Issued by NWS Charleston, SC
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000 FXUS62 KCHS 211751 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 1251 PM EST SAT NOV 21 2009 .SYNOPSIS... HIGH PRES WILL REMAIN N OF THE REGION THROUGH TONIGHT. AN AREA OF LOW PRES WILL TRACK ALONG THE GULF COAST STATES AND INTO THE ATLC OFF THE SOUTHEAST GEORGIA COAST SUNDAY AND SUNDAY NIGHT. THIS SYSTEM WILL THEN MOVE NE ALONG THE EASTERN SEABOARD...AS HIGH PRES HOLDS IN PLACE INLAND. HIGH PRES WILL SLOWLY BECOME MORE DOMINANT FROM MID-LATE WEEK. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... MID AND HIGH CLOUDS WILL CONTINUE TO INCREASE FROM THE SW TODAY AHEAD OF AN APPROACHING UPPER TROUGH. MEANWHILE THE LOW STRATUS FROM THIS MORNING SHOULD SLOWLY BURN OFF...BUT LOW-LEVEL CLOUDS WILL PROBABLY INCREASE LATE IN THE DAY AS ISENTROPIC LIFT BEGINS. THERE WILL BE LITTLE IN THE WAY OF FORCING FOR PRECIP EXCEPT JUST OFF THE CHARLESTON COUNTY COAST WHERE AN ISOLATED SHOWER COULD DEVELOP IN THE COASTAL TROUGH. ONGOING FORECAST OF HIGHS IN THE MIDDLE TO UPPER 60S LOOKED GOOD BASED ON LATEST TRENDS SO WE MADE ONLY MINOR TWEAKS TO TODAY/S GRAPHICS. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... THE VIGOROUS SOUTHERN STREAM SYSTEM WILL LIFT FROM SE TEXAS TODAY INTO THE LOWER MISSISSIPPI VALLEY REGION BY 12Z ON SUNDAY. DYNAMIC INFLUENCES WILL GUIDE SIGNIFICANT RAINS INTO WESTERN AND N GEORGIA BY DAWN WITH ISENTROPIC LIFT AND DEEP MOISTURE GRADUALLY INCREASING OVERNIGHT OVER OUR FORECAST AREA. THE GFS IS NOW A FASTER OUTLIER IN SPREADING RAINS THROUGH OUR ENTIRE FORECAST AREA AFTER MIDNIGHT TONIGHT. THE BAROCLINIC ZONE WILL LIKELY PERSIST OFF THE COAST AND DEEP LAYERED CONVERGENCE AND UPPER FORCING WILL LIKELY INCITE RAINFALL DEVELOPMENT SW OF THE SAVANNAH RIVER SHORTLY BEFORE DAYBREAK ON SUNDAY. COASTAL SOUTH CAROLINA APPEARS TO BE BETWEEN THE APPROACHING DYNAMICS AND THE OFFSHORE FRONTAL ZONE AND WE TRIMMED POPS BACK TO A LOW END CHANCE LATE TONIGHT. CLOUDS AND A LIGHT/STEADY NE FLOW SHOULD KEEP TEMPERATURES IN THE LOWER TO MID 50S COASTAL AREAS WITH SOME UPPER 40S CLOSER TO THE CSRA AND MIDLANDS WHERE THE SURFACE WEDGE WILL BE ENTRENCHED. && .LONG TERM /SUNDAY THROUGH FRIDAY/... MEDIUM RANGE GUIDANCE IS IN GOOD AGREEMENT THROUGH EARLY NEXT WEEK...WITH SOLUTIONS THEN DIVERGING DURING THE LAST SEVERAL DAYS OF THE EXTENDED FORECAST PERIOD. AS A RESULT...CONFIDENCE IS NEAR NORMAL THROUGH MONDAY...THEN DROPS BELOW NORMAL TUESDAY THROUGH FRIDAY. GUIDANCE HAS COME INTO GOOD AGREEMENT BRINGING A WEAKENING WAVE OF LOW PRESSURE ACROSS THE NORTHERN GULF COAST STATES SUNDAY AND TRACKING ACROSS THE GEORGIA/FLORIDA BORDER SUNDAY NIGHT. AS THIS LOW PASSES BY TO THE SOUTH OF THE FORECAST AREA...A WEDGE OF HIGH PRESSURE WILL HOLD IN PLACE ACROSS MUCH OF THE EASTERN THIRD OF THE COUNTRY. THE AREA OF LOW PRESSURE WILL THEN TRACK NORTHEASTWARD OFF THE EASTERN SEA BOARD AND INTO THE SOUTHERN NEW ENGLAND MONDAY THROUGH TUESDAY...HELPING TO RE-ENFORCE THE EXISTING HIGH PRESSURE WEDGE ACROSS THE FORECAST AREA. MODEL SOLUTIONS THEN REMAIN SPLIT ON DEGREE OF PHASING BETWEEN THE NORTHERN AND SOUTHERN STREAM WEDNESDAY THROUGH THURSDAY...WITH THE GFS AND CANADIAN SOLUTIONS SHOWING LITTLE PHASING. AS A RESULT...THESE MODELS KEEP THE FORECAST AREA UNDER THE INFLUENCE OF HIGH PRESSURE WITH LITTLE CHANCE FOR RAIN. ON THE OTHER-HAND...THE ECMWF AND UKMET MODELS DEPICT THE TWO STREAMS PHASING...WHICH RESULTS IN A WAVE OF LOW PRESSURE TRACKING THROUGH THE SOUTHEAST STATES ALONG WITH RAIN CHANCES WEDNESDAY THROUGH THURSDAY. GUIDANCE THEN COMES INTO AGREEMENT BUILDING HIGH PRESSURE INTO THE AREA THURSDAY NIGHT AND FRIDAY. AS FOR FORECAST DETAILS...WILL CARRY LIKELY/CATEGORICAL POPS SUNDAY AND SUNDAY NIGHT AS GOOD FORCING FOR ASCENT ALONG WITH DEEP MOISTURE AND ISENTROPIC UPGLIDE OVERSPREAD THE FORECAST AREA. MODEL SOLUTIONS DEPICT SOME DECENT MID-LEVEL LAPSE RATES ON THE ORDER OF 6.5 C/KM MOVING THROUGH SUNDAY AFTERNOON THROUGH SUNDAY EVENING. ALTHOUGH A STRONG SURFACE HIGH PRESSURE WEDGE WILL BE IN PLACE...GIVEN THE DEGREE OF FORCING MOVING OVERHEAD...CAN/T RULE OUT ELEVATED CONVECTION...SO WILL CARRY THUNDER MENTION IN THE FORECAST AS WELL. AS SURFACE HIGH PRESSURE WEDGE IS RE-ENFORCED MONDAY THROUGH TUESDAY AS THE SURFACE AREA OF LOW PRESSURE LIFTS NORTHWARD TOWARD NEW ENGLAND...NOT ANTICIPATING MUCH IF ANY CLEARING ACROSS THE FORECAST AREA. MODEL TIME HEIGHT RELATIVE HUMIDITY CROSS SECTIONS INDICATE THAT THE 1000-850 MB LAYER WILL REMAIN NEARLY SATURATED THROUGH THIS PERIOD...SO WILL CARRY SLIGHT CHANCE/LOW CHANCE POPS AND MENTION OF DRIZZLE ACROSS THE FORECAST AREA. HAVE SIGNIFICANTLY UNDERCUT MOS GUIDANCE MAX TEMPERATURES AS WELL...AS LIGHT WIND FIELDS COMBINED WITH THE LOW SUN ANGLE SHOULD LEAD TO PRIMARILY OVERCAST CONDITIONS AND SMALL DIURNAL TEMPERATURE SWINGS. FORECAST CONFIDENCE GOES DOWNHILL QUICKLY WEDNESDAY AND THURSDAY WITH GUIDANCE SOLUTIONS SPLIT ON EVENTUAL PATTERN EVOLUTION. FOR NOW WILL STICK WITH A SOLUTION CLOSE TO THE GFS AND CANADIAN SOLUTIONS WHICH MAINTAINS GOOD FORECAST CONTINUITY AND AWAIT BETTER MODEL AGREEMENT BEFORE MAKING ANY SWEEPING CHANGES. THIS RESULTS...IN A RAIN FREE FORECAST WEDNESDAY THROUGH FRIDAY. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... KCHS...CEILING/VSBY RESTRICTIONS NOT LOOKING AS LIKELY TONIGHT DUE TO THE HIGH PRESSURE SYSTEM TO THE NORTH PUSHING FARTHER TO THE SOUTH. WE SHOULD PROBABLY SEE SOME LIGHT RAIN INCREASING IN COVERAGE FROM SW TO NE DURING THE DAY SUNDAY SO WE INCLUDED -RA STARTING AT 1330Z. CEILING RESTRICTIONS ARE SIGNIFICANTLY MORE LIKELY BY LATE SUNDAY AFTERNOON INTO SUNDAY EVENING. KSAV...VFR CONDITIONS SHOULD BEGIN THE PERIOD BUT CEILINGS ARE EXPECTED TO LOWER LATE TONIGHT AS MOISTURE INCREASES FROM THE APPROACHING SURFACE LOW. STARTED MVFR CEILINGS AT 08Z. RAIN SHOULD BEGIN TO SPREAD IN FROM THE SW AFTER 08Z WITH COVERAGE BECOMING MORE WIDESPREAD BY DAYBREAK. BROUGHT IN PREVAILING MVFR CEILINGS AND VISIBILITIES BEYOND 13Z. EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CIGS AND VSBYS LIKELY SUNDAY NIGHT THROUGH TUE. VFR CONDITIONS MAY RETURN ON WED. && .MARINE... THE PRES GRADIENT WAS PINCHING PRE-DAWN FROM AROUND BUOY 41004 UP TO THE BUOY 41013. NNE FLOW AVERAGING 10 TO 15 KT OVER OUR WATERS THIS MORNING...EXCEPT COASTWARD FROM GRAYS REEF WHERE SPEEDS WERE FAIRLY LIGHT IN A WEAKER GRADIENT. SEAS WERE ELEVATED BEYOND 40 NM OFF THE COAST ACCORDING TO STEADY WAVEWATCH OUTPUT AND FETCH LENGTH AND PERSISTENCE PROGS. WE WILL CONTINUE WITH MARGINAL SMALL CRAFT ADVISORY CONDITIONS...MAINLY FOR SEAS OVER OUR OUTER GEORGIA WATERS. THE COASTAL CAROLINAS PRES GRADIENT WILL TIGHTEN TONIGHT AND WITH STEADY NE FLOW...IT APPEARS SEAS WILL BUILD INTO SCA RANGE OFF AT LEAST THE CHARLESTON COUNTY COAST OVERNIGHT. THE MODELS COULD BE UNDERDONE WITH THE PINCHING AND FORECAST CONDITIONS MAY TURN OUT SOMEWHAT OPTIMISTIC. OUTER GEORGIA WATERS WILL REMAIN IN SCA CONDITIONS TONIGHT. AN AREA OF LOW PRESSURE WILL TRACK EASTWARD ALONG THE NORTHERN GULF COAST AND GEORGIA/FLORIDA BORDER REGION SUNDAY AND SUNDAY NIGHT...THEN NORTHEASTWARD ALONG THE EASTERN SEABOARD AND INTO SOUTHERN NEW ENGLAND MONDAY THROUGH TUESDAY. HIGH PRESSURE WILL THEN BUILD IN FROM THE WEST ON WEDNESDAY. EXPECT A FAIRLY STRONG PINCHED GRADIENT ACROSS THE WATERS SUNDAY AND SUNDAY NIGHT...WITH SMALL CRAFT CONDITIONS CONTINUING ACROSS PORTIONS OF THE WATERS. AS LOW PRESSURE PULLS AWAY FROM THE REGION MONDAY...EXPECT IMPROVING CONDITIONS WITH WINDS AND SEAS REMAINING BELOW SMALL CRAFT THRESHOLDS THROUGH WEDNESDAY. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EST MONDAY FOR AMZ374. SMALL CRAFT ADVISORY FROM 10 PM THIS EVENING TO 5 AM EST MONDAY FOR AMZ350. && $$ JRL
