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Area Forecast Discussion

361
FXUS62 KCHS 210524
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
124 AM EDT SUN SEP 21 2014

.SYNOPSIS...
AN AREA OF LOW PRESSURE WILL LIFT NORTHWARD OFF THE SOUTHEAST
COAST THROUGH TONIGHT. HIGH PRESSURE WILL BRIEFLY BUILD OVER THE
AREA SUNDAY. A COLD FRONT WILL THEN ADVANCE THROUGH THE REGION
MONDAY AND WILL STALL SOUTH OF THE AREA THROUGH NEXT WEEKEND. A
BROAD AREA OF HIGH PRESSURE WILL BUILD IN FROM THE NORTH TUESDAY
AND WILL PREVAIL INTO NEXT WEEKEND...WHILE A TROUGH OF LOW
PRESSURE DEVELOPS AND STRENGTHENS OFF THE COAST.

&&

.NEAR TERM /THROUGH TODAY/...
RAIN CONTINUES TO DIMINISH AS DRIER AIR WORKS IN FROM THE WEST AND
SOUTHWEST. ADJUSTED POPS TO MATCH CURRENT TRENDS WITH ONLY SLIGHT
CHANCES ACROSS NE CHARLESTON COUNTY FOR LIGHT AMOUNTS THROUGH 3 AM.

IT WAS A FALL LIKE ACROSS THE LOWCOUNTRY AND COASTAL EMPIRE TODAY
WITH CLOUDY...COOL AND SOMEWHAT DAMP CONDITIONS. HIGHS ONLY
REACHED 75 AT THE CHARLESTON AIRPORT AND 77 AT SAVANNAH...WHICH IS
THE COOLEST AFTERNOON HIGH HAVE BEEN SINCE MID-MAY.

LOW PRESSURE NOTED WELL OFFSHORE OF THE GEORGIA COAST WILL
CONTINUE TO MOVE AWAY FROM THE REGION TONIGHT AS THE ASSOCIATED
UPPER LOW BEGINS TO WEAKEN...OPEN UP AND EJECT OFF THE NORTHEAST
AS MUCH A MUCH STRONGER SHORTWAVE DIGS ACROSS THE GREAT LAKES AND
NORTHERN OHIO VALLEY. DRY AIR WILL GRADUALLY FILTER IN FROM THE
NORTHWEST IN THE WAKE OF THE SURFACE LOW. THIS IS ALREADY
BEGINNING TO TAKE PLACE AS RETURNS ON THE KCLX RADAR ARE STEADILY
DIMINISHING. OTHER THAN A FEW SHOWERS POTENTIALLY BRUSHING THE
SOUTH CAROLINA COASTAL ZONES THROUGH MIDNIGHT...EXPECT MOST OF THE
AREA TO REMAIN DRY. CLOUDS WILL BE SLOW TO CLEAR AS IT WILL TAKE
SOME TIME FOR DOWNSLOPE TRAJECTORIES TO TAKE HOLD. THE LATEST
MODEL SIMULATED CLOUD PRODUCTS SHOW SKIES CLEARING ACROSS THE FAR
INTERIOR AFTER MIDNIGHT...BUT HOLDING ON FOR MUCH OF THE NIGHT
ALONG THE COAST. ADJUSTED THE FORECAST ACCORDINGLY.

LOWS WILL RANGE FROM THE MID 60S INLAND TO THE LOWER 70S AT THE
BEACHES.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...
SUNDAY COULD START WITH LINGERING CLOUDS BUT WILL TURN OUT MOSTLY
SUNNY AS ZONAL FLOW...SURFACE HIGH PRESSURE AND DEEP LAYER
SUBSIDENCE SPREAD INTO THE REGION IN THE WAKE OF DEPARTING LOW
PRESSURE. OFFSHORE WINDS INCLUDING A W/NW FLOW AROUND 850 MB WILL
YIELD A DOWNSLOPE COMPONENT...ENHANCING WARMING PROVIDED BY
INSOLATION. AS DEWPOINTS FALL INTO THE 50S/60S...TEMPERATURES WILL
RECOVER INTO THE UPPER 80S/AROUND 90F.

EXPECT A TRANQUIL SUNDAY NIGHT WITH LOW TEMPS RANGING FROM 60-65F
WELL INLAND TO 70-75 ON THE BEACHES WHERE W/SW WINDS WILL PERSIST.

ON MONDAY A SHORTWAVE TROUGH WILL DROP INTO NORTHERN GA/SC DURING
THE AFTERNOON...PUSHING A COLD FRONT INTO THE AREA FROM THE N/NW.
MODELS DIFFER REGARDING THE POSITION OF THE UPPER TROUGH/ASSOCIATED
SHORTWAVE BY LATE MONDAY...WHILE MODEL SOUNDINGS DEPICT A PERSISTENT
SUBSIDENCE INVERSION WELL INTO THE DAY. THUS...TIMING/COVERAGE OF
PRE-FRONTAL CONVECTION REMAINS UNCERTAIN. HOWEVER...MOST 20/12Z
GUIDANCE DEPICTS A FLARE-UP OF SHOWERS/THUNDERSTORMS JUSTIFYING AT
LEAST SLIGHT CHANCE/CHANCE POPS MONDAY AFTERNOON. IF HIGH TEMPS IN
THE UPPER 80S/LOWER 90S CONTRIBUTE TO POOLING INSTABILITY AHEAD OF
THE COLD FRONT WE CANNOT RULE OUT A FEW SEVERE TSTMS MONDAY
AFTERNOON/EARLY EVENING.

FORECAST UNCERTAINTY INCREASES SIGNIFICANTLY MONDAY NIGHT INTO
TUESDAY. MOST GUIDANCE MAINTAINS A PROGRESSIVE UPPER
TROUGH/SHORTWAVE AND PUSHES ASSOCIATED SURFACE COLD FRONT SOUTH OF
THE REGION BY TUESDAY MORNING...FOLLOWED BY EXPANSIVE HIGH PRESSURE
BUILDING FROM THE NORTH AND WEST. THIS SCENARIO SUGGESTS THAT A
SHARP MOISTURE GRADIENT WOULD DEVELOP AND LIMIT SLIGHT CHANCE/CHANCE
POPS TO SOUTHERN COUNTIES ESPECIALLY TUESDAY AFTERNOON.
HOWEVER...THE 20/12Z OPERATIONAL EUROPEAN DEPICTS A MUCH
SLOWER/DEEPER UPPER TROUGH STILL LOCATED UPSTREAM MONDAY NIGHT AND
ADVANCING INTO THE REGION TUESDAY...SUPPORTING GREATER AND MORE
PERSISTENT COVERAGE OF SHOWERS/THUNDERSTORMS. THE LATEST FORECAST
REPRESENTS A COMPROMISE LEANING IN FAVOR OF A MORE PROGRESSIVE
SOLUTION. OTHERWISE...COLD ADVECTION WILL PUSH TEMPERATURES INTO THE
60S MOST AREAS MONDAY NIGHT...PERHAPS INCLUDING SOME UPPER 50S WELL
INLAND. TUESDAY HIGHS ARE EXPECTED TO RANGE FROM THE UPPER
70S/AROUND 80F NORTH TO THE LOWER TO MID 80S SOUTH.

&&

.LONG TERM /WEDNESDAY 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 /06Z SUNDAY THROUGH THURSDAY/...
VFR.

EXTENDED AVIATION OUTLOOK...BRIEF SLIGHT RESTRICTIONS POSSIBLE IN
THUNDERSTORMS MONDAY AFTERNOON INTO MONDAY NIGHT.

&&

.MARINE...
WINDS ARE SLOWLY DIMINISHING AS THE PRESSURE GRADIENT RELAXES.
REMOVED THE SMALL CRAFT ADVISORY FROM THE CHARLESTON HARBOR WERE
AVAILABLE OBSERVATIONS SUGGEST WINDS ARE BELOW 20 KT. THE
COMBINATION OF WINDS AND SEAS NECESSITATES KEEPING FLAGS IN PLACE FOR
THE REMAINDER OF THE COASTAL WATERS THROUGH THE NIGHT.

ELEVATED WINDS/SEAS WILL PERSIST SUNDAY MORNING. PER LATEST
GUIDANCE DEPICTING LONGER PERSISTENCE OF 6 FT SEAS ...EXTENDED
ONGOING SMALL CRAFT ADVISORIES FOR AMZ350 AND AMZ374 UNTIL 11 AM.

A WEAKER PRESSURE GRADIENT WILL TRANSLATE TO LIGHTER W/SW WINDS
SUNDAY AFTERNOON. THEN...A SURGE OF S/SW WINDS AS HIGH AS 15-20 KT
WILL LIKELY DEVELOP SUNDAY NIGHT AHEAD OF AN APPROACHING COLD FRONT.
WINDS WILL BRIEFLY DIMINISH AS THE COLD FRONT CROSSES THE WATERS
LATE MONDAY INTO MONDAY NIGHT. THEN...LATE MONDAY NIGHT INTO
TUESDAY...STRONG HIGH PRESSURE WILL BUILD IN FROM THE NORTH AND
NORTHEAST WINDS WILL SURGE INTO THE REGION.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...
THE PROBABILITY FOR SHALLOW SALT WATER FLOODING WILL DIMINISH
SUNDAY INTO EARLY NEXT WEEK...BUT TIDES WILL REMAIN ELEVATED AND
WILL PEAK CLOSE TO ADVISORY LEVELS DURING EACH HIGH TIDE CYCLE.
AFTER COLD FROPA...STRONGER ONSHORE WINDS WILL INCREASE THE CHANCE
FOR SHALLOW SALT WATER FLOODING AROUND HIGH TIDE MID THROUGH LATE
WEEK.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EDT EARLY THIS MORNING FOR
     AMZ352-354.
     SMALL CRAFT ADVISORY UNTIL 11 AM EDT THIS MORNING FOR AMZ350-
     374.

&&

$$

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






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