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Area Forecast Discussion

438
FXUS62 KCHS 200035
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
835 PM EDT FRI SEP 19 2014

.SYNOPSIS...
LOW PRESSURE WILL DEVELOP SOUTH OF THE AREA TONIGHT AND TRACK
NORTHEAST OFF THE SOUTHEAST COAST SATURDAY AND SATURDAY NIGHT. HIGH
PRESSURE WILL BRIEFLY BUILD OVER THE REGION SUNDAY. A COLD FRONT
WILL ADVANCE INTO THE REGION MONDAY AND WILL STALL SOUTH OF THE
AREA THROUGH LATE NEXT WEEK. HIGH PRESSURE WILL FOLLOW THE COLD
FRONT AND WILL BUILD FROM THE NORTH. A TROUGH OF LOW PRESSURE
COULD DEVELOP OFF THE SOUTHEAST COAST MID TO LATE NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
THE HIGH PRESSURE WEDGE IS BUILDING SOUTH WHILE THE COASTAL TROUGH
SHARPENS JUST OFF THE COAST. LOW-LEVEL CONVERGENCE CONTINUES TO
INCREASE PER MSAS MOISTURE FLUX CALCULATIONS...WHICH HAS RESULTED
IN A GRADUAL INCREASE IN SHOWER/TSTM ACTIVITY OVER THE COASTAL
WATERS AND COAST. THIS TREND WILL PERSIST OVERNIGHT WITH NUMEROUS
SHOWERS/TSTMS IMPACTING THE COASTAL COUNTIES THROUGH SUNRISE.
INCREASED POPS TO 70 PERCENT ACROSS MOST COASTAL ZONES...TAPERING
TO 20-30 PERCENT WELL INLAND. THE REST OF THE FORECAST IS ON
TRACK.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
A BROAD UPPER LOW WILL ADVANCE NORTHEAST ACROSS THE REGION AND AN
ASSOCIATED SURFACE LOW WILL TRACK NORTHEAST OFF THE SOUTHEAST COAST
SATURDAY INTO SATURDAY NIGHT. DAYTIME POPS ARE TAPERED FROM LOW
CHANCE WELL INLAND TO LIKELY NEAR THE COAST...AND HIGHER POPS COULD
EVENTUALLY BE REQUIRED FOR COASTAL COUNTIES. UPPER LEVEL SUPPORT AS
DEPICTED BY 700-300 MB Q VECTOR CONVERGENCE/UPPER LEVEL DIVERGENCE
WILL COMBINE WITH PWATS AROUND 2 INCHES TO SUPPORT A CHANCE FOR
LOCALLY HEAVY RAIN ESPECIALLY IF/WHERE CONVECTIVE PRECIPITATION
PERSISTS OVER COASTAL LOCALES.

LATE SATURDAY THROUGH SATURDAY NIGHT...AS DEEP-LAYERED LOW PRESSURE
PULLS AWAY FROM THE REGION...DRIER AIR AND LOWERING POPS WILL
OVERSPREAD THE REGION FROM W/NW TO E-NE. 19/12Z GUIDANCE VARIES
REGARDING TIMING OF THIS DRYING TREND...RANGING FROM THE NAM WHICH
SUGGESTS POPS WILL FALL TO ZERO BY LATE NIGHT AS STRONG
SUBSIDENCE/DRYING PUNCHES AGGRESSIVELY INTO THE REGION...TO THE MUCH
SLOWER EUROPEAN WHICH SUGGESTS SHOWERS COULD PERSIST INTO SUNDAY
MORNING ALONG THE COAST. THE LATE FRIDAY AFTERNOON FORECAST
REPRESENTS A COMPROMISE...WITH POPS BELOW 15 PERCENT ALONG THE COAST
AFTER 12Z SUNDAY.

REGARDLESS OF THE TIMING OF IMPROVEMENT LATE SATURDAY NIGHT INTO
SUNDAY...HIGH PRESSURE AND STRONG SUBSIDENCE/DRYING WILL ENSURE THAT
MUCH OF THE DAY WILL TURN OUT WARM AND DRY UNDER MOSTLY SUNNY SKIES.
TEMPERATURES WILL RECOVER INTO THE UPPER 80S/AROUND 90F...AND
DOWNSLOPE FLOW/MIXING WILL PROMOTE DEWPOINTS DROPPING INTO THE LOWER
60S BY AFTERNOON.

AFTER A TRANQUIL SUNDAY NIGHT FEATURING LOWS IN THE LOWER/MID 60S
INLAND AND THE UPPER 60S/LOWER 70S AT THE COAST...A SHORTWAVE DIGGING
FROM THE OHIO VALLEY WILL PUSH A COLD FRONT TOWARD THE REGION FROM
THE NORTHWEST. AS THE FRONT PUSHES THROUGH THE REGION MONDAY
AFTERNOON AND MONDAY NIGHT...SLIGHT CHANCE/CHANCE POPS ARE
JUSTIFIED. IF INSTABILITY POOLS AHEAD OF THE FRONT AS DEPICTED BY
THE 19/12 NAM...ISOLATED STRONG/SEVERE THUNDERSTORMS COULD OCCUR
MONDAY PM. OTHERWISE...PRE-FRONTAL COMPRESSION WILL YIELD HIGH TEMPS
AT LEAST IN THE MID 80S TO NEAR 90F.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
COOL AND DRY HIGH PRESSURE WILL BUILD DOWN THE EASTERN SEABOARD AT
THE SURFACE WHILE A COASTAL TROUGH LINGERS JUST OFFSHORE. THERE WILL
PROBABLY BE A SHARP PRECIP GRADIENT FOR MUCH OF THE PERIOD WITH
INLAND AREAS DRY AND CONSIDERABLE SHOWER AND TSTM COVERAGE OVER THE
WATERS AND IMMEDIATE COASTAL AREAS. THERE ARE INDICATIONS THE
COASTAL TROUGH WILL STRENGTHEN WEDNESDAY OR THURSDAY AND SHIFT
INLAND WHICH WOULD INCREASE SKY COVER AND RAIN POTENTIAL FARTHER TO
THE WEST. TEMPS WILL GENERALLY BE AT OR SLIGHTLY BELOW NORMAL DURING
THE PERIOD.

&&

.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
VERY UNSETTLED PATTERN WILL AFFECT THE TERMINAL THROUGH SATURDAY
AFTERNOON. SHOWERS WILL AFFECT KCHS THROUGH ABOUT 02Z...BUT LIKELY
MISS KSAV FOR A FEW MORE HOURS. MODELS SUGGEST RAIN COULD FALL
JUST ABOUT ANYTIME DURING THE NEXT 24 HOURS ACROSS THE COASTAL
CORRIDOR SO THE CHALLENGE IS TRYING TO PIN DOWN WHEN THE WINDOWS
OF GREATEST IMPACTS WILL OCCUR. GUIDANCE GENERALLY FAVORS VFR CIGS
FOR THE NEXT 24 HOURS...BUT VSBYS WILL DROP TO MVFR TO OCCASIONALLY
IFR DURING PERIODS OF RAIN. ATTM IT APPEARS THE WINDOW OF GREATEST
OPPORTUNITY FOR IMPACTS WILL BE FROM 09-12Z AT KCHS AND 05-10Z AT
KSAV. LOW CONFIDENCE OVERALL ON THIS SCENARIO HOWEVER. MORE
SHOWERS/TSTMS LIKELY AFTER 12Z SATURDAY...ESPECIALLY IN THE
AFTERNOON. LIMITED CONDITIONS TO LOW END VFR FOR NOW. FURTHER
REFINEMENTS WILL BE NEEDED.


EXTENDED AVIATION OUTLOOK...PRIMARILY VFR.

&&

.MARINE...
TONIGHT...BETWEEN HIGH PRESSURE TO THE NORTH AND A DEVELOPING AREA
OF LOW PRESSURE TO THE SOUTH THE GRADIENT WILL CONTINUE TO TIGHTEN
ACROSS THE LOCAL WATERS. THIS WILL RESULT IN A STRENGTHENING EAST TO
NORTHEAST FLOW...ESPECIALLY LATE TONIGHT. IN FACT...THE SMALL CRAFT
ADVISORY HAS BEEN EXPANDED TO INCLUDE ALL WATERS AS WELL AS THE
CHARLESTON HARBOR TONIGHT. CONDITIONS ARE ALREADY BORDERLINE
ADVISORY THRESHOLDS...AND AS THE GRADIENT TIGHTENS WINDS WILL ONLY
INCREASE AND SO WILL CORRESPONDING SEAS. EXPECT FREQUENT GUSTS TO 25
KT IN THE HARBOR AND A SOLID 20-25 KT ELSEWHERE. WE MAY EVEN SEE A
GUSTS TO 30 KT OVER THE OUTER GEORGIA WATERS LATE. SEAS WILL RAMP UP
AND REACH UP TO 5-6 FT OVER THE OUTER WATERS LATE AND 3-5 FT FOR THE
NEARSHORE WATERS. NUMEROUS TO WIDESPREAD SHOWERS AND ISOLATED
THUNDERSTORMS WILL ALSO OCCUR ACROSS THE WATERS TONIGHT.

AN IMPRESSIVE SURFACE PRESSURE GRADIENT BETWEEN A WEDGE OF HIGH
PRESSURE ANCHORED EAST OF THE APPALACHIANS AND LOW PRESSURE PASSING
OFFSHORE WILL CONTINUE TO SUPPORT ELEVATED WINDS/SEAS SATURDAY AND
SATURDAY NIGHT. SMALL CRAFT ADVISORIES WILL CONTINUE INTO SATURDAY
NIGHT FOR MOST AREAS...AND PERSISTENT SEAS AS HIGH AS 6 FT AND THE
ASSOCIATED SCA WILL CONTINUE INTO SATURDAY MORNING OVER AMZ374.

A MUCH WEAKER PRESSURE GRADIENT WILL TRANSLATE TO A MUCH MORE
TRANQUIL WIND/SEA REGIME SUNDAY INTO MONDAY. THEN...IN THE WAKE OF
COLD FROPA...HIGH PRESSURE WILL BUILD FROM THE NORTH AND WILL SUPPORT
A PROLONGED SURGE OF ELEVATED NORTHEAST WINDS AND BUILDING SEAS
TUESDAY THROUGH FRIDAY. SMALL CRAFT ADVISORIES ARE INCREASINGLY
LIKELY WEDNESDAY AND THEREAFTER.

&&

.TIDES/COASTAL FLOODING...
EXTRATROPICAL SURGE GUIDANCE SUGGESTS THAT THE TIDE WILL EXCEED 7
FT MLLW ON CHARLESTON HARBOR SATURDAY EVENING...REASONABLE GIVE
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 SATURDAY FOR AMZ330.
     SMALL CRAFT ADVISORY UNTIL 5 AM EDT SUNDAY FOR AMZ350-352-354.
     SMALL CRAFT ADVISORY UNTIL 8 AM EDT SUNDAY FOR AMZ374.

&&

$$

ST





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