FXUS62 KCHS 201530 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 1030 AM EST TUE JAN 20 2009 .SYNOPSIS... LOW PRESSURE OFF THE EASTERN NORTH CAROLINA COAST WILL CONTINUE TO TRACK NORTHEASTWARD AND AWAY FROM THE AREA TODAY. HIGH PRESSURE WILL THEN BUILD BACK INTO THE AREA TONIGHT THROUGH FRIDAY...BEFORE THE NEXT COLD FRONT MOVES INTO THE REGION OVER THE UPCOMING WEEKEND. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... CURRENTLY AT THE SURFACE...AN AREA OF LOW PRESSURE CONTINUES TO INTENSIFY OFF THE EASTERN NORTH CAROLINA COAST AS IT TRACKS NORTHEASTWARD AND AWAY FROM THE FORECAST AREA. UPSTREAM A VIGOROUS SHORTWAVE TROUGH OVER NORTHERN ALABAMA...IS BEGINNING TO ROUND THE BASE OF A HIGHLY AMPLIFIED EASTERN CONUS UPPER TROUGH. RADAR IMAGERY SHOWS SCATTERED RETURNS PRIMARILY NORTH OF THE SAVANNAH RIVER. CALLS RECEIVED FROM THE PUBLIC AS WELL AS REPORTS RELAYED BY LOCAL MEDIA CONFIRM THAT PRECIPITATION HAS CHANGED OVER TO PRIMARILY SNOW ACROSS THE AREA. MEANWHILE VISIBLE SATELLITE IMAGERY SHOWS GENERALLY CLOUDY CONDITIONS OVER A GOOD 2/3RDS OF THE FORECAST AREA WITH PARTLY CLOUDY TO MOSTLY SUNNY CONDITIONS ALONG THE ALTAMAHA. MAIN NEAR TERM CONCERN FOR THE REMAINDER OF THE DAY REVOLVES AROUND WINTER WEATHER AND POTENTIAL SNOW ACCUMULATIONS. THE AREA OF LOW PRESSURE OFF THE NORTH CAROLINA COAST WILL CONTINUE TO TRACK NORTHEASTWARD AND AWAY FROM THE FORECAST AREA THIS AFTERNOON...WHILE A POTENT UPPER LEVEL DISTURBANCE PUSHES INTO THE REGION. GUIDANCE SHOWS A LAYER OF NEGATIVE SATURATED EQUIVALENT POTENTIAL VORTICITY UNDERNEATH A LAYER OF FRONTOGENETICAL FORCING BETWEEN H5 AND H4 HOLDING ON THROUGH MID AFTERNOON PRIMARILY NORTH OF THE SAVANNAH RIVER AND FOCUSED ACROSS FAR NORTHWESTERN SOUTH CAROLINA ZONES IN ASSOCIATION WITH LINGERING DEFORMATION ZONE FROM STORM SYSTEM OFF THE NORTH CAROLINA COAST. ADDITIONALLY...MODELS SHOW PRETTY DECENT H85-H5 LAYER QG FORCING PUSHING THROUGH THE REGION IN ASSOCIATION WITH APPROACHING SHORTWAVE TROUGH THIS AFTERNOON AND INTO EARLY THIS EVENING. LIMITING MECHANISMS BEHIND ANY ORGANIZED CONVECTIVE BANDING HOWEVER WILL LIKELY BE DRY AIR IN PLACE IN THE H7 TO H5 LAYER THROUGH MID TO LATE AFTERNOON AND IN THE H85-H7 LAYER THROUGH ROUGHLY 16/17Z. THIS ALONG WITH NORTHWESTERLY DOWNSLOPE FLOW AND SMALL SATURATED REGION IN THE -12 TO -16C DENDRITIC GROWTH RANGE AS SEEN IN MODEL SOUNDINGS...SHOULD LIMIT ANY POTENTIAL SNOW ACCUMULATIONS TO LESS THAN AN INCH AND BELOW WARNING CRITERIA. ONE CAVEAT THAT MAY COME INTO PLAY HOWEVER IS THE ROUGHLY 25 TO 30 NAUTICAL MILE FETCH OVER LAKES MARION AND MOULTRIE WITH A NORTHWESTERLY LOW LEVEL FLOW LATE THIS AFTERNOON AND THIS EVENING. COULD SEE SOME ENHANCEMENT GIVEN LINGERING SYNOPTIC FORCING FOR ASCENT IN ASSOCIATION WITH APPROACHING SHORTWAVE TROUGH...PLENTY OF SYNOPTIC MOISTURE WITH H85-H7 RH GREATER THAN 60 PERCENT...AND LAKE SURFACE-H85 DELTA T/S AROUND 20 TO 21C. 1000-850 MB WIND FIELDS WILL NOT REMAINED LOCKED IN A FIXED DIRECTION FOR A PROLONGED PERIOD OF TIME HOWEVER TO TAKE ADVANTAGE OF OPTIMUM FETCH. THEREFORE...DON/T ANTICIPATE ANY ACCUMULATIONS REACHING WARNING CRITERIA OVER A LARGE ENOUGH AREA TO WARRANT A WINTER STORM WARNING...BUT WOULDN/T BE SURPRISED TO SEE A FEW ISOLATED REPORTS OF AROUND 2 INCHES DOWNWIND OF LAKE MOULTRIE. AS FOR HEADLINES...WILL KEEP ONGOING WINTER WEATHER ADVISORIES IN TACT BUT EXTEND THEM THROUGH 7 PM THIS EVENING AS SYNOPTIC MOISTURE AND FORCING ARE EXPECTED TO REMAIN IN PLACE INTO EARLY THIS EVENING. WILL NEED TO MONITOR CHARLESTON COUNTY CLOSELY FOR A POTENTIAL NEED FOR A WINTER WEATHER ADVISORY DURING THE EVENING RUSH...BUT AT THIS TIME IT APPEARS ANY WIDESPREAD ACCUMULATING SNOW...ALBEIT VERY SMALL AMOUNTS...WILL REMAIN JUST OFF TO THE NORTHWEST. TEMPERATURES AREN/T EXPECTED TO MOVE MUCH THROUGH THE REMAINDER OF THE DAY WITH ONGOING STRONG COLD AIR ADVECTION. AFTER EARLY MORNING HIGHS IN MOST LOCALES...TEMPERATURES SHOULD HOLD IN THE 30S ACROSS MOST OF THE AREA...WITH SOME LOWER 40S EXPECTED ALONG THE ALTAMAHA WHERE MORE SUNSHINE WILL BE SEEN. LAKE WINDS...STRONG PRESSURE RISES /5-6 MB PER 3 HR/ HAVE DEVELOPED OVER THE PAST COUPLE OF HOURS OVER BERKELEY COUNTY AND SURROUNDING AREAS. THE TIGHTENED SFC PRES GRADIENT AS A RESULT BETWEEN INCOMING HIGH PRES AND STRENGTHENING OFFSHORE LOW PRES WILL MAINTAIN STRONG NW WINDS OVER LAKE MOULTRIE THROUGH THIS AFTN. WILL MAINTAIN THE LAKE WIND ADVY AS IS...WITH ITS CURRENT EXPIRATION AT 5PM...AND LOOK FOR WINDS TO STEADILY DIMINISH LATE THIS AFTN. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... CLEARING SKIES AND ONGOING COLD AIR ADVECTION WILL RESULT IN CHILLY NIGHT ACROSS THE REGION WITH LOWS DROPPING INTO THE UPPER TEENS WELL INLAND WITH MID 20S CLOSER TO THE COAST. COULD SEE FLURRIES LINGER ACROSS BERKELEY AND CHARLESTON COUNTY UNTIL SUNSET BUT NO ADDITIONAL ACCUMULATIONS WILL OCCUR. WIND CHILL VALUES WILL DROP TO 10-15 INLAND AND CLOSER TO 15-20 ALONG THE COAST...WHICH IS ABOVE WIND CHILL ADVISORY THRESHOLDS. ANY SNOW OR WET GROUNDS THAT REMAIN AFTER SUNSET WILL QUICKLY REFREEZE AND THERE IS A POSSIBILITY THAT BLACK ICE COULD BECOME AN ISSUE IN PLACES. THIS WILL BE CAREFULLY MONITORED AS A WINTER WEATHER ADVISORY FOR BLACK ICE MAY BE NEEDED. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... HIGH PRESSURE WILL BUILD INTO THE REGION FROM THE WEST ON WEDNESDAY...AS THE AXIS OF THE DEEP UPPER TROUGH SHIFTS INTO THE ATLANTIC. THIS PATTERN WILL ALLOW FOR RAIN FREE WEATHER AND A WARMING TREND THROUGH LATE WEEK. HIGH TEMPERATURES WILL RETURN TO NEAR SEASONAL NORMALS BY THURSDAY...REACHING THE UPPER 50S ACROSS THE FORECAST AREA. THE CENTER OF HIGH PRESSURE WILL GRADUALLY SLIP TO THE SOUTHEAST BY FRIDAY...WHILE A COLD FRONT APPROACHES FROM THE WEST. THE REGION WILL FALL WITHIN WARM SOUTHERLY FLOW AHEAD OF THE FRONT...AND EXPECT HIGH TEMPERATURES TO REACH THE LOWER TO MID 60S WITHIN THE WARM SECTOR FRIDAY. THERE ARE SOME TIMING DIFFERENCES BETWEEN THE ECMWF AND GFS REGARDING COLD FRONTAL PASSAGE OVER THE WEEKEND...WITH GFS BEING THE FASTER OF THE TWO SOLUTIONS. AS A COMPROMISE BETWEEN THE MODELS...HAVE INTRODUCED A SLIGHT CHANCE FOR SHOWERS AFTER MIDNIGHT EARLY SATURDAY MORNING....WITH HIGHEST RAIN CHANCES LATER ON SATURDAY. WILL CAP POPS AT 30 PERCENT...AS MODELS WILL LIKELY ADJUST TIMING IN SUBSEQUENT RUNS. DECREASE RAIN CHANCES FROM WEST TO EAST AS THE FRONT DROPS TO THE SOUTHEAST OFFSHORE ON SUNDAY. GFS RAPIDLY BUILDS HIGH PRESSURE DOWN FROM THE NORTH BEHIND THE FRONT ON SUNDAY...SETTING UP A STRONG INLAND WEDGE/COASTAL TROUGH PATTERN. A QUICK GLANCE AT 00Z ECMWF APPEARS TO SUGGEST IT ALSO COMING AROUND TO A SIMILAR PATTERN. HAVE THUS REINTRODUCED SLIGHT CHANCE POPS ACROSS THE AREA LATE SUNDAY NIGHT INTO MONDAY. COOLER HIGH TEMPERATURES IN THE 50S ARE POSSIBLE WITHIN ANY INLAND WEDGE ON SUNDAY. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/... KCHS...ANY PRECIP THAT FALLS IS EXPECTED TO BE IN THE FORM OF SNOW. WILL MAINTAIN MENTION OF 4SM -SHSN BETWEEN 15-18Z AS ACTIVITY UPSTREAM CONTINUES TO DEVELOP. MVFR CEILINGS WILL PERSIST ALONG WITH GUSTY WINDS THROUGH MUCH OF THE AFTERNOON BEFORE VFR CONDITIONS RETURN BY 00Z AS CLOUD COVER GETS SCOURED OUT BEHIND DEPARTING LOW PRESSURE. KSAV...SATELLITE OBSERVATIONS CONTINUE TO INDICATE LOW CLOUDS ON THE VERGE OF OVERSPREADING THE KSAV TERMINAL. EXPECT MVFR CEILINGS TO OCCUR BY 18Z OVER THE TERMINAL AND PERSIST THIS AFTERNOON ALONG WITH GUSTY WINDS. CEILINGS SHOULD RECOVER TO VFR BY THE EVENING AS CLOUD COVER BEGINS TO SCOUR OUT BEHIND DEPARTING LOW PRESSURE. NO PRECIPITATION IS EXPECTED AT THE KSAV TERMINAL OUTSIDE OF A FEW FLURRIES THIS MORNING...BUT CERTAINLY NOT ENOUGH TO INCLUDE A MENTION ON THE 12Z TAF. EXTENDED AVIATION OUTLOOK...VFR CONDITIONS ARE EXPECTED THROUGH FRIDAY NIGHT. COULD SEE MVFR CONDITIONS RETURN SATURDAY AS A COLD FRONT PUSHES INTO THE AREA. && .MARINE... HAVE OBSERVED CONTINUED STRONG PRES RISES OVER THE SC/GA COASTAL REGION THE PAST FEW HOURS AND THIS HAS BEEN REFLECTED IN SEEING GALE TO NEAR GALE FORCE WINDS AT BOTH BUOYS 41004 AND 41008. THE STRONGER WINDS WERE EARLIER THIS MORNING SEEN ALONG AND S OF THE ALTAMAHA RIVER BUT THAT HAS NOW TRANSITIONED TO WRN PORTIONS OF THE CWFA AS THE OFFSHORE LOW STRENGTHENS AND HIGH PRES TRIES TO COME IN. LATEST MRP AND MMG GUIDANCE INDICATES SOLID SUSTAINED SCA WINDS BUT OVER THE WARMER WATERS NEAR THE GULF STREAM...AM CONCERNED THAT FREQUENT GUSTS TO GALE FORCE CAN BE MAINTAINED THROUGH THE DAY. THUS...HAVE OPTED TO EXTEND THE GALE WARNING FOR THE OFFSHORE GA WATERS THROUGH THIS AFTN...AND WILL LOOK FOR WINDS TO SUBSIDE BELOW GALE FORCE BY LATE THIS AFTN. WILL ALSO KEEP SOLID SCA CONDITIONS GOING FOR ALL THE NEARSHORE WATERS...INCLUDING CHAS HARBOR. BY TONIGHT...WINDS WILL WILL DIMINISH AS HIGH PRESSURE TRIES TO BUILD IN FROM THE WEST. ONLY THE GA OFFSHORE WATERS LOOK TO REMAIN WITHIN SCA THRESHOLDS. WEDNESDAY THROUGH SUNDAY...HIGH PRESSURE WILL BUILD BACK INTO THE REGION WEDNESDAY THROUGH FRIDAY...WITH THE NEXT COLD FRONT EXPECTED TO APPROACH FROM THE WEST FRIDAY NIGHT AND PUSH THROUGH THE MARINE AREA LATE SATURDAY. STRONG WEST WINDS EARLY WEDNESDAY WILL GRADUALLY DIMINISH WITHIN BUILDING HIGH PRESSURE...THEN BECOME SOUTHWEST AHEAD OF THE FRONT BY EARLY FRIDAY. GFS MODEL DEVELOPS A TIGHT PRESSURE GRADIENT AS THE FRONT DROPS SOUTH THROUGH THE AREA LATE SATURDAY...SUGGESTING WINDS AND SEAS COULD REACH SMALL CRAFT ADVISORY CRITERIA. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...LAKE WIND ADVISORY UNTIL 5 PM EST THIS AFTERNOON FOR SCZ045. WINTER WEATHER ADVISORY UNTIL 7 PM EST THIS EVENING FOR SCZ043>045. MARINE...SMALL CRAFT ADVISORY UNTIL 5 PM EST THIS AFTERNOON FOR AMZ330- 350-352-354. GALE WARNING UNTIL 5 PM EST THIS AFTERNOON FOR AMZ374. && $$ JRJ/JPC/DPB