000 FXUS62 KCHS 222122 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 422 PM EST SUN NOV 22 2009 .SYNOPSIS... AN AREA OF LOW PRESSURE WILL TRACK OFFSHORE ALONG THE SOUTHEAST U.S. COAST TONIGHT...BEFORE LIFTING NORTHEAST OF THE AREA MONDAY INTO TUESDAY. MEANWHILE...A HIGH PRESSURE WEDGE WILL REMAIN FIRMLY ENTRENCHED OVER COASTAL SOUTH CAROLINA AND GEORGIA THROUGH TUESDAY. ANOTHER AREA OF LOW PRESSURE IS EXPECTED TO MOVE ACROSS THE EASTERN GULF OF MEXICO ON WEDNESDAY THEN ACROSS THE SOUTHWEST ATLANTIC ON THURSDAY. HIGH PRESSURE WILL THEN GRADUALLY BUILD OVER THE REGION FROM THE WEST THROUGH NEXT WEEKEND. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... AN UPPER LOW LIFTING UP THE SPINE OF THE APPALACHIANS WILL SPUR SURFACE CYCLOGENESIS OFF THE SC COAST TONIGHT. MEANWHILE...RESIDUAL LOW PRESSURE IN THE EASTERN GULF WILL CONTINUE TO WEAKEN TONIGHT. A LOBE OF VORTICITY ON THE EASTERN SIDE OF THE SHORTWAVE IS RESPONSIBLE FOR MUCH OF THE PRECIPITATION CURRENTLY OVERSPREADING THE AREA. WEAK NVA ON THE BACK SIDE OF THIS FEATURE IS CLEARING MOST OF THE PRECIPITATION OUT OF OUR SOUTHWESTERN ZONES. SOME ADDITIONAL ISENTROPIC LIFT ACROSS SOUTHERN SC MAY OCCUR THIS EVENING AS THE COASTAL TROUGH STRENGTHENS AND MOISTURE STREAMS OFF THE ATLANTIC. HOWEVER PRECIP SHOULD START TAPERING OFF FROM SW TO NE OVERNIGHT AS THE MAJORITY OF THE FORCING PUSHES NE OF THE AREA. PRODIGIOUS CLOUD COVER AND COLD ADVECTION WILL DROP TEMPS INTO THE UPPER 40S INLAND TO LOWER 50S COASTAL. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... NORTHERLY FLOW WILL PERSIST INTO MONDAY BEHIND A DEPARTING SURFACE LOW WHILE A WEAK COASTAL TROUGH REMAINS OFF THE COAST. WITH HIGH PRESSURE CONTINUING TO RIDGE IN FROM THE NORTH...WEAK ISENTROPIC ASCENT WILL MAINTAIN CLOUDY SKIES IN MOST AREAS WITH AREAS OF DRIZZLE POSSIBLE. THE BRUNT OF THE HEAVIER RAINFALL SHOULD BE WELL NE OF THE AREA BY MID-MORNING. COLD ADVECTION WILL MAINTAIN BELOW- NORMAL TEMPERATURES /MID TO UPPER 50S/. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... LOW PRESSURE WILL CONTINUE TO TRACK NORTH-NORTHEAST OFF THE MID-ATLANTIC AND NEW ENGLAND COASTS MONDAY NIGHT INTO TUESDAY. AS THIS OCCURS...THE RESULTING NORTHERLY FLOW OVER THE REGION WILL ACT TO REINFORCE THE EXISTING WEDGE...WHICH SHOULD CONTINUE TO EXTEND SOUTHWARD TO JUST NORTH OF THE ALTAMAHA RIVER. MODEL TIME/HEIGHT CROSS SECTIONS CONTINUE TO DEPICT ABUNDANT MOISTURE WITHIN THE BOUNDARY LAYER TRAPPED BENEATH THE LOW LEVEL INVERSION...WHICH SHOULD BE ENOUGH TO SUPPORT CLOUDY SKIES AND AREAS OF DRIZZLE INTO TUESDAY MORNING. IN ADDITION...LOW CEILINGS MAY DROP TO THE SURFACE MONDAY NIGHT INTO TUESDAY MORNING IN A WEAK LOW LEVEL WIND REGIME... RESULTING IN AREAS OF FOG WHICH COULD BECOME DENSE IN SOME LOCATIONS. THIS IS ALSO SUPPORTED BY THE SHORT RANGE ENSEMBLE FORECAST /SREF/ WHICH HAS RATHER HIGH PROBABILITIES OF LOW VISIBILITIES ACROSS THE AREA. AS FOR TEMPERATURES...CONTINUE TO FORECAST HIGHS WELL BELOW GUIDANCE ON TUESDAY AS MODELS LIKELY DO NOT HAVE A GOOD HANDLE OF THE STRENGTH OF THE WEDGE. SOME UNCERTAINTY DEVELOPS DURING THE MID-WEEK PERIOD AS MODELS HAVE BEGUN TO TREND TOWARD THE SOLUTION OF LOW PRESSURE TRACKING ACROSS THE EASTERN GULF OF MEXICO ON WEDNESDAY...THEN SHIFTING/REDEVELOPING OVER THE SOUTHWEST ATLANTIC ON THURSDAY. ACCOUNTING FOR THE LATEST MODEL TRENDS...THE FORECAST REFLECTS A SLIGHT CHANCE OF SHOWERS ACROSS NORTH COASTAL GEORGIA ON WEDNESDAY...ACROSS ALL COASTAL AREAS WEDNESDAY NIGHT...THEN SHIFTING NORTH AND EAST OF THE AREA FOR THANKSGIVING DAY. IF THE MODELS CONTINUE THE TREND IN THIS DIRECTION...THEN RAIN CHANCES MAY NEED TO BE INCREASED AND/OR EXPANDED ACROSS MORE OF THE FORECAST AREA. MEDIUM RANGE GUIDANCE IS IN GOOD AGREEMENT DEPICTING A BROAD UPPER LEVEL TROUGH SHIFTING OVER THE EASTERN U.S. FRIDAY INTO SATURDAY...AS STRONG HIGH PRESSURE AT THE SURFACE GRADUALLY BUILDS FROM THE WEST. THIS WILL RESULT IN DRY CONDITIONS...HOWEVER ROUNDS OF STRONG COLD AIR ADVECTION WILL DEVELOP AS REINFORCING COLD FRONTS SHIFT THROUGH AT THE SURFACE. THIS WILL LEAD TO MUCH BELOW NORMAL TEMPERATURES. THERE MAY EVEN BE SOME FROST/FREEZE CONCERNS INLAND...WITH THE COOLEST TEMPERATURES EXPECTED FRIDAY NIGHT. THE SURFACE HIGH WILL SETTLE OVER THE REGION ON SUNDAY WITH WEAK RIDGING BUILDING ALOFT. THIS WILL ALLOW FOR SLOWLY MODERATING TEMPERATURES BUT WILL ALSO KEEP DRY CONDITIONS IN PLACE. && .AVIATION /21Z SUNDAY THROUGH FRIDAY/... BACK EDGE OF THE STEADY RAINS WILL MOVE THROUGH KSAV THIS EVENING...BUT WILL TAKE THE FIRST PART OF TONIGHT AT KCHS BEFORE TAPERING TO DRIZZLE. MVFR AND/OR IFR CONDITIONS WILL PREVAIL THROUGH THE ENTIRE FORECAST PERIOD...ALTHOUGH IF WE ARE ABLE TO OBTAIN ENOUGH OF A DOWNSLOPING NW WIND AT KSAV THERE MIGHT BE A SMALL PERIOD OF VFR CIGS MONDAY AFTERNOON. THAT HOWEVER APPEARS UNLIKELY AS OF SUNDAY AFTERNOON. VAD WIND PROFILE SUGGESTS THAT WIND SHEAR IS OCCURRING AT KSAV...WHICH WILL CONTINUE THROUGH 03Z BEFORE ENDING. EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CONDITIONS ARE LIKELY MONDAY INTO TUESDAY. VFR CONDITIONS SHOULD BECOME PREVALENT WEDNESDAY THROUGH FRIDAY. && .MARINE... GRADIENT IS EXPECTED TO TIGHTEN THIS EVENING AS THE COASTAL TROUGH STRENGTHENS OFF OUR COAST. BRIEF NE WIND SURGE WILL PROBABLY TAKE PLACE BEFORE A CLOSED SURFACE LOW DEVELOPS NE OF THE WATERS AFTER MIDNIGHT. WILL CONTINUE SMALL CRAFT ADVISORIES FOR ALL NEARSHORE WATERS AND THE HARBOR THROUGH MIDNIGHT WITH AN ADVISORY RUNNING THROUGH 11 AM MONDAY FOR THE OFFSHORE GA WATERS FOR 6 FT SEAS. RAPIDLY IMPROVING CONDITIONS WILL OCCUR ON MONDAY AS FLOW BECOMES OFFSHORE AND GRADIENT SIGNIFICANTLY WEAKENS. MONDAY NIGHT THROUGH FRIDAY...A WEDGE OF HIGH PRESSURE INLAND WILL KEEP A NORTHERLY FLOW IN PLACE OVER THE COASTAL WATERS THROUGH WEDNESDAY NIGHT WITH WINDS LESS THAN 15 KT. A COLD FRONT WILL SHIFT OFFSHORE THURSDAY...WITH STRONG COLD AIR ADVECTION DEVELOPING IN ITS WAKE. THIS COULD RESULT IN THE DEVELOPMENT OF SMALL CRAFT ADVISORY CONDITIONS...ESPECIALLY OVER THE OUTER GEORGIA WATERS...BY LATE IN THE WORK WEEK. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR AMZ330-350- 352-354. SMALL CRAFT ADVISORY UNTIL 11 AM EST MONDAY FOR AMZ374. && $$ JRL/JAQ