Issued by NWS Charleston, SC
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-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --000 FXUS62 KCHS 220210 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 910 PM EST SAT NOV 21 2009 .SYNOPSIS...-- Changed Discussion --HIGH PRESSURE WILL REMAIN NORTH OF THE REGION THROUGH TONIGHT. AN AREA OF LOW PRESSURE WILL TRACK ALONG THE GULF COAST STATES AND INTO THE ATLANTIC OFF THE SOUTHEAST GEORGIA COAST SUNDAY NIGHT AND MONDAY...THEN MOVE NORTHEAST ALONG THE EASTERN SEABOARD. A HIGH PRESSURE WEDGE WILL THEN DOMINATE INTO MID WEEK. LOW PRESSURE WILL PASS OFF THE SOUTHEAST U.S. COAST ON THURSDAY...THEN WILL BE FOLLOWED BY COLDER HIGH PRESSURE FROM THE WEST INTO THE WEEKEND.-- End Changed Discussion --&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...-- Changed Discussion --A 1007 MB LOW IS FOUND ALONG THE SOUTHERN COAST OF LA...MOVING SLOWLY EASTWARD...WHILE 1025 MB HIGH IS CENTERED OVER LAKE ERIE AND IS WEDGING SOUTH INTO NORTHERN FLORIDA. THE CENTER OF THE HIGH WILL SHIFT EASTWARD THROUGH THE NIGHT...WITH THE WEDGE BECOMING FURTHER ESTABLISHED ACROSS THE AREA AS RAINS START BREAKING OUT ACROSS SOUTHERN GA IN ASSOCIATION WITH A DEVELOPING WARM FRONT TO THE SOUTH AND SHORT WAVES MOVING NE FROM THE GULF. AS THE WEDGE STRENGTHENS AND ISENTROPIC LIFT INCREASES WE LOOK FOR AN INCREASE IN VERTICAL MOISTURE THROUGH THE NIGHT...MAINLY FAR SOUTH...AS PWATS RISE TO 1-1.25 INCHES SOUTH OF I-16 IN SE GA LATE. GIVEN THAT THE LEADING EDGE OF THE PRECIP IS ALREADY A LITTLE FURTHER NE THAN MODEL GUIDANCE DEPICTS...I FEEL COMPELLED TO ADD AT LEAST SMALL POPS TO FAR SW ZONES LATE TONIGHT...BEGINNING AROUND 4 AM. CLOUD COVER HAS DISPERSED A BIT SINCE THIS AFTERNOON IN SOME AREAS...AND HAS ALLOWED FOR TEMPS TO FALL CLOSE TO OUR EARLIER MIN TEMP FORECAST. BUT WITH CONSIDERABLE MID AND HIGH LEVEL CLOUDINESS JUST UPSTREAM...TEMP FALL WILL SLOW OR EVEN REVERSE OVERNIGHT. WE WILL HOWEVER TWEAK TEMPS DOWN INTO THE UPPER 40S AND LOWER 50S MOST INLAND LOCATIONS...BUT HOLDING IN THE MID AND UPPER 50S CLOSER TO THE COAST WITH MODERATE NE WINDS DEVELOPING.-- End Changed Discussion --&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... A COASTAL TROUGH WILL FURTHER STRENGTHEN OFF THE NC/SC COAST SUNDAY MORNING...INCREASING ISENTROPIC ASCENT FROM THE E. MEANWHILE THE RESIDUAL GULF LOW WILL MOVE INTO THE FL PANHANDLE...ADVECTING QUITE A BIT OF LOW-LEVEL MOISTURE INTO SOUTHEAST GA AND CENTRAL SC. RAIN SHOULD OVERSPREAD THE ENTIRE AREA BY LATE MORNING. WE LEFT ISOLATED THUNDERSTORMS ACROSS FAR SOUTHEASTERN PORTIONS IN THE AFTERNOON DUE TO A SLUG OF 6C 700-500 MB LAPSE RATES PROGGED TO TREK THROUGH. COLD ADVECTION AND EVAPORATIONAL COOLING WILL KEEP TEMPERATURES FROM INCREASING VERY MUCH AFTER DAYBREAK. HIGHS WILL PROBABLY NOT GET ABOVE 60F. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... GUIDANCE IS IN GOOD AGREEMENT EJECTING WEAKENING SURFACE LOW PRESSURE OFF THE GEORGIA COAST SUNDAY NIGHT...THEN TRACKING IT NORTHEAST ALONG THE CAROLINA COAST INTO MONDAY. AS THIS OCCURS...OVERRUNNING PRECIPITATION WILL ALLOW A HIGH PRESSURE WEDGE TO BECOME SOLIDLY ENTRENCHED TO THE LEE OF THE APPALACHIANS AS THE PARENT HIGH STRENGTHENS OVER NEW ENGLAND. THERE IS SOME UNCERTAINTY AS TO HOW QUICKLY THE WIDESPREAD RAIN WILL TAPER OFF FROM THE SOUTHWEST SUNDAY NIGHT...AND SOME CONSIDERATION NEEDS TO BE GIVEN TO THE NAM WHICH SHOWS A DRY SLOT DEVELOPING OVER THE AREA. AS A RESULT...THE FORECAST WILL SHOW CATEGORICAL RAIN CHANCES TOWARD THE SANTEE RIVER DECREASING TO ONLY A CHANCE OF RAIN TOWARD THE ALTAMAHA RIVER SUNDAY EVENING. RAIN CHANCES LATE SUNDAY NIGHT WILL CONTINUE TO DIMINISH AS THE DEEPEST MOISTURE SHIFTS NORTH AND EAST OF THE AREA. AS FOR MONDAY...DEEP MOISTURE WILL BE LACKING BUT PLENTY OF LOW LEVEL MOISTURE WITHIN THE WEDGE COULD SUPPORT A SLIGHT CHANCE OF LIGHT RAIN AND/OR DRIZZLE ACROSS THE AREA. TEMPERATURES WILL ALSO BE SEVERELY IMPACTED BY THE WEDGE...AND GUIDANCE APPEARS MUCH TOO WARM GIVEN THE EXPECTED SCENARIO...THEREFORE WILL FORECAST HIGHS IN THE MID 50S WELL INLAND TO AROUND 60 TOWARD THE COAST ON MONDAY. THE HIGH PRESSURE WEDGE SHOULD PERSIST INTO TUESDAY...BEFORE GRADUALLY ERODING AS THE PARENT HIGH SHIFTS OFF THE NEW ENGLAND COAST. ABUNDANT LOW LEVEL MOISTURE WILL KEEP CONSIDERABLE CLOUDINESS IN PLACE...ALONG WITH THE POTENTIAL FOR AT LEAST PATCHY DRIZZLE. HIGH TEMPERATURES TUESDAY MAY MODERATE SOME...BUT SHOULD REMAIN BELOW NORMAL AND WELL BELOW WHAT GUIDANCE SUGGESTS. WEDNESDAY WILL BE A TRANSITION DAY...WITH DRY CONDITIONS AND NEAR NORMAL TEMPERATURES EXPECTED. LOW PRESSURE IS EXPECTED TO TRACK OFFSHORE ALONG THE SOUTHEAST U.S. COAST WEDNESDAY NIGHT INTO THURSDAY...BUT THERE IS SOME MODEL DISAGREEMENT AS TO HOW FAR OFFSHORE. THEREFORE...WILL SHOW SOME ISOLATED SHOWERS IN THE FORECAST FOR WEDNESDAY NIGHT. COOL AND DRY HIGH PRESSURE WILL THEN BUILD FROM THE WEST LATE IN THE WORK WEEK THROUGH THE FIRST PART OF THE WEEKEND. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... KCHS...HIGH PRESSURE WEDGE HOLDS IN PLACE TONIGHT WITH ANTICYCLONIC FLOW...AND ONLY LIMITED MOISTURE TO BE FOUND IN PLACE. THUS WE HAVE MAINTAINED VFR WEATHER THROUGH SUNDAY MORNING. HOWEVER...SHORT WAVE NOW MOVING INTO THE NE GULF OF MEXICO WILL MOVE THROUGH AFTER DAYBREAK AND RESULT IN SOME LIGHT RAINS DEVELOPING AROUND 14Z...AND ALSO FOR A LOWERING OF THE CIGS TO AROUND 5K FEET. THE COMBINATION OF INCREASING LOW LEVEL CONVERGENCE...ISENTROPIC LIFT AND SHORT WAVES ALOFT WILL PRODUCE A HEAVIER AND STEADIER RAIN OVERSPREADING THE AREA DURING THE AFTERNOON...AND WITH IT OUR CONDITIONS WILL DROP TO MVFR AT 18Z AND TO IFR AT 22Z. THE RISK OF WIND SHEAR IS MARGINAL BUT DOES BEAR WATCHING SUNDAY AFTERNOON. KSAV...THE HIGH PRESSURE WEDGE AND ITS RESULTING ANTICYCLONIC FLOW EARLY TONIGHT WILL GIVE WAY TO A SHORT WAVE AND MORE CYCLONIC CURVATURE OF THE ISOBARS LATE TONIGHT. THIS ALLOWS FOR A STEADY MOISTENING OF THE LOWER LEVELS AND EVENTUALLY LIGHT RAINS AND A LOWERING OF THE CIGS TO OCCUR...FALLING TO MVFR AT 14Z. A MORE SUBSTANTIAL INCREASE IN OVERALL MOISTURE AND LOW LEVEL CONVERGENCE AND ISENTROPIC LIFT FORMS FOR THE AFTERNOON...AND IFR WEATHER WILL PREVAIL FROM 18Z ONWARDS. LATER SHIFTS WILL NEED TO KEEP WATCH FOR A LITTLE ELEVATED CONVECTION THAT WILL ATTEMPT TO DEVELOP LATE SUNDAY AS SHOWALTER INDICES DROP TO -1 OR -2 AND MID LEVEL LAPSE RATES APPROACH 6C/KM. ALSO...THE RISK OF WIND SHEAR IS MARGINAL BUT DOES BEAR WATCHING DURING SUNDAY. EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CONDITIONS ARE LIKELY SUNDAY NIGHT THROUGH TUESDAY. VFR CONDITIONS MAY RETURN ON WEDNESDAY. && .MARINE...-- Changed Discussion --THERE WILL CONTINUE TO BE ENOUGH PINCHING OF THE PRESSURE GRADIENT TONIGHT BETWEEN THE INLAND HIGH PRESSURE WEDGE AND THE DEVELOPING WARM FRONT TO THE SOUTH...THAT 20 KT WINDS ARE COMMON ACROSS THE ATLANTIC WATERS TONIGHT. EVEN CHARLESTON HARBOR WILL HAVE A SOLID 15 KT. SEAS WILL BUILD TO 5 OR 6 FEET WITHIN 20 NM OF THE COAST...AND UP TO 6 OR 7 FEET ACROSS THE OUTER GA WATERS. THE WATERS WILL BE QUITE AGITATED WITH THE NE OR NORTH WIND WAVES...AND A 2-4 FOOT SWELL WAVE FROM THE EAST EVERY 8-9 SECONDS. WE WILL CONTINUE THE SCA/S FOR AMZ350-352-374...AND CAREFULLY KEEP WATCH ON AMZ330-354 SHOULD THE PINCHING GET STRONGER AND INDUCE HIGHER WINDS/SEAS. THE PRESSURE GRADIENT WILL INCREASE FURTHER SUNDAY AS THE WEDGE OF HIGH PRESSURE STRENGTHENS FROM THE NORTH WHILE AN APPROACHING UPPER TROUGH STRENGTHENS A TROUGH AT THE SURFACE. 20 OR 25 KT WINDS WILL BE COMMON...WHILE SEAS WILL BE AT LEAST AS HIGH AS 5 TO 7 FEET. SCA/S MAY BE NEEDED FOR AMZ354 AND AMZ330...OTHERWISE SOLID SCA WILL OCCUR ELSEWHERE. CONDITIONS WILL BEGIN TO SLOWLY IMPROVE OVER THE WATERS SUNDAY NIGHT DUE TO A WEAKENING SURFACE PRESSURE GRADIENT AS LOW PRESSURE EJECTS OFF THE GEORGIA COAST AND TRACKS JUST OFF THE COAST OF SOUTH CAROLINA. WINDS AND SEAS SHOULD FALL BELOW SMALL CRAFT ADVISORY CRITERIA OVER THE FAR SOUTHERN SOUTH CAROLINA NEAR SHORE WATERS SUNDAY EVENING...THEN OVER THE REMAINING SOUTH CAROLINA NEAR SHORE WATERS AND OUTER GEORGIA WATERS BY LATE SUNDAY NIGHT. A NORTHERLY FLOW WILL THEN PERSIST THROUGH MID WEEK...WITH SPEEDS 15 KT OR LESS. AN STRENGTHENING OFFSHORE FLOW IS EXPECTED ON THURSDAY AS STRONG COLD AIR ADVECTION DEVELOPS OVER THE WATERS.-- End Changed Discussion --&& .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL MIDNIGHT EST SUNDAY NIGHT FOR AMZ352. SMALL CRAFT ADVISORY UNTIL 5 AM EST MONDAY FOR AMZ350-374. && $$ 33
