FXUS62 KCHS 111753
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
1253 PM EST FRI DEC 11 2009
.SYNOPSIS...
HIGH PRESSURE WILL MOVE FROM THE TENNESSEE VALLEY INTO THE
SOUTHERN MID-ATLANTIC REGION TODAY INTO TONIGHT. HIGH PRESSURE
WILL REMAIN WEDGED EAST OF THE APPALACHIANS OVER THE WEEKEND...AS
A STORM SYSTEM TRACKS ACROSS THE GULF OF MEXICO ON SATURDAY...THEN
OFF THE SOUTHEAST COAST SUNDAY. HIGH PRESSURE WILL BRIEFLY REBUILD
ON MONDAY...WITH A COLD FRONT EXPECTED TO AFFECT THE AREA INTO THE
MIDDLE OF NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
NO MAJOR CHANGES ARE PLANNED TO THE GOING FORECAST. MOSTLY CLOUDY
SKIES WILL PREVAIL TODAY WITH HIGHS RUNNING A SOLID 10-15 DEGREES
BELOW NORMAL. WILL NUDGE TEMPERATURES UP ABOUT A DEGREE ALONG THE
I-95 CORRIDOR FROM SAVANNAH SOUTH PER INCOMING 12Z BUFR SOUNDINGS...
BUT STILL EXPECT UPR 40S-LWR 50S TO PREVAIL. KJAX/KTLH RADARS ARE
PICKING UP ON A FEW ELEVATED RETURNS OVER FAR SOUTHERN GEORGIA AND
NORTHERN FLORIDA. FORECAST SOUNDINGS SHOW EXTENSIVE DRY AIR IN
PLACE SO THE PROSPECTS FOR MEASURABLE RAINFALL ARE VERY LOW...
ALTHOUGH A FEW SPRINKLES CAN NOT BE COMPLETELY RULED OUT ALONG THE
COUNTIES BORDERING THE ALTAMAHA RIVER.
THE FREEZE WARNING APPEARS TO HAVE VERIFIED WELL THIS MORNING.
INCOMING DATA SUGGEST THE FREEZING LINE MADE IT AS FAR SOUTH AS A
TARBORO-SPRINGFIELD-STATESBORO-COBBTOWN LINE. WE ARE STILL
EVALUATING THE DATA TO SEE IF ADDITIONAL ZONES CAN BE REMOVED FROM
THE FROST/FREEZE PROGRAM.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
SURFACE HIGH PRESSURE WILL SHIFT ACROSS THE CENTRAL APPALACHIANS
TONIGHT...WHILE A WEAK COASTAL TROUGH BEGINS TO TAKE SHAPE OFF THE
SOUTHEAST U.S. COAST.
WEAK LOW LEVEL CONVERGENCE ALONG THE DEVELOPING TROUGH MAY SUPPORT
AN ISOLATED SHOWER ALONG THE GEORGIA COAST THIS EVENING...OTHERWISE
THE AIR MASS WILL BE TOO DRY TO SUPPORT ANY PRECIPITATION THIS
EVENING ACROSS THE REMAINDER OF THE FORECAST AREA. DURING THE
OVERNIGHT HOURS...WARM AIR ADVECTION AND ISENTROPIC ASCENT WILL
DEVELOP ACROSS THE AREA...WITH DEEPER MOISTURE SHIFTING INTO THE
REGION FROM THE SOUTH. MODEL GUIDANCE HAS CONTINUED THE TREND OF
DELAYING THE ONSET OF PRECIPITATION...WHICH APPEARS VERY
REASONABLE AS IT WILL TAKE SOME TIME FOR THE DRY LOW LEVELS OF THE
ATMOSPHERE TO MOISTEN SUFFICIENTLY FOR ANY RAIN TO REACH THE
SURFACE. ALTHOUGH THE MODELS TEND TO HAVE A BIAS OF DEVELOPING
PRECIPITATION TOO SLOWLY IN OVERRUNNING PATTERNS...FEEL THE DRY
AIR MASS WARRANTS LOWERING FORECAST CHANCES OF RAIN RATHER SIGNIFICANTLY
THROUGH THE NIGHT. TOWARD DAYBREAK SATURDAY...THE POTENTIAL FOR
RAINFALL RANGES FROM A LOW CHANCE NEAR THE ALTAMAHA RIVER TO A
SLIGHT CHANCE AROUND THE SAVANNAH RIVER...WITH NO PRECIPITATION
FORECAST FARTHER NORTH.
LOW TEMPERATURES SHOULD BE IN THE MID TO UPPER 30S ACROSS THE AREA
TONIGHT. PRECIPITATION-TYPE ISSUES ARE NOT EXPECTED AS THE AREA
WHERE THE COOLEST TEMPERATURES ARE FORECAST DOES NOT COINCIDE WITH
WHERE ANY PRECIPITATION IS EXPECTED TO OCCUR.
&&
.LONG TERM /SATURDAY THROUGH THURSDAY/...
MEDIUM RANGE MODEL GUIDANCE IS IN RATHER GOOD AGREEMENT IN THE
OVERALL PATTERN THIS PERIOD BUT THERE ARE SOME DIFFERENCES THAT
PRESENT MORE OF A CHALLENGE...ESPECIALLY FOR THE MIDDLE OF NEXT WEEK.
ISENTROPIC ASCENT/DEEP MOISTURE WILL CONTINUE TO INCREASE ACROSS THE
AREA SATURDAY AND THEN REMAIN IN PLACE THROUGH SATURDAY NIGHT AS A
COASTAL TROUGH DEVELOPS OFF THE SOUTH CAROLINA AND GEORGIA COASTS.
IT APPEARS AS THOUGH THE LATEST MODEL GUIDANCE HAS SLOWED DOWN THE
PROGRESSION OF THE RAIN INTO OUR AREA AND THUS HAVE LOWERED POPS A
TAD ACCORDINGLY...ESPECIALLY THROUGH SATURDAY MORNING ACROSS
SOUTHERN SOUTH CAROLINA. HOWEVER...WILL CONTINUE TO INDICATE
DEFINITE RAIN CHANCES ACROSS MOST AREAS SATURDAY AFTERNOON.
DEVELOPING LOW PRESSURE ALONG THE TROUGH AS WELL AS SHORTWAVE ENERGY
ALOFT AND FAVORABLE UPPER-LEVEL JET PLACEMENT SHOULD CONTINUE SOLID
RAIN CHANCES THROUGH SATURDAY NIGHT. RAINFALL TOTALS OF 1 TO 2
INCHES OR MORE ARE POSSIBLE...AS PRECIPITABLE WATER VALUES INCREASE
TO 1.5 TO 1.75 INCHES...WHICH IS ABOUT 2 STANDARD DEVIATIONS ABOVE
NORMAL FOR THIS TIME OF YEAR.
DRIER AIR SHOULD MOVE INTO THE AREA SUNDAY BEHIND THE DEPARTING LOW
PRESSURE UP THE COAST AND WILL INDICATE DECREASING RAIN CHANCES.
WEAK HIGH PRESSURE SHOULD THEN MOVE IN AND PROVIDE DRY WEATHER
SUNDAY NIGHT THROUGH MOST OF MONDAY BEFORE ANOTHER STORM SYSTEM
APPROACHES THE REGION FROM THE WEST. CANNOT RULE OUT SOME SHOWERS
MOVING INTO THE AREA MONDAY AFTERNOON BUT PREFER TO KEEP THINGS DRY
FOR NOW DUE TO UNCERTAINTY IN THE STRENGTH OF THE OVERRUNNING.
GUIDANCE IS NOT IN THE BEST AGREEMENT WITH THE PROGRESSIVENESS OF A
COLD FRONT THROUGH THE AREA IN THE TUESDAY/TUESDAY NIGHT TIME FRAME.
WILL MAINTAIN CONTINUITY FOR NOW AND CONTINUE TO INDICATE THE
FRONTAL PASSAGE LATE TUESDAY...AS PER THE QUICKER GFS AND GEM. MAY
EVEN SEE A FEW THUNDERSTORMS TUESDAY AS LONG AS SURFACE DEWPOINTS
RECOVER SO HAVE ADDED MENTION TO THE FORECAST...MOSTLY FOR SOUTHEAST
GEORGIA AND COASTAL SOUTH CAROLINA. WILL KEEP A RAIN-FREE FORECAST
WEDNESDAY THROUGH THURSDAY WITH FAIRLY COLD HIGH PRESSURE BUILDING
IN FROM THE NORTHWEST.
TEMPERATURES WILL BE TRICKY MUCH OF THE PERIOD BUT HAVE CONTINUE TO
INDICATE VALUES BELOW MOS GUIDANCE SATURDAY...ALTHOUGH NOT QUITE AS
COLD AS PREVIOUS FORECASTS DUE TO THE SLOWER PROGRESSION OF THE
RAIN. TEMPERATURES SHOULD THEN RISE TO NEAR TO ABOVE NORMAL LEVELS
THROUGH TUESDAY NIGHT BEFORE RETURNING BACK TO BELOW NORMAL
WEDNESDAY AND THURSDAY. IN FACT...WE MAY BE DEALING WITH FREEZING
TEMPERATURES INLAND THURSDAY MORNING.
&&
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
BKN TO OVC VFR CIGS WILL CONTINUE TO AFFECT BOTH CHS AND SAV
TERMINALS THIS AFTERNOON AS STRONG UPPER LVL WINDS TRANSPORT MID
AND UPPER LVL MOISTURE TO THE AREA. NORTH TO NORTHEAST FLOW BETWEEN
5 TO 10 KNOTS WILL BE ONGOING FRIDAY AFTERNOON THROUGH SATURDAY
MORNING BEFORE HIGH PRESSURE SHIFTS TO THE NORTHEAST AND A LOW
PRESSURE SYSTEM SHIFTS TOWARDS THE SOUTHEAST.
AS THIS SYSTEM DEVELOPS AND APPROACHES THE SOUTHEAST...NORTHEAST WINDS
WILL BECOME EAST BETWEEN 10 TO 15 KNOTS BY NOON SATURDAY. VFR CIGS
BETWEEN 3 TO 4 KFT WILL CONTINUE TO DEVELOP EARLY AFTERNOON AS
LOW LVL MOISTURE RETURNS AHEAD OF THE SYSTEM. MVFR CIGS ALONG WITH
SHOWERS ARE NOT OUT OF THE QUESTION BY 18Z SAT...ESPECIALLY AT THE
SAV TERMINAL. HAVE NOT INCLUDED MVFR CIGS OR SHOWERS FOR THIS TAF
ISSUANCE DUE TO TIME OF ARRIVAL UNCERTAINTY LATE IN THE TAF PERIOD.
EXTENDED AVIATION OUTLOOK...PERIODS OF IFR OR LOWER CONDITIONS
POSSIBLE SATURDAY NIGHT THROUGH SUNDAY. MVFR OR LOWER CONDITIONS
ARE POSSIBLE SUNDAY NIGHT THROUGH TUESDAY.
&&
.MARINE...
HIGH PRESSURE SHIFTING FROM THE TENNESSEE VALLEY INTO THE CENTRAL
APPALACHIANS TODAY WILL CAUSE WINDS OVER THE COASTAL WATERS TO
VEER NORTHEAST. STRONG COLD AIR ADVECTION EARLY THIS MORNING HAS
LIKELY RESULTED IN WINDS AND POSSIBLY SEAS REACHING SMALL CRAFT
ADVISORY LEVELS IN THE OUTER GEORGIA WATERS. ALTHOUGH THE COLD AIR
ADVECTION WILL WANE AS THE MORNING PROGRESSES...A TIGHT SURFACE
PRESSURE GRADIENT WILL KEEP WINDS ELEVATED THROUGH THE DAY. AS THE
FETCH BECOMES MORE FAVORABLE LATE THIS AFTERNOON...SEAS SHOULD
REBUILD AS WELL. AS A RESULT...HAVE INITIALIZED WITH A SMALL CRAFT
ADVISORY FOR THE OUTER GEORGIA WATERS. ELSEWHERE ACROSS THE
WATERS... WINDS SHOULD REMAIN JUST BELOW 20 KT WITH SEAS NO HIGHER
THAN 3 OR 4 FT IN THE NEAR SHORE WATERS.
THE HIGH WILL SHIFT FARTHER EAST INTO THE SOUTHERN MID-ATLANTIC
REGION TONIGHT...ALLOWING A TROUGH TO DEVELOP OFF THE SOUTHEAST
COAST. THIS WILL CAUSE THE SURFACE PRESSURE GRADIENT TO TIGHTEN
FURTHER ACROSS THE COASTAL WATERS AND THE NORTHEAST FLOW TO
STRENGTHEN. SEAS WILL BUILD AS A RESULT...LIKELY REACHING 6 FT OVER
THE NEAR SHORE GEORGIA WATERS WHERE A SMALL CRAFT ADVISORY HAS
BEEN ISSUED BEGINNING AT MIDNIGHT.
SATURDAY THROUGH TUESDAY...A TROUGH OF LOW PRES WILL BE IN PLACE
OVER THE WEEKEND OFF THE SC AND GA COASTS. A WAVE OF LOW PRESSURE IS
EXPECTED TO TRACK NORTHEAST ALONG THIS TROUGH SATURDAY NIGHT INTO
SUNDAY. HIGH PRESSURE IS THEN EXPECTED TO BUILD BACK INTO THE REGION
SUNDAY NIGHT INTO TUESDAY BEFORE A COLD FRONT BEGINS TO APPROACH THE
AREA LATE IN THE PERIOD. EXPECT A RATHER TIGHT PRESSURE GRADIENT TO
DEVELOP THROUGH SATURDAY NIGHT...AND SMALL CRAFT ADVISORIES ARE
EXPECTED FOR ALL WATERS. AFTER SEAS DROP BELOW 6 FT ACROSS THE OUTER
GA WATERS SUNDAY NIGHT...CONDITIONS SHOULD REMAIN BELOW ADVISORY
CRITERIA THROUGH TUESDAY.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EST MONDAY FOR AMZ374.
SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 11 AM EST SUNDAY
FOR AMZ354.
&&
$$
