FXUS62 KCHS 150513
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
1213 AM EST TUE DEC 15 2009
.SYNOPSIS...
HIGH PRESSURE TO THE NORTH WILL SHIFT OFFSHORE INTO TUESDAY. A
COLD FRONT WILL MOVE THROUGH THE AREA TUESDAY NIGHT...FOLLOWED BY
HIGH PRESSURE THROUGH THURSDAY. A SLOW MOVING STORM SYSTEM WILL
THEN LIFT NORTHEAST OFF THE SOUTH CAROLINA AND GEORGIA COAST
FRIDAY THROUGH 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 THIS MORNING/...
HAVE UPDATED TO CANCEL THE DENSE FOG ADVISORY FOR PARTS OF SE GA AS
AREA OBSERVATIONS INDICATE VISIBILITIES ABOVE 1/4 MILE AND WITH
THE SHOWERS/ISO THUNDERSTORMS MOVING IN...DENSE FOG IS NO LONGER
EXPECTED.
RAIN WILL CONTINUE TO EXPAND IN BOTH COVERAGE AND INTENSITY
AS INCREASING ISENTROPIC AND SYNOPTIC LIFT OVERSPREAD THE AREA.
EARLY EVENING SURFACE ANALYSIS PLACED THE WARM FRONT RIGHT ALONG
THE ALTAMAHA RIVER WITH TEMPERATURES QUICKLY RISING INTO THE 70S
SOUTH OF THE BOUNDARY. WITH WIDESPREAD HEAVY RAIN FALLING TO THE
NORTH OF THE FRONT WE DO NOT EXPECT THE WARM FRONT TO LIFT NORTH
VERY QUICKLY TONIGHT. HOWEVER...MODELS STILL INSIST THE FRONT WILL
EVENTUALLY PUSH THROUGH THE FORECAST AREA AND BECOMING POSITIONED
ACROSS THE MIDLANDS AND PEE DEE BY TUESDAY MORNING. THINK LOWS
HAVE JUST ABOUT BEEN ACHIEVED FOR MOST LOCATIONS WITH TEMPERATURES
LIKELY TO HOLD STEADY OR SLOWLY RISE OVERNIGHT.
LIGHTNING PLOTS SHOW QUITE A BIT OF TSTM ACTIVITY IN THE WARM
SECTOR WITH SOME ELEVATED TSTMS MOVING ACROSS CENTRAL GEORGIA.
WE UPDATED THE FORECAST EARLIER TO INCLUDE A CHANCE FOR TSTMS AND
THIS STILL LOOKS TO BE THE CASE. IN FACT...SEVERAL BOWING
CONVECTIVE LINE SEGMENTS MOVING ACROSS SOUTHERN ALABAMA AND
GEORGIA HAVE HAD A HISTORY OF PRODUCING WIND DAMAGE AND TORNADOES.
SOME OF THIS ACTIVITY COULD BRUSH THE FAR SOUTHERN AREAS NEAR THE
VICINITY OF THE WARM FRONT OVER THE NEXT SEVERAL HOURS. INCREASED
POPS TO NEAR 100 PERCENT FOR AREAS NORTH OF A REIDSVILLE TO
SAVANNAH LINE WHERE THE AXIS OF HEAVIEST RAINS EXPECTED TO SETUP
WITH POPS TRENDING DOWN TO 50 PERCENT ACROSS THE DARIEN AND SAPELO
ISLAND AREAS.
&&
.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM/...
STRONG WARM ADVECTION TUESDAY AHEAD OF AN APPROACHING COLD FRONT
WILL BRING LOWER TO MIDDLE 60S DEWPOINTS INTO THE REGION AND PUSH
TEMPS CLOSE TO 70. CONSIDERABLE UPPER-LEVEL DIVERGENCE SHOULD
CONTINUE WHILE SHORTWAVE ENERGY MOVES THROUGH. WITH A MOIST
BOUNDARY LAYER AND DEEP-LAYERED FORCING WE SHOULD CONTINUE TO SEE
WIDESPREAD COVERAGE OF RAIN SHOWERS...POSSIBLY HEAVY AT TIMES.
MID-LEVEL LAPSE RATES OF AROUND 6.5C MAY SUPPORT ISOLATED
THUNDERSTORMS SO WE HAVE INCLUDED THOSE IN THE FORECAST.
&&
.LONG TERM /TONIGHT THROUGH MONDAY/...
MEDIUM RANGE GUIDANCE IS IN GOOD AGREEMENT THROUGH
THURSDAY...BEFORE DIVERGING IN A NUMBER OF DIFFERENT DIRECTIONS
OVER THE WEEKEND AND INTO EARLY NEXT WEEK...RESULTING IN
PROGRESSIVELY LOWERING CONFIDENCE THROUGH THE EXTENDED PERIODS.
MODEL SOLUTIONS SHOW A STRONG COLD FRONT MOVING THROUGH THE REGION
TUESDAY NIGHT...AND WITH DECENT SHORTWAVE ENERGY ALONG WITH ENTRANCE
REGION FORCING FROM A 160+ KNOT H25 JET MOVING THROUGH THE REGION AS
WELL AS MID-LEVEL LAPSE RATES STEEPENING TO BETWEEN 6.5 AND 7
C/KM...WILL CARRY HIGH CHANCE/LOW END LIKELY POPS FOR
SHOWERS/THUNDERSTORMS.
HIGH PRESSURE WILL THEN BUILD INTO THE REGION WEDNESDAY THROUGH
THURSDAY...RESULTING IN RAIN FREE CONDITIONS.
GUIDANCE SOLUTIONS THEN BEGINS TO DIVERGE WITH THE HANDLING OF A
STORM SYSTEM PASSING BY TO THE SOUTH AND EAST OF THE FORECAST AREA
DURING THE THURSDAY NIGHT THROUGH SATURDAY TIME FRAME. GUIDANCE IS
TYPICALLY TOO QUICK IN RETURNING DEEP MOISTURE INTO THE REGION...SO
DESPITE THE MODELS SPREADING RAIN BACK INTO THE AREA THURSDAY
NIGHT...WILL KEEP THE FORECAST RAIN FREE FOR NOW. THE MODELS ALL
SHOW A GOOD SHOT FOR RAIN IN THE FRIDAY THROUGH SATURDAY TIME
FRAME...BUT TIMING DETAILS ARE NOT IN THE BEST AGREEMENT. FOR NOW
HAVE INCREASED POPS IN THE FRIDAY THROUGH SATURDAY NIGHT TIME
FRAME...BUT HAVE KEEP THEM IN THE SLIGHT CHANCE/LOW CHANCE RANGE DUE
TO LOW CONFIDENCE ON TIMING DETAILS.
STILL SOME DISAGREEMENT AMONGST THE MODELS WITH RESPECT TO THE EXACT
SYNOPTIC PATTERN EVOLUTION IN THE SUNDAY THROUGH MONDAY TIME
FRAME...BUT GENERALLY HAVE FOLLOWED A SOLUTION BUILDING HIGH
PRESSURE BACK INTO THE AREA. HAVE THEREFORE KEPT THE FORECAST RAIN
FREE SUNDAY THROUGH MONDAY.
TEMPERATURES THROUGH THE EXTENDED PERIODS ARE EXPECTED TO RUN NEAR
TO SLIGHTLY BELOW CLIMO.
&&
.AVIATION /06Z TODAY THROUGH SATURDAY/...
POOR CONDITIONS /IFR OR LESS/ WILL CONTINUE AT KCHS/KSAV THROUGH
MOST OF THE 06Z/15 TAF CYCLE AS A WIDESPREAD AREA OF RAIN MOVES
ACROSS THE REGION AHEAD OF AN APPROACHING COLD FRONT. THE RAIN HAS
RECENTLY LED TO IMPROVED VSBYS AT KSAV...BUT THIS MAY ONLY BE
TEMPORARY. THERE COULD BE A BRIEF BREAK IN THE RAIN FROM LATE
MORNING INTO THE EARLY AFTERNOON BEFORE ADDITIONAL RAIN MOVES IN
FROM THE WEST WITH THE COLD FRONT ITSELF. CANNOT RULE OUT A FEW
THUNDERSTORMS BUT THINK CHANCES ARE TOO LOW TO MENTION IN THIS TAF
SET.
EXTENDED AVIATION OUTLOOK...MVFR/IFR CONDITIONS ARE EXPECTED
THROUGH TUESDAY EVENING. VFR CONDITIONS ARE EXPECTED WEDNESDAY
THROUGH THURSDAY NIGHT. COULD SEE SOME MVFR OR LOWER CIGS AND/OR
VSBYS FRIDAY THROUGH SATURDAY AS A STORM SYSTEM PASSES BY OFF THE
SOUTHEAST COAST.
&&
.MARINE...
THE MARINE DENSE FOG ADVISORY WILL REMAIN IN PLACE. UNFORTUNATELY
WE HAVE HAD NO OBSERVATIONS OVER THE MARINE AREA SINCE MID
AFTERNOON AND VIEWS FROM COASTAL WEBCAMS ARE INCONCLUSIVE ON HOW
EXTENSIVE THE SEA FOG STILL IS. UNTIL WE CAN GET A BETTER HANDLE
ON HOW MUCH SEA FOG IS STILL OUT OVER THE COASTAL WATERS...THE
ADVISORY WILL BE MAINTAINED. OTHERWISE WINDS WILL REMAIN LESS THAN
10 KT AND SEAS 3-4 FT NEARSHORE WATERS AND 4-5 FT OFFSHORE WATERS.
A COLD FRONT WILL PUSH THROUGH THE WATERS TUESDAY NIGHT...
FOLLOWED BY HIGH PRESSURE BUILDING BACK INTO THE AREA THROUGH
THURSDAY NIGHT. A STORM SYSTEM WILL THEN PASS BY TO THE EAST OF
THE WATERS FRIDAY THROUGH SATURDAY. EXPECT A PRETTY GOOD SURGE IN
THE WINDS FOLLOWING THE COLD FRONT TUESDAY NIGHT AND
WEDNESDAY...AND WILL LIKELY NEED SMALL CRAFT ADVISORIES ACROSS ALL
WATERS. IMPROVING CONDITIONS ARE EXPECTED WEDNESDAY NIGHT...BUT
SEAS MAY LINGER IN THE SMALL CRAFT RANGE THROUGH THURSDAY EVENING
ACROSS THE OUTER GEORGIA WATERS. DEPENDING ON THE STRENGTH AND
PROXIMITY OF THE STORM SYSTEM PASSING BY OFF THE SOUTH CAROLINA
AND GEORGIA COAST FRIDAY THROUGH SATURDAY...SMALL CRAFT ADVISORIES
MAY ONCE AGAIN BE NECESSARY FOR PORTIONS OF THE WATERS.
&&
.HYDROLOGY...
HAVE ISSUED A FLASH FLOOD WATCH FOR MUCH OF THE FORECAST AREA
VALID THROUGH LATE MORNING AS MODERATE TO HEAVY RAIN WILL IMPACT
MUCH OF THE FORECAST AREA. MODELS SHOW THE AXIS OF HEAVIEST RAIN
SETTING UP ALONG A LINE FROM STATESBORO TO CHARLESTON. THIS IS
WHERE THE STRONGEST 850MB MOISTURE FLUX CONVERGENCE AND PWATS AS
HIGH AS 1.7 INCHES WILL BE FOUND. MUCH OF THIS AREA HAS ALREADY
SEEN BETWEEN 3-5 INCHES OF RAIN DURING THE PAST SEVERAL DAYS AND
WITH THE POTENTIAL FOR SEVERAL MORE INCHES OVERNIGHT WE FELT IT
BEST TO GO AHEAD AND RAISE THE FLASH FLOOD WATCH. HAVING SOME DRY
WEATHER FOR THE PAST 24 HOURS HAS ALLOWED FLASH FLOOD GUIDANCE
VALUES TO RECOVER SOMEWHAT...BUT WITH CONDITIONS FAVORING A
CONCENTRATED LINE OF ELEVATED CONVECTION THESE VALUES COULD
EASILY BE EXCEED.
THE GREATEST RISK FOR FLASH FLOODING WILL BE ACROSS INTERIOR
SOUTHEAST GEORGIA AND THE COUNTIES BORDERING THE SAVANNAH
RIVER WHERE THE HEAVIEST RAINS HAVE FALLEN OVER THE PAST FEW
DAYS...BUT OPTED TO THE INCLUDE OUR SOUTH CAROLINA COASTAL
COUNTIES SINCE HEAVY RAINS COULD FALL DURING A SOMEWHAT ELEVATED
HIGH TIDE CYCLE /6.1 FT IN THE CHARLESTON HARBOR/.
MANY RIVERS REMAIN IN FLOOD...INCLUDING THE OHOOPEE...OGEECHEE...
CANOOCHEE...SAVANNAH AND SOUTH SANTEE. RIVER FLOOD WARNINGS REMAIN
IN EFFECT.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...FLASH FLOOD WATCH UNTIL 11 AM EST TUESDAY FOR GAZ087-088-
099>101-114>116-118.
SC...FLASH FLOOD WATCH UNTIL 11 AM EST TUESDAY FOR SCZ040-042-043-
047>051.
MARINE...DENSE FOG ADVISORY UNTIL NOON EST TUESDAY FOR AMZ354.
&&
$$
