COASTAL HIGH TEMPS TODAY SOME AND WOULD
NOT BE SURPRISED IF MANY AREAS FAIL TO REACH 80.

POPS IN A GRADIENT FROM CATEGORICAL COAST TO PERHAPS SLIGHT CHANCE
FROM REIDSVILLE TO ALLENDALE.

GUSTY WINDS WILL ACCOMPANY SHOWERS WITH EVEN SOME BREEZY WEATHER
WELL INLAND THIS AFTERNOON IF IT MIXES WITH A BIT OF AFTERNOON
HEATING. NOT A NICE DAY ON AREA BEACHES...WINDS AND OCCASIONAL
RAINS...HEAVY AT TIMES. A STRONG LONG-SHORE CURRENT IS ALSO
EXPECTED.

THE NON-TROPICAL COASTAL LOW SHOULD BEGIN TO SCOOT OFF ALONG THE
GULF STREAM LATER TODAY AND TONIGHT...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 FRIDAY/...
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 /08Z SATURDAY THROUGH WEDNESDAY/...
VERY UNSETTLED PATTERN WILL AFFECT THE TERMINALS THROUGH THIS
AFTERNOON...DECREASING CHANCES FOR RAIN THIS EVENING NEARING THE
END OF THE 06Z TAF CYCLE. OCCASIONAL SHOWERS WILL IMPACT BOTH
KCHS AND KSAV OVERNIGHT THROUGH TODAY WITH BEST COVERAGE AT KCHS
THROUGH DAYBREAK...ALTHOUGH ACTIVITY LOOKS TO BE ON THE INCREASE
AROUND KSAV AT 05Z. NE FLOW WILL BE GUSTY AT TIMES IN SHOWERS
WITH 20-25 KT GUSTS IN SHOWERS A GOOD BET LATER TODAY. CIGS ARE
QUITE TRICKY WITH MVFR/IFR CIGS LIKELY CLOSER TO THE OCEAN AND
DIMINISHING RETURNS FOR LOWER CIGS FURTHER INLAND.


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...
SHORT TERM...JHP/JRL
LONG TERM...JRL
AVIATION...
MARINE...JRL
TIDES/COASTAL FLOODING...






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