022 FXUS62 KCHS 271148 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 648 AM EST FRI DEC 27 2013 .SYNOPSIS... A LARGE RIDGE OF HIGH PRESSURE WILL REMAIN SITUATED JUST NORTH OF THE REGION INTO TONIGHT. LOW PRESSURE WILL DEVELOP IN THE GULF OF MEXICO SATURDAY AND TRACK NORTHEAST OVER OR NEAR THE AREA THROUGH EARLY SUNDAY...BRINGING A PERIOD OF WET WEATHER TO SOUTHERN SC AND SOUTHEAST GA THIS WEEKEND. A WEAK COLD FRONT WILL MOVE THROUGH MONDAY NIGHT...FOLLOWED BY HIGH PRESSURE WHICH WILL LINGER THROUGH THE LATTER PART OF NEXT WEEK. && .NEAR TERM /THROUGH TONIGHT/... TODAY...A DAMPENING AND FAST-MOVING SHORT WAVE TROUGH WILL PROGRESS FROM THE EASTERN GREAT LAKES AND OHIO VALLEY EARLY ON TO THE MID-ATLANTIC REGION AND NEW ENGLAND FOR THE AFTERNOON. AS THIS OCCURS IT ALLOWS FOR A DEEP-LAYERED SW ATLANTIC RIDGE TO POKE A LITTLE NORTH AND NW ATOP THE LOCAL AREA. SIMULTANEOUSLY THIS KEEPS AN EXPANSIVE SURFACE RIDGE IN PLACE OVER A GOOD PORTION OF THE EASTERN HALF OF THE COUNTRY. THIS IN TURN BLOCKS A NOTICEABLE BAROCLINIC ZONE WITH A CONSTANT FEED OF SUB-TROPICAL MOISTURE NOT TOO FAR OFF TO THE SE AND SOUTH. WE/LL BE LOCATED WITHIN THE ANTICYCLONIC INFLUENCES OF THE BLOCKING SURFACE HIGH...WHICH WILL KEEP THE BEST LOW LEVEL CONVERGENCE JUST OUTSIDE THE CWFA TO THE SOUTH. THERE ARE HOWEVER A FEW WEAK MID LEVEL IMPULSES THAT WILL TRAVEL BRISKLY THROUGH THE SW AND WEST FLOW ALOFT THAT WILL SKIRT OUR SOUTHERNMOST ZONES. SO A FEW SHOWERS COULD SNEAK NORTH ACROSS THE ALTAMAHA RIVER THIS AFTERNOON...AND/OR THE LOW LEVEL FLOW MAY VEER A LITTLE FURTHER EAST THAN IS NOW ANTICIPATED. THIS WOULD ENHANCE LOW LEVEL CONVERGENCE FROM OFF THE ATLANTIC...PERHAPS GENERATING ISOLATED SHOWERS. FOR NOW WE HAVE HELD POPS TO LESS THAN 15 PERCENT...THUS NO MENTION OF SHOWERS IN THE ACTUAL FORECAST PRODUCTS. SKY COVER AND THE RESULTING TEMPERATURE FORECAST WILL BE THE TWO MAIN FORECAST CHALLENGES TODAY. WHILE VARYING AMOUNTS OF JET INDUCED MID/HIGH LEVEL CLOUDS WILL FILTER THE SUNSHINE AT TIMES OVER NORTHERN AND CENTRAL ZONES...THE MORE OPAQUE STRATOCUMULUS CLOUDS WILL RESIDE OVER SOUTHERN COMMUNITIES. THIS LOOKS TO BE APPROXIMATELY SOUTH OF A LINE FROM ABOUT HILTON HEAD TO SPRINGFIELD...CLAXTON AND REIDSVILLE. ANY FLUCTUATION NORTH OR SOUTH WITH THIS THICKER CLOUD COVER WILL HAVE AN IMPACT ON OUR MAX TEMPS. SO TODAY MIGHT BE ONE OF THOSE RARE DAYS WHEN IT/S ACTUALLY WARMER NORTH THAN SOUTH DEPENDING UPON HOW SKY COVER TRENDS PAN OUT. FOR NOW WE HAVE SHOWN UNIFORM TEMPS EVERYWHERE...BUT AT LEVELS NEAR OR SLIGHTLY BELOW CLIMATOLOGICAL NORMALS WITHIN A NORTH AND NE SYNOPTIC FLOW. TONIGHT...RIDGING ALOFT AND THE SURFACE HIGH TO THE NORTH WILL PERSIST...WHICH HOLDS OFF ON ANY NORTHERN PROGRESSION OF THE BAROCLINIC ZONE UNTIL CLOSER TO DAYBREAK SATURDAY. WE/LL BE CLOSELY WATCHING A SHORT WAVE ARRIVING IN THE GULF OF MEXICO FROM THE BAY OF CAMPECHE OVERNIGHT...WHICH ALLOWS FOR WEAK CYCLOGENESIS LATE IN THE PERIOD. WIND FIELDS LOCALLY IN THE LOWEST LEVELS ABOVE THE NOCTURNAL INVERSION VEER AROUND TO THE EAST...WHILE A SW AND WEST FLOW PREVAILS THROUGHOUT MUCH OF THE VERTICAL ABOVE 4-5K FT. THIS PRODUCES AN INCREASE IN ISENTROPIC UPGLIDE AND ALSO CAUSES A LITTLE COASTAL CONVERGENCE OVER FAR SE GEORGIA. THAT MAY BE ENOUGH TO GENERATE ISOLATED SHOWERS AFTER 4 OR 5 AM IN MCINTOSH AND COASTAL LIBERTY COUNTIES...BUT THE VAST MAJORITY OF THE REGION WILL BE RAINFREE. A DECENT SPREAD IN CLOUD COVER WILL BE IN PLACE...WITH SKY COVERAGE RANGING FROM 40-50 PERCENT NW TO 80-90 PERCENT SE. AS WINDS DECOUPLE OVER NW SECTIONS IN CLOSER PROXIMITY TO THE CENTER OF THE RIDGE TO THE NORTH...MINIMUMS WILL FALL INTO THE MIDDLE 30S. BUT WITH CONTINUED AIR MASS WARMING OUR TEMPS WILL TREND HIGHER AS ONE HEADS SE. UPPER 30S AND LOWER 40S WILL BE COMMON IN THE CORRIDOR FROM MCCLELLANVILLE TO CHARLESTON... BEAUFORT...SAVANNAH AND CLAXTON. WARMEST WILL BE ALONG THE IMMEDIATE COASTS OF SC AND THE SE QUADRANT OF GEORGIA WHERE UPPER 40S TO NEAR 50 WILL PREVAIL. && .SHORT TERM /SATURDAY THROUGH MONDAY/... SATURDAY AND SATURDAY NIGHT...ALOFT THE PATTERN WILL BEGIN TO AMPLIFY BETWEEN RIDGING OVER THE BAHAMAS/CARIBBEAN AND A CLOSED LOW/SHORTWAVE OVER THE SOUTHERN PLAINS. AS THE SHORTWAVE OVER THE SOUTHERN PLAINS AND DEVELOPING JET ENERGY EVOLVE...A WEAK SURFACE LOW IS POISED TO DEVELOP OVER THE CENTRAL/NORTHERN GULF OF MEXICO ON SATURDAY. THIS LOW IS THEN EXPECTED TO TRACK TO THE NORTHEAST AND ACROSS OR JUST INLAND OF THE FORECAST AREA THROUGH THE OVERNIGHT. SATURDAY WILL BRING ABOUT A SIGNIFICANT MOISTENING OF THE AREA AS DEEP MOISTURE STREAMS IN WITHIN THE DEEP SOUTHWEST FLOW. IN FACT...PWATS SURGE TO AROUND 1.75 INCHES LATE SATURDAY NIGHT WHICH IS NEAR THE 99TH PERCENTILE FOR LATE DECEMBER. INCREASING ISENTROPIC LIFT WILL KICK OFF THE INCREASING POP TREND ON SATURDAY...AND RAIN CHANCES INCREASE FROM SOUTH TO NORTH THROUGH THE DAY. THE PRIMARY PERIOD OF RAINFALL WILL TAKE PLACE SATURDAY NIGHT INTO SUNDAY MORNING AS THE SURFACE LOW DRAWS CLOSER AND DEEPER FORCING INVADES FROM THE WEST. PRECEDING THE DEVELOPING LOW...HIGH PRESSURE WILL BE POSITIONED JUST TO THE NORTH OF THE AREA SATURDAY MORNING. A CAD EVENT WILL BE UNDERWAY...AND WILL LIKELY THEN BE DIABATICALLY REINFORCED BY ANY PRECIP THAT OCCURS SATURDAY AFTERNOON EVENING. THE QUESTION THEN BECOMES WHAT IS THE EXACT TRACK OF THE LOW AND HOW QUICKLY IS THE CAD SCOURED OUT AND REPLACED BY A WARM SECTOR AND SOUTHERLY FLOW. THIS HAS SIGNIFICANT IMPLICATIONS FOR TEMPERATURES...THUNDERSTORMS...AND ANY SEVERE WEATHER POTENTIAL. THE MODELS CONTINUE TO SHOW SIGNIFICANT TRACK/TIMING DIFFERENCES SO OVERALL CONFIDENCE IN THE DETAILS IS QUITE LOW. REGARDING THE SEVERE WEATHER POTENTIAL...LATE SATURDAY NIGHT A STRENGTHENING WIND FIELD WILL SPREAD ACROSS THE REGION. THE MAIN QUESTION WILL BE THE DEGREE OF SURFACE DESTABILIZATION...IF ANY...IN WHAT WILL CERTAINLY BE A HIGH SHEAR/LOW CAPE ENVIRONMENT. GIVEN THE QUESTIONS SURROUNDING THE TRACK AND TIMING OF THE LOW...I WILL NOT MENTION ANYTHING IN THE HWO. SUNDAY THROUGH MONDAY...THE SURFACE LOW WILL MAKE ITS CLOSEST PASS SUNDAY MORNING AND THE FORECAST AREA SHOULD BECOME SOLIDLY WARM SECTORED AT SOME POINT. THE BULK OF PRECIP WILL OCCUR IN THE MORNING WITH A DIMINISHING TREND IN THE AFTERNOON. CATEGORICAL POPS REMAIN IN PLACE SUNDAY MORNING...WITH MUCH OF THE AREA GOING DRY IN THE LATE AFTERNOON/EVENING HOURS. THE DAMPENING SHORTWAVE ALOFT WILL RAPIDLY MOVE AWAY FROM THE AREA ON SUNDAY THOUGH MEAN TROUGHING WILL CONTINUE THROUGH MONDAY. BEFORE THE COLD FRONT PASSES THROUGH ON SUNDAY...TEMPS WILL SURGE TO THE UPPER 60S IN MANY AREAS WITHIN THE WARM SECTOR. MONDAY WILL THEN BE MORE OF A TRANSITION DAY IN BETWEEN THE DEPARTING SURFACE LOW AND ANOTHER MORE SUBTLE COLD FRONT ALONG THE LEADING EDGE OF HIGH PRESSURE BUILDING IN FROM THE WEST. MONDAY WILL BE DRY AS THE ANOMALOUSLY HIGH PWATS DIMINISH BACK TO AROUND 0.5 INCHES. TEMPS WILL COOL BACK CLOSER TO CLIMO WITH HIGHS IN THE LOW 60S EXPECTED. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... OVERALL...THE LONG TERM PERIOD CONTINUES TO LOOK QUIET. A DRY COLD FRONT WILL CROSS THE AREA MONDAY NIGHT FOLLOWED BY HIGH PRESSURE BUILDING IN BRIEFLY THROUGH TUESDAY NIGHT. THEN IN THE MID WEEK TIME PERIOD...A CLIPPER TYPE LOW WILL DRAG ANOTHER DRY COLD FRONT THROUGH THE AREA WEDNESDAY/WEDNESDAY NIGHT. MORE HIGH PRESSURE WILL THEN RETURN TO THE AREA THROUGH THE END OF THE PERIOD. TEMPERATURES WILL STAY ON THE COOL SIDE FOR THE LONG TERM...WITH HIGHS AND LOWS RUNNING SEVERAL DEGREES BELOW CLIMO FOR EARLY JANUARY. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... HIGH PRESSURE TO THE NORTH WILL BE THE INFLUENCING FACTOR AT THE TERMINALS THROUGH 12Z SATURDAY...KEEPING ANY RISK FOR SHRA TO THE SOUTH. WHILE WE DO ANTICIPATE VFR CONDITIONS WILL OCCUR...CEILINGS WILL BE AS LOW AS 3500-6000 FT AT KSAV BUT ABOVE 12K FT AT KCHS. NORTH-NE WINDS WILL AVERAGE 5-10 KT THROUGHOUT. EXTENDED AVIATION...INCREASING CHANCES OF MVFR OR LOWER CIGS/VSBYS SATURDAY EVENING INTO SUNDAY WITH WIDESPREAD SHOWERS ASSOCIATED WITH A PASSING SURFACE LOW AND COLD FRONT. GUSTY WINDS AND/OR LOW LEVEL WIND SHEAR ARE POSSIBLE LATE SATURDAY NIGHT INTO SUNDAY. THERE IS LOW RISK OF TSRA SUNDAY MORNING...MAINLY AT THE SAV TERMINAL. VFR CONDITIONS WILL RETURN BY LATE SUNDAY THROUGH MONDAY. && .MARINE... TODAY AND TONIGHT...A SPRAWLING 1030 MILLIBAR HIGH ALIGNED EAST/NE TO WEST/SW WILL COVER A LARGE AREA FROM SOUTH OF NEW ENGLAND TO TEXAS...WITH THE LOCAL MARINE COMMUNITY SITUATED NEAR THE SE PERIPHERY OF THIS MAIN SYNOPTIC FEATURE. THERE WILL REMAIN A FAIRLY DECENT PACKING OF THE ISOBARS BETWEEN THE RIDGE AND A STATIONARY FRONT ACROSS EXTREME SOUTH FLORIDA AND THE FLORIDA STRAITS...ENOUGH FOR SUSTAINED NORTH AND NE WINDS HOLD FAIRLY STEADY AROUND 15 OR 20 KT OVER THE ATLANTIC AND 10 OR 15 KT IN CHARLESTON HARBOR. THE WAVE SPECTRUM WILL BE COMPRISED OF A MIXTURE OF WIND DRIVEN WAVES AND 10-11 SECOND SWELLS...PRODUCING SIGNIFICANT WAVE HEIGHTS OF 3-5 FT WITHIN 20 NM OF SHORE AND UP TO 6 FT FURTHER OUT. THEREFORE WE HAVE MAINTAINED THE ONGOING SMALL CRAFT ADVISORY FOR AMZ374. SATURDAY THROUGH TUESDAY...TO START THE PERIOD HIGH PRESSURE WILL SIT OFF TO THE NORTH OF THE WATERS AS AN AREA OF LOW PRESSURE DEVELOPS OVER THE GULF OF MEXICO. THIS WILL MAINTAIN ELEVATED NE FLOW ACROSS THE WATERS IN THE LOCALLY PINCHED GRADIENT INTO SATURDAY NIGHT. AS THE LOW TRACKS NORTHEASTWARD SATURDAY NIGHT...A WARM FRONT FEATURE WILL LIFT NORTHWARD AS INCREASING SOUTHERLY FLOW SPREADS ACROSS THE WATERS. THE FLOW WILL THEN BECOME MORE SOUTHWESTERLY ON SUNDAY AND STRENGTHEN THANKS TO A STRONG LOW LEVEL WIND FIELD. CONDITIONS WILL THEN LEVEL OFF MONDAY BEFORE A DRY COLD FRONT MOVES THROUGH AND BRINGS ABOUT A SHOT OF COOLER AIR AND AN ASSOCIATED SURGE. THE SMALL CRAFT ADVISORY FOR THE OUTER GEORGIA WATERS HAS BEEN EXTENDED THROUGH SUNDAY NIGHT TO ACCOUNT FOR A COMBINATION OF WINDS AND SEAS. ALL OR PARTS OF THE NEARSHORE WATERS AND THE CHARLESTON HARBOR WILL LIKELY SEE ADVISORY LEVEL CONDITIONS SUNDAY INTO SUNDAY NIGHT...BEFORE IMPROVING ON MONDAY. THE TUESDAY SURGE WILL ONCE AGAIN BRING MARGINAL ADVISORY CONDITIONS BACK TO THE LOCAL WATERS. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 7 AM EST MONDAY FOR AMZ374. && $$ SYNOPSIS... NEAR TERM... SHORT TERM... LONG TERM...BSH AVIATION... MARINE...