Issued by NWS Charleston, SC
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-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --000 FXUS62 KCHS 202059 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 359 PM EST FRI NOV 20 2009 .SYNOPSIS...-- Changed Discussion --HIGH PRESSURE WILL BUILD NORTH OF THE REGION THROUGH TONIGHT. A DEVELOPING GULF LOW PRESSURE SYSTEM WILL MOVE UP THE SOUTHEAST COAST SATURDAY NIGHT AND SUNDAY. HIGH PRESSURE WILL REBUILD BEGINNING EARLY NEXT WEEK AND REMAIN THE DOMINANT WEATHER FEATURE THROUGH MID-WEEK.-- End Changed Discussion --&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...-- Changed Discussion --HIGH PRESSURE WILL CONTINUE TO RIDGE IN TO THE NORTH TONIGHT... MAINTAINING A NORTHEAST ONSHORE FLOW OVERNIGHT. THE GRADIENT LOOKS TO TIGHTEN OVERNIGHT WITH GEOSTROPHIC WINDS INCREASING TO 20-25 KT BY 12Z SATURDAY. WE SHOULD SEE ANOTHER BATCH OF LOW STRATUS ADVECT IN FROM THE NORTH OVERNIGHT WHICH WILL PROBABLY AFFECT SIMILAR PORTIONS OF SOUTHERN SC AS THIS MORNING. FORTUNATELY...FOG DOES NOT APPEAR TO BE AN ISSUE BECAUSE STRONGER LOW-LEVEL WINDS SHOULD KEEP THE LOWEST FEW HUNDRED FEET MIXED FAIRLY WELL. LOWS WILL BE IN THE MIDDLE 40S INLAND WHERE THE WINDS WILL BE LIGHTEST AND SKY COVER WILL BE MOST LIMITED. ELSEWHERE UPPER 40S TO LOWER 50S EXPECTED.-- End Changed Discussion --&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...-- Changed Discussion --GUIDANCE HAS REALLY BACKED OFF ON BRINGING THE ISENTROPIC PRECIPITATION INTO THE REGION ON SATURDAY. THE GFS IS NOW THE OUTLIER SHOWING SOME LIGHT PRECIP BY AFTERNOON AS MOISTURE SPREADS OUT AHEAD OF THE DEVELOPING GULF LOW. DUE TO THE AMOUNT OF DRY AIR TO CONTEND WITH...THE CONSENSUS NAM/ECMWF/GEM/HPC SOLUTION OF MAINLY DRY CONDITIONS THROUGH SATURDAY SEEMS REASONABLE. WE RESTRICTED THE SLIGHT CHANCE POPS TO SOUTHEAST GA WHERE THE SLIGHTLY DEEPER MOISTURE WILL EXIST. ALTHOUGH SOME POCKETS OF SUN ARE LIKELY...IN GENERAL SKIES WILL BE MOSTLY CLOUDY DURING THE DAY. HIGHS SHOULD REMAIN IN THE UPPER 60S.-- End Changed Discussion --&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...-- Changed Discussion --MEDIUM RANGE GUIDANCE IS STILL IN DISAGREEMENT MUCH OF THE PERIOD LEADING TO LOWER THAN NORMAL CONFIDENCE. GUIDANCE SOLUTIONS ARE IN GOOD AGREEMENT DEVELOPING AN AREA OF LOW PRESSURE ALONG THE NORTHERN GULF COAST SATURDAY NIGHT...BUT THEN DIFFER SOMEWHAT ON THE EVOLUTION/TRACK OF THE SYSTEM. LATEST THINKING IS THAT THE INITIAL LOW WILL MOVE INLAND ALONG THE GULF COAST AND DISSIPATE WHILE THEN TRANSFERRING IT/S ENERGY TO THE SE US COAST SUN NIGHT. DESPITE THE DISAGREEMENT IN TIMING OF THE SYSTEM THROUGH THE AREA...THE MODELS ARE IN GENERAL AGREEMENT SHOWING AN INCREASE IN DEEP MOISTURE...PRESENCE OF SOME UPPER LEVEL FORCING...AND DECENT ISENTROPIC ASCENT OVERSPREADING THE AREA...MAINLY EARLY SUN THROUGH SUN NIGHT. HAVE LOWERED POPS A BIT SAT NIGHT BASED ON THE LATEST SREF AND PREFERRED WRF GUIDANCE. STILL LOOKS LIKE THE BEST CHANCES OF RAIN WILL COME SUN...WITH DECREASING CHANCES SUN NIGHT. MON LOOKS RATHER DRY EXCEPT FOR FAR NORTHERN PORTIONS OF THE FORECAST AREA. MODEL SOLUTIONS CONTINUE TO DIFFER THROUGH MUCH OF THE UPCOMING WORK WEEK BUT HAVE GENERALLY FOLLOWED THE DRIER GFS WHICH KEEPS A SOUTHERN STREAM SYSTEM OFF THE SE US COAST LATE IN THE WEEK.-- End Changed Discussion --&& .AVIATION /21Z FRIDAY THROUGH WEDNESDAY/... AT KCHS...LOW CLOUDS FINALLY SHIFTED AWAY FROM THE TERMINAL LATE THIS MORNING BUT ANOTHER ROUND OF LOW CLOUDS EXPECTED TO MOVE IN FROM THE NNE LATE THIS EVENING AND OVERNIGHT. WITH HIGH PRESSURE REMAINING CENTERED TO THE NORTH...20-25 KT NE GEOSTROPHIC WINDS WILL MAINTAIN A STEADY MOISTURE FEED ABOVE THE INVERSION. ENOUGH LOW-LEVEL MIXING SHOULD PREVENT THE FORMATION OF MUCH SURFACE FOG. HOWEVER A PERIOD OF IFR CEILINGS APPEAR INCREASINGLY LIKELY...MAINLY A FEW HOURS PRIOR TO DAYBREAK SATURDAY. INCLUDED A TEMPO IFR CEILING FROM 09-12Z. LATER ISSUANCES MAY NEED TO INCLUDE PREVAILING IFR CEILINGS. PARTIAL CLEARING EXPECTED BY LATE SAT MORNING ALTHOUGH SOME SCT CLOUDS BELOW 3 KFT MAY PERSIST. AT KSAV...MOST OF THE LOW CLOUDS TONIGHT ARE EXPECTED ACROSS SOUTHERN SC BUT THEY COULD EASILY ENCROACH ON THE SAV TERMINAL OVERNIGHT. WE KEPT VFR CONDITIONS IN THE 18Z TAFS SINCE WE EXPECT THERE TO BE ENOUGH LOW-LEVEL MIXING TO PRECLUDE FOG FORMATION AND THE CHANCE FOR STRATUS AT SAV IS TOO LOW AT THIS TIME. EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CEILINGS AND VSBYS POSSIBLE SAT THROUGH MON. VFR THEREAFTER. && .MARINE...-- Changed Discussion --HIGH PRESSURE WILL CONTINUE TO STRENGTHEN NORTH OF THE REGION OVERNIGHT INTO SATURDAY WITH THE PRESSURE GRADIENT TIGHTENING ACROSS THE COASTAL WATERS. ALTHOUGH NORTHEAST WINDS ARE EXPECTED TO REMAIN BELOW 20 KNOTS...SEAS SHOULD BUILD TO 4 TO 6 FT OVER THE OFFSHORE GA WATERS DURING THE DAY SATURDAY. HAVE HOISTED A SMALL CRAFT ADVISORY BEGINNING AT 6 AM SATURDAY FOR THESE WATERS. SATURDAY NIGHT THROUGH WEDNESDAY...AN AREA OF LOW PRESSURE ALONG THE GULF COAST REGION WILL TRANSFER ITS ENERGY TO A DEVELOPING LOW PRESSURE SYSTEM OFF THE SE US COAST SUN NIGHT/MON. HIGH PRESSURE WILL THEN BUILD BACK INTO THE AREA FROM THE WEST THROUGH THE MIDDLE OF NEXT WEEK. SMALL CRAFT ADVISORY CONDS ARE POSSIBLE ACROSS PORTIONS OF THE WATERS SATURDAY NIGHT INTO EARLY SUNDAY WITH IMPROVING CONDITIONS THEN EXPECTED THROUGH WEDNESDAY AS HIGH PRESSURE TAKES CONTROL.-- End Changed Discussion --&& .CHS WATCHES/WARNINGS/ADVISORIES...-- Changed Discussion --GA...NONE. SC...NONE. MARINE...SMALL CRAFT ADVISORY FROM 6 AM SATURDAY TO 10 AM EST SUNDAY FOR AMZ374.-- End Changed Discussion --&& $$ JRL/RJB
