FXUS62 KCHS 050551
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
1251 AM EST SAT DEC 5 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 THIS MORNING/...
RADAR AND SURFACE OBSERVATIONS INDICATE MEASURABLE RAINS HAVE
OVERSPREAD THE ENTIRE FORECAST AREA THIS EVENING. THIS COMBINED
WITH TEMPERATURES IN THE MID-UPR 40S IS MAKING FOR A RATHER CHILLY
AND RAW EVENING. THE RAIN HAS BEEN PRETTY LIGHT SO FAR WITH
AMOUNTS RANGING FROM A FEW HUNDREDTHS OF AN INCH INLAND WITH AS
MUCH AS 0.25 INCH AT THE COAST.
THE RAIN HAS BEEN PRIMARY ISENTROPICALLY DRIVEN SO FAR...BUT AS
THE ATMOSPHERE BECOMES INCREASINGLY DIFLUENT AHEAD OF THE
ENERGETIC SHORTWAVE MOVING ACROSS EASTERN TEXAS THE RAIN SHOULD
BECOME STEADIER AND HEAVIER OVER THE NEXT SEVERAL HOURS AS
SECONDARY CYCLOGENESIS INITIATES OFF THE SOUTHEAST U.S. COAST.
FAVORABLE POSITIONING OF A 130 KT JET STREAK OVER THE WESTERN
CAROLINAS WILL ALSO AID IN THIS ENHANCEMENT. INCREASING RETURNS
OFF THE KTLH RADAR SUGGEST RAINS ARE ALREADY BECOMING ENHANCED
ACROSS THE NORTHEAST GULF OF MEXICO AND PARTS OF NORTHERN FLORIDA
WHICH CORRESPONDS NICELY WITH THE COOLING CLOUD TOPS NOTED ON GOES
INFRARED IMAGERY. EXPECT GENERAL RAINFALL AMOUNTS OF 1/2 TO 1 INCH
THROUGH SUNRISE WITH LOCALIZED HEAVIER AMOUNTS POSSIBLE. NEARLY
SATURATED CONDITIONS AND A LACK OF SIGNIFICANT THERMAL ADVECTION
SUGGESTS TEMPERATURES WILL ONLY FALL A FEW MORE DEGREES WITH LOWS
BOTTOMING OUT IN THE MID-UPR 40S...COOLEST FAR WEST.
OTHER THAN INCREASING POPS TO 100 PERCENT ACROSS THE BOARD AND
MAKING MINOR TWEAKS TO TEMPERATURES...NO MAJOR CHANGES WILL BE
MADE TO THE GRIDS OR TEXT PRODUCTS THIS EVENING.
&&
.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/...
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 /SUNDAY 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 /06Z SATURDAY THROUGH WEDNESDAY/...
AT KCHS...WE THINK IFR CIGS WILL BE DEVELOPING OVERNIGHT AS RAIN
CONTINUES. VSBYS SHOULD BE MAINLY MVFR WITH A FEW PERIODS OF IFR
POSSIBLE IN HEAVIER RAIN AT TIMES. RAINS SHOULD BE ENDING DURING
THE EARLY TO MID AFTERNOON HOURS. SURFACE WINDS WILL SHIFT NW IN
THE WAKE OF THE DEPARTING COASTAL LOW PRES SAT AFTERNOON AND
BEGIN TO LIFT CIGS TO MVFR. A RETURN TO VFR CONDITIONS ARE LIKELY
AROUND OR SHORTLY AFTER SUNSET.
AT KSAV...IFR CIGS WERE ALREADY SOCKED IN AND AS SUBSIDENCE ALOFT
DEVELOPS SAT MORNING IN THE WAKE OF THE DEPARTING COASTAL LOW...
WE THINK IFR CIGS MAY TAKE UNTIL EARLY AFTERNOON TO LIFT TO MVFR
BEFORE SCATTERING OUT POSSIBLY BEFORE DUSK. RAIN SHOULD BE ENDING
IN THE MID TO LATE MORNING HOURS. VFR CONDITIONS EXPECTED FOR
00Z-06Z/06 PORTION OF THIS THIS TAF CYCLE.
EXTENDED AVIATION OUTLOOK...VFR LATE SAT NIGHT AND SUNDAY.
OCCASIONAL MVFR OR LOWER CONDITIONS MAY DEVELOP MON...THEN
PREVAILING MVFR OR LOWER CONDITIONS SHOULD DEVELOP TUE.
&&
.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 THIS AFTERNOON TO 5 AM EST
SUNDAY FOR AMZ374.
&&
$$
