FXUS62 KCHS 042203
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
502 PM EST FRI DEC 4 2009

.SYNOPSIS...
A DEVELOPING AREA OF LOW PRESSURE IN THE GULF OF MEXICO WILL SHIFT
NE ACROSS FLORIDA TONIGHT TO OFFSHORE THE SOUTH CAROLINA COAST BY
SATURDAY MORNING. THE LOW PRESSURE AREA WILL QUICKLY SHIFT
NORTHEAST DURING THE DAY SATURDAY...ALLOWING HIGH PRESSURE TO
RETURN FROM THE W SATURDAY NIGHT. HIGH PRESSURE WILL REMAIN OVER
THE REGION THROUGH SUNDAY. A TROUGH OF LOW PRESSURE WILL DEVELOP
NEAR THE COAST BY MONDAY. A WARM FRONT WILL APPROACH THE REGION
FROM THE SOUTH TUESDAY...THEN A COLD FRONT WILL ADVANCE THROUGH
THE SOUTHEAST WEDNESDAY. HIGH PRESSURE WILL THEN BUILD OVER THE
REGION FOR THE LATTER HALF OF NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
AN UPDATE TO THE GRIDS AND TEXT PRODUCTS HAS BEEN ISSUED TO REFLECT 
GOING TRENDS. MEDIA REPORTS OUT OF THE SAVANNAH AREA AS WELL AS 
SURFACE OBSERVATIONS ACROSS MUCH OF FAR SOUTHEAST GEORGIA SUGGEST 
MEASURABLE RAINS ARE BEGINNING TO FALL. RADAR DATA FROM 
KVAX/KTLH/KJAX SHOW LIGHT TO OCCASIONALLY MODERATE RAIN EXTENDS TO 
THE SOUTHWEST WELL INTO THE GULF OF MEXICO. ISENTROPIC LIFT WILL 
CONTINUE TO INTENSIFY TONIGHT AS SECONDARY CYCLOGENESIS OCCURS OFF 
THE SOUTH CAROLINA AND GEORGIA COASTS TONIGHT. HAVE UPDATED THE 
GRIDS TO REFLECT A QUICKER INCREASE IN POPS AND QPF THIS EVENING 
WITH ALL AREAS BECOMING CATEGORICAL AFTER MIDNIGHT. THE HEAVIEST 
RAIN AMOUNTS IS STILL ON TRACK TO FALL ALONG THE COAST WITH A 
SECONDARY MAX OCCURRING ALONG THE WESTERN WALL OF THE GULF STREAM.

AS FOR TEMPS...THE EXTENSIVE CLOUD COVER TODAY HELD TEMPS
DOWN...ESPECIALLY OVER SE GA. WITH THE CLOUDS REMAINING
OVERNIGHT...THERE WILL NOT BE MUCH OF A DROP TO FORECAST LOWS
TONIGHT. WILL LOOK FOR LOWS IN THE MID 40S AWAY FROM THE COAST AND
THE UPPER 40S NEAR THE SHORES.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
MUCH IF NOT ALL THE CHS CWFA SHOULD BE EXPERIENCING RAIN AT THE
START OF THE DAY SATURDAY. AS THE UPPER LEVEL TROUGH SHIFTS NE
INTO THE MID ATLC STATES DURING THE DAY...THE OFFSHORE LOW PRES
CENTER WILL QUICKLY SHIFT NE AND AWAY FROM THE AREA WITH HIGH
PRESSURE FILLING IN BEHIND IT. QPF/S IN THE MORNING MAY STILL BE
IN THE 1/4 TO 1/2 INCH RANGE SINCE THIS WILL BE THE TIME OF
DEEPEST MOISTURE AND CLOSEST PROXIMITY OF THE LOW...BUT RAIN
CHANCES WILL DWINDLE THROUGH THE EARLY AFTN HOURS WITH JUST ABOUT
ALL THE RAIN OUT OF THE AREA BY THE LATTER HALF OF THE AFTN...IF
NOT EARLIER DEPENDING ON THE DEPARTURE SPEED OF THE LOW PRESSURE
SYSTEM AND DRIER AIR MOVING IN FROM THE W.

WINDS WILL CONTINUE OUT OF THE NORTH...BACKING FROM NE TO NW AS
THE DAY PROGRESSES. THIS WILL KEEP A COOL FLOW OF AIR INTO THE
AREA WHICH WILL BE REFLECTED IN MAX TEMPS NOT COMING OUT OF THE
LOW 50S. SOME MODEL GUIDANCE IS DEPICTING MAX TEMPS IN THE UPR 40S
WHICH WOULD NOT BE UNREASONABLE IF CLOUDS HOLD ON LONGER THAN
EXPECTED.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
PERSISTENCE OF WRAP AROUND LOW CLOUDS REMAINS HIGHLY UNCERTAIN 
SATURDAY NIGHT. POSSIBLE SCENARIOS RANGE FROM CLEARING ACROSS THE 
ENTIRE REGION EARLY...TO N WINDS HOLDING LOW CLOUDS OVER MOST AREAS 
WELL INTO THE NIGHT. WHERE CLEARING OCCURS...FROST WILL LIKELY 
DEVELOP. INCLUDED A MENTION OF FROST WHERE CLEARING IS MOST LIKELY 
ACROSS S/W COUNTIES...AWAY FROM THE COAST. A 30 PERCENT CHANCE FOR
A KILLING FROST APPEARS REASONABLE...THUS WILL ALSO MENTION THE
POTENTIAL FOR FROST FOR COUNTIES WHERE THE GROWING SEASON HAS NOT
YET ENDED...EXCLUDING GA COASTAL COUNTIES. HOWEVER...THE POTENTIAL
FOR LOW TEMPERATURES 32F OR LOWER WITHIN THESE AREAS REMAINS BELOW
50 PERCENT...TOO LOW TO JUSTIFY A FREEZE WATCH.

HIGH PRESSURE WILL ADVANCE TO A POSITION N/NE OF THE FORECAST AREA 
SUNDAY THROUGH MONDAY...AND A TROUGH OF LOW PRESSURE WILL LIKELY 
DEVELOP OVER THE COASTAL WATERS. AS USUAL...MODEL DEPICTIONS OF
THIS FEATURE DIFFER...AND THESE DEPICTIONS IMPACT CLOUD COVER
AND POPS THROUGH EARLY NEXT WEEK. THE 12Z NAM DEPICTS A
WELL/DEFINED TROUGH PUSHING ONSHORE AND SUPPORTING SHOWERS ACROSS
MUCH OF THE REGION SUNDAY NIGHT INTO MONDAY. WHILE THIS SOLUTION
APPEARS TOO AGGRESSIVE/WET...MAINTAINED SLIGHT CHANCE POPS OVER
COASTAL WATERS SUNDAY/SUNDAY NIGHT...THEN PER INCREASED DEEP LAYER
MOISTURE/WEAK ASCENT PROVIDED BY A SUBTLE SHORTWAVE EMBEDDED
WITHIN WSW FLOW ALOFT...AREA CONSENSUS FOR SLIGHT CHANCE POPS
APPEARS REASONABLE FOR ALL AREAS MONDAY.

TUESDAY...MODELS HAVE TRENDED TOWARD AN EARLIER ONSET OF ISENTROPIC 
ASCENT/INCREASED DEEP LAYER MOISTURE AHEAD OF LOW PRESSURE TRACKING 
THROUGH THE US MIDSECTION. THUS...INTRODUCED CHANCE POPS 
TUESDAY...AND RAISED POPS TUESDAY NIGHT. THEN...AS A SECONDARY LOW 
PRESSURE DEVELOPS ALONG THE MID ATLANTIC COAST...A COLD FRONT WILL 
PUSH THROUGH THE REGION WITH SHOWERS AND PERHAPS EVEN A FEW 
THUNDERSTORMS WEDNESDAY. UNCERTAINTY REGARDING FORECAST DETAILS 
PRECLUDES POPS HIGH THEN AROUND 50 PERCENT WITHIN THIS 
FORECAST...BUT HIGHER POPS WILL EVENTUALLY BE REQUIRED FOR SOME 
PORTION OF THE TUESDAY-WEDNESDAY TIME FRAME. 

IN THE WAKE OF COLD FROPA HIGH PRESSURE WILL PROVIDE RAIN FREE 
CONDITIONS AT LEAST THROUGH THURSDAY...AND PERHAPS THROUGH THE END 
OF THE WEEK AS WELL. HOWEVER...IF THE HIGH RETREATS E/NE EARLIER 
THAN EXPECTED ISENTROPIC ASCENT COULD SUPPORT ANOTHER ROUND OF RAIN 
BEFORE NEXT WEEKEND.

&&

.AVIATION /22Z FRIDAY THROUGH WEDNESDAY/...
VFR CONDITIONS WILL PERSIST THIS AFTERNOON AT BOTH CHS AND SAV
TERMINALS. MVFR CIGS AND LIGHT RAIN WILL THEN ENCOUNTER BOTH
TERMINALS EARLY TONIGHT AS SURFACE LOW PRESSURE DEVELOPS OFF THE
COAST. EXPECT MVFR CIGS AND LIGHT RAIN TO REACH THE SAV TERMINAL
BY 02Z SATURDAY AND AN HOUR OR TWO LATER AT THE CHS TERMINAL.
CIGS WILL CONTINUE TO LOWER TO IFR CRITERIA AS LOW LVL MOISTURE
INCREASES OVERNIGHT. EXPECT IFR CIGS BY 05Z AT THE SAV TERMINAL
AND THEN AN HOUR OR TWO LATER AT THE CHS TERMINAL. IFR
CONDITIONS WILL REMAIN THROUGH LATE MORNING SATURDAY...14Z AT
SAV...15Z AT CHS...BEFORE A GRADUAL UPWARD TREND IN SFC WINDS
OCCURS RESULTING IN MVFR CIGS THROUGH LATE MORNING INTO EARLY
AFTERNOON.

EXTENDED AVIATION OUTLOOK...IMPROVEMENT BY VFR BY SATURDAY NIGHT
OR SUNDAY MORNING. OCCASIONAL MVFR OR LOWER CONDITIONS MAY
DEVELOP MONDAY...THEN PREVAILING MVFR OR LOWER CONDITIONS SHOULD
DEVELOP TUESDAY.

&&

.MARINE...
WHILE A STORM SYSTEM DEVELOPS OVER THE NE GULF OF MEXICO
TONIGHT...RESIDUAL HIGH PRESSURE OVER THE WATERS WILL KEEP
CONDITIONS ACROSS THE WATERS RATHER BENIGN WITH N WINDS AOB 15 KT
AND SEAS CAPPED AT 3 FT. AS THE AREA OF LOW PRESSURE SHIFTS NE
ACROSS FLORIDA INTO THE ATLANTIC WATERS SATURDAY...NW FLOW WILL
INCREASE AS THE PRESSURE GRADIENT TIGHTENS. SEAS WILL BUILD OVER
ALL WATERS...INCLUDING SEAS REACHING 6 FT IN THE OUTER GA WATERS.
IN ADDITION...WINDS OVER THE WATERS BEYOND 20 NM FROM SHORE WILL
EXCEED 20 KT LATE DAY TOMORROW SO HAVE RAISED A SMALL CRAFT
ADVISORY FOR AMZ374. THE NEARSHORE ZONES ARE QUITE TRICKY. MODEL
GUIDANCE SHOWS 41008 REACHING 21 KT WITH 41004 REACHING 25 KT.
TRANSLATING THAT TO THE ZONES WITHIN 20 NM OF SHORE...CONFIDENCE
IS NOT HIGH ENOUGH THAT SCA WINDS WOULD BE REACHED SO WILL CAP
WIND SPEEDS AT 20 KT ATTM AND HOLD OFF ANY NEARSHORE ZONE
HEADLINES.

SMALL CRAFT ADVISORY CONDITIONS SHOULD END ACROSS THE OUTER GA 
WATERS BY DAYBREAK SUNDAY. THEN...WIND/SEA FORECAST CONFIDENCE 
REMAINS LOW SUNDAY THROUGH MONDAY AND WILL DEPEND GREATLY ON THE 
POSITION/STRENGTH OF A SURFACE TROUGH EXPECTED TO DEVELOP OVER OR 
CLOSE TO THE WATERS. THEN...WINDS WILL INCREASE AND SEAS WILL 
BUILD...PERHAPS AS EARLY AS TUESDAY AS AN INLAND WEDGE/COASTAL 
TROUGH SCENARIO DEVELOPS...BUT MORE LIKELY TUESDAY NIGHT/EARLY 
WEDNESDAY AS S/SW WINDS SURGE INTO THE REGION AHEAD OF AN 
APPROACHING COLD FRONT. SMALL CRAFT ADVISORY CONDITIONS ARE
POSSIBLE AND GALES CANNOT BE RULED OUT. AS THE COLD FRONT PASSES
THROUGH THE WATERS WEDNESDAY...WINDS WILL SHIFT TO THE W/NW...AND
SCA OR GALES COULD PERSIST WITHIN THE COLD ADVECTION REGIME.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 3 PM SATURDAY TO 5 AM EST SUNDAY FOR 
     AMZ374.

&&

$$