FXUS62 KCHS 152318
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
618 PM EST TUE DEC 15 2009

.SYNOPSIS...
A COLD FRONT WILL MOVE THROUGH THE AREA TONIGHT...THEN HIGH
PRESSURE WILL BUILD INTO THE AREA WEDNESDAY AND THURSDAY. A SLOW
MOVING STORM SYSTEM WILL MOVE FROM THE NORTHEAST GULF OF MEXICO ON
FRIDAY TO OFF THE GEORGIA AND SOUTH CAROLINA COAST ON SATURDAY.
HIGH PRESSURE IS THEN EXPECTED TO BUILD BACK INTO THE AREA FOR THE
TAIL END OF THE WEEKEND INTO EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A COLD FRONT CONTINUES TO SLOWLY SHIFT SOUTHEAST TOWARD THE REGION
THIS AFTERNOON. ASSOCIATED PRECIPITATION HAS BEEN RATHER LIMITED
THUS FAR...DESPITE QUITE A BIT OF LOW-LEVEL MOISTURE AND UPPER
LEVEL ENERGY. THIS IS PROBABLY DUE TO THE WSW FLOW IN THE LOW-
LEVELS LIMITING SURFACE-BASED CONVERGENCE ALONG THE BOUNDARY. WE
MAY SEE A BIT OF AN INCREASE IN ACTIVITY AS THE BOUNDARY PUSHES
INTO OUR AREA WHERE TEMPERATURES CLIMBED WELL INTO THE 70S WITH
DEWPOINTS IN THE LOWER 60S. NEVERTHELESS...EXPECT SCATTERED
SHOWERS THROUGH THIS EVENING WITH THE FRONT...HIGHEST ACROSS
WESTERN AREAS WHERE RAIN HAS ALREADY DEVELOPED. MOST OF THE PRECIP
SHOULD BE PUSHING OFF THE COAST BY MIDNIGHT TONIGHT WITH MUCH
COOLER...DRIER AIR FILTERING IN BEHIND. EXPECT LOW TEMPS WED
MORNING TO BE IN THE MIDDLE TO UPPER 40S.

WINDS ON LAKE MOULTRIE SHOULD PICK UP PRETTY QUICKLY LATE THIS
EVENING AS COLD AIR MOVES IN AND GEOSTROPHIC WINDS INCREASE TO
20-25 KT OVER THE SOMEWHAT WARMER LAKE WATERS. WE HAVE A LAKE WIND
ADVISORY BEGINNING AT 8 PM.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
A DRY...COOL AND MOSTLY SUNNY DAY IS EXPECTED WEDNESDAY AS A LARGE
RIDGE OF HIGH PRESSURE BUILDS IN. SUSTAINED COLD ADVECTION
THROUGHOUT THE DAY SHOULD OFFSET SOME WEAK DOWNSLOPE COMPRESSION.
WE TRIED TO ACCOUNT FOR THE USUAL WARM BIAS IN THE STATISTICAL
GUIDANCE DURING POST-FROPA COLD ADVECTION EVENTS AND THEREFORE
UNDERCUT FORECAST HIGHS BY A FEW DEGREES. THIS YIELDS UPPER 50S
FOR MOST AREAS.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
MEDIUM RANGE GUIDANCE HAS COME INTO MUCH BETTER AGREEMENT WITH THE 
SURFACE PRESSURE PATTERN THROUGH THE EXTENDED PERIODS THIS 
AFTERNOON...BUT THE HIGHLY ANOMALOUS PATTERN CONTINUES TO LEAD TO 
SLIGHTLY BELOW NORMAL FORECAST CONFIDENCE.

MODEL SOLUTIONS ALL AGREE THAT HIGH PRESSURE WILL BUILD OVER THE 
AREA WEDNESDAY NIGHT THROUGH THURSDAY...RESULTING IN RAIN FREE 
WEATHER. CONDITIONS THEN DETERIORATE THURSDAY NIGHT AS DEEP MOISTURE 
AND PLENTY OF FORCING FOR ASCENT FROM UPPER LEVEL SHORTWAVE ENERGY 
AND THE ENTRANCE REGION OF A 140+ KNOT H25 JET SPREADS OVER THE 
FORECAST AREA. COULD SEE SOME LIGHT PRECIPITATION DEVELOP IN THE 
EVENING ACROSS PRIMARILY SOUTHEAST GEORGIA...SO HAVE INCLUDED SLIGHT 
CHANCE POPS HERE. AFTER MIDNIGHT...AS THE DEEPER MOISTURE AND BETTER 
FORCING ARRIVE HAVE BUMPED POPS TO HIGH CHANCE SOUTH AND SLIGHT 
CHANCE ACROSS THE FAR NORTH. OF CONCERN IS HOW DRY THE AIRMASS IN 
PLACE WILL BE AS THE PRECIPITATION ARRIVES...WITH MODEL SOUNDINGS 
SHOWING MOISTURE EXTENDING PLENTY HIGH ENOUGH FOR ICE NUCLEATION. AT 
THE SAME TIME...WITH THE DRY LOW LEVEL AIRMASS IN PLACE AS SEEN IN 
MODEL SOUNDINGS...CAN/T RULE OUT A PERIOD OF GRAUPLE OR SLEET 
ESPECIALLY ACROSS FAR INTERIOR PORTIONS OF THE FORECAST AREA PRIOR 
TO PRECIPITATION TRANSITIONING TO RAIN. FOR NOW...WILL KEEP ANY 
MENTION OF WINTRY PRECIP OUT OF THE FORECAST...BUT THIS WILL NEED TO 
BE CLOSELY MONITORED OVER THE NEXT COUPLE OF DAYS.

ANY POTENTIAL WINTRY PRECIP SHOULD CHANGE OVER TO ALL RAIN QUICKLY 
FRIDAY MORNING AS WARMER AIR ALOFT AND WINDS SHIFTING SLIGHTLY MORE 
ONSHORE ALLOW FOR TEMPERATURES TO WARM THROUGHOUT THE BOUNDARY 
LAYER. EXPECT A FAIRLY STEADY RAIN FRIDAY AND FRIDAY NIGHT AS AN 
AREA OF LOW PRESSURE TRACKS FROM THE NORTHWEST GULF OF MEXICO TO OFF 
THE SOUTH CAROLINA COAST BY DAYBREAK SATURDAY. HAVE BUMPED POPS UP 
ACCORDINGLY INTO THE LIKELY RANGE AREA WIDE. 

MODEL GUIDANCE IS IN GOOD AGREEMENT LIFTING THE AREA OF LOW PRESSURE 
NORTHEAST AND AWAY FROM THE AREA SATURDAY AND SATURDAY NIGHT...BUT 
DO SO AT VARYING SPEEDS. GIVEN PLENTY OF WRAP AROUND MOISTURE IN 
PLACE ACROSS THE AREA SATURDAY...HAVE BUMPED POPS UP INTO THE CHANCE 
RANGE ALL AREAS...AND HAVE KEPT SLIGHT CHANCE/LOW CHANCE POPS IN 
PLACE SATURDAY NIGHT.  

HIGH PRESSURE IS THEN EXPECTED TO BUILD BACK INTO THE AREA SUNDAY 
THROUGH TUESDAY...WITH RAIN FREE WEATHER EXPECTED.

TEMPERATURES ARE EXPECTED TO RUN WELL BELOW NORMAL THROUGHOUT THE 
EXTENDED PERIODS.

&&

.AVIATION /23Z TUESDAY THROUGH SUNDAY/...
UPSTREAM OBSERVATION SHOW THE COLD FRONT IS MOVING QUICKLY THROUGH
NORTHERN GEORGIA AND THE NORTH CAROLINA MOUNTAINS AND IS ON TARGET
TO MOVE THROUGH THE TERMINALS LATER TONIGHT. FROPA IS ANTICIPATED 
AT KCHS AROUND 05Z AND KSAV AROUND 07Z. EXPECT A WIND SHIFT TO
THE NORTH ALONG WITH GUSTY WINDS WITH THE FRONTAL PASSAGE. SPOTTY 
SHOWER ACTIVITY IS APPROACHING FROM THE WEST BUT IT HAS BEEN VERY
LIGHT IN INTENSITY AND SHOULD HAVE LITTLE IMPACT ON VSBYS PRIOR TO
FROPA. COME MVFR CIGS WILL LIKELY MOVE IN AHEAD OF THE FRONT...BUT
NOTHING MORE THAN MVFR IS ANTICIPATED. EXPECT VFR CONDITIONS TO
PREVAIL BY LATE TONIGHT INTO THE DAY WEDNESDAY. AFTER AN INITIAL
PERIOD GUSTY WINDS WITH THE FRONTAL PASSAGE...EXPECT WINDS TO DIE
DOWN A BIT PRIOR TO SUNRISE THEN BECOME GUSTY AGAIN BY 14Z.

EXTENDED AVIATION OUTLOOK...VFR CONDITIONS ARE EXPECTED WEDNESDAY 
NIGHT THROUGH THURSDAY NIGHT. MVFR OR LOWER CIGS AND/OR VSBYS ARE 
LIKELY FRIDAY AND FRIDAY NIGHT...AND POSSIBLE SATURDAY AND SATURDAY 
NIGHT AS A STORM SYSTEM PASSES BY OFF THE SOUTHEAST COAST. VFR 
CONDITIONS ARE EXPECTED TO RETURN FOR SUNDAY.

&&

.MARINE...
A STRONG COLD FRONT WILL MOVE SE THROUGH THE WATERS THIS EVENING AND
OVERNIGHT WITH STRONG COLD ADVECTION IN ITS WAKE. BOUNDARY LAYER
AIR TEMPERATURES SHOULD DROP INTO THE LOWER TO MIDDLE 50S BY 12Z
WED WHILE WATER TEMPS RANGE FROM MIDDLE 50S WITHIN ABOUT 10 MILES
OF SHORE TO THE LOWER 70S OUT OVER THE GULF STREAM. THE MODELS ARE
NEARLY UNANIMOUS WITH APPROXIMATELY 30 KTS OF 1000 MB GEOSTROPHIC
WINDS FROM 09-15Z WED OVER THE WATERS. OUR LOCAL STABILITY FACTOR
TABLE INDICATES A MIX-DOWN OF 70 TO 80 PERCENT OF THE GEOSTROPHIC
WINDS GIVEN THE AIR AND SEA TEMPERATURES. THUS...EVEN WITH THE
ASSUMPTION THAT THE GUIDANCE IS 5-10 KT TOO WEAK WITH WINDS...DO
NOT EXPECT TO SEE WIDESPREAD GALES TONIGHT. HOWEVER THE OFFSHORE
GA WATERS WILL BE DARN CLOSE...AT LEAST WITH FREQUENT GUSTS TO 35
KT. SINCE THAT ZONE EXTENDS OUT 60 NM...THE OUTER 15-20 NM OF THE
ZONE INCLUDES THE GULF STREAM WATERS WITH WATER TEMPS IN THE LOWER
70S. QUITE FREQUENTLY IN STRONG COLD ADVECTION REGIMES WE SEE GALE
FORCE GUSTS OVER THE OFFSHORE GA WATERS DESPITE FAIRLY
UNIMPRESSIVE GEOSTROPHIC WINDS. WE PLAN TO ISSUE A GALE WARNING
FOR THAT ZONE RUNNING FROM MIDNIGHT THROUGH NOON WED. FOR THE SC
NEARSHORE ZONES WE WILL INCLUDE GUSTS TO 30 KT OVER OUTER PORTIONS
WHERE THE WATER IS WARMER. BASED ON MODEL WIND PROGS...THIS EVENT
LOOKS FAIRLY SHORT-LIVED AND BY WED AFTERNOON WINDS AND SEAS
SHOULD BE SUBSIDING.

HIGH PRESSURE WILL BUILD BACK INTO THE REGION WEDNESDAY NIGHT AND
THURSDAY...BEFORE A DEVELOPING STORM SYSTEM IN THE GULF OF MEXICO
BEGINS TO TAKE AIM ON THE AREA THURSDAY NIGHT AND FRIDAY. THIS
STORM SYSTEM WILL THEN LIFT NORTHEAST OFF THE SOUTH CAROLINA AND
GEORGIA COAST FRIDAY NIGHT AND SATURDAY...WITH HIGH PRESSURE
BUILDING BACK INTO THE REGION FOR SUNDAY. SMALL CRAFT CONDITIONS
FOR WINDS/SEAS OR A COMBINATION OF THE TWO MAY LINGER ACROSS THE
OUTER GEORGIA WATERS THROUGH SUNDAY.

THE BIGGER STORY MAY BE A NEED FOR GALE WARNINGS OVER PORTIONS OF 
THE WATERS FRIDAY THROUGH SATURDAY...MUCH DEPENDENT ON THE STRENGTH 
AND TRACK OF THE LOW HOWEVER. GIVEN THE UNCERTAINTY...HAVE KEPT 
WINDS AND WIND GUSTS BELOW GALE FORCE...BUT FUTURE SHIFTS WILL NEED 
TO CLOSELY MONITOR THIS DEVELOPING WEATHER SITUATION.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...LAKE WIND ADVISORY UNTIL NOON EST WEDNESDAY FOR SCZ045.
MARINE...SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM EST 
     WEDNESDAY FOR AMZ350-352.
     SMALL CRAFT ADVISORY FROM 6 AM WEDNESDAY TO MIDNIGHT EST 
     WEDNESDAY NIGHT FOR AMZ354.
     GALE WARNING FROM MIDNIGHT TONIGHT TO NOON EST WEDNESDAY FOR 
     AMZ374.
     SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO NOON EST 
     WEDNESDAY FOR AMZ330.

&&

$$