FXUS62 KCHS 260242
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
852 PM EST FRI DEC 25 2009
.SYNOPSIS...
A DEPARTING AREA OF LOW PRESSURE OVER NC WILL CONTINUE TO MOVE
FURTHER NE TONIGHT. COLD HIGH PRESSURE WILL BUILD IN THROUGH MUCH
OF NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
COLD FRONT PUSHING INTO THE ATLANTIC WATERS THIS EVENING...AND IN
ITS WAKE SKIES HAVE CLEARED ENOUGH WHERE WE HAVE SEEN PATCHY FOG
DEVELOPING. NOT ENOUGH TO WARRANT CHANGING THE FORECAST...BUT
ENOUGH SO THAT A SHORT TERM FORECAST HAS BEEN ISSUED TO ADDRESS
SOME VISIBILITIES DOWN TO 1 MILE OR LESS. AS WINDS INCREASE
THROUGH THE NIGHT WITH THE COLD AIR ADVECTION...THE RISK OF ANY
FOG WILL END.
HIGH PRESSURE CENTERED IN SW LA WILL CONTINUE TO BUILD EASTWARD
TONIGHT AS THE NC LOW MOVES INTO SE VA. GRADUALLY THE CYCLONIC
FLOW AROUND THE LOW WILL GIVE WAY TO ANTICYCLONIC CURVATURE AS A
RESULT OF THE HIGH PRESSURE SYSTEM. THIS ALONG WITH WEST AND NW
FLOW OFF THE APPALACHIANS WILL LIMIT THE AMOUNT OF STRATOCUMULUS
FROM IMPACTING THE REGION. BUT WITH STRONG UPPER JET TO CONTINUE
TRANSPORTING OCCASIONAL CIRRUS FROM THE GULF ACROSS THE SE AND
PLENTY OF MOISTURE WITHIN THE BOUNDARY LAYER TO ALLOW FOR PATCHY
STRATUS...OVERALL WE LOOK FOR PARTLY CLOUDY SKIES TO PREVAIL.
BUT THE CLOUD COVER WILL HAVE LITTLE EFFECT ON TEMPS...WHICH IS
DRIVEN PURELY BY THE COLD AIR ADVECTION. TEMPS THIS EVENING IN
MANY AREAS ARE ALREADY LOWER THAN THEY WERE LAST NIGHT...AND WITH
CONTINUED COLD AIR ADVECTION WE LOOK FOR A STEADY FALL THROUGH THE
NIGHT. LOWEST READINGS BY MORNING WILL BE DOWN IN THE MID AND
UPPER 30S FAR NW TIER...BUT IN THE LOWER AND MIDDLE 40S CLOSER TO
THE COAST.
HIGH SURF...WE WILL MAINTAIN THE HIGH SURF ADVISORY FOR THE SC
COASTAL AREAS WITH BREAKERS STILL AS HIGH AS 5 OR 6 FEET FROM THE
FOLLY BEACH PIER. BUT WITH A CONTINUED OFFSHORE FETCH THESE
BREAKERS WILL COME DOWN THROUGH THE NIGHT.
&&
.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
COLD HIGH PRESSURE WILL MAINTAIN DRY CONDITIONS ON SATURDAY WITH
HIGHS IN THE MID TO UPPER 50S. MOST OF THE LOW AND MID CLOUDS
SHOULD BE CLEARED OUT BY MORNING HOWEVER A 140 KT JET STREAK
MOVING ACROSS THE AREA SHOULD BRING IN A FAIRLY EXTENSIVE CIRRUS
VEIL BY MID-MORNING.
&&
.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
A DEEP UPPER TROF WILL START THE PERIOD OVER THE GREAT LAKE
STATES...WITH STRONG UPPER ZONAL FLOW OVER THE SE U.S. AND GULF OF
MEXICO. THE UPPER LOW WILL MOVE OFF THE NEW ENGLAND COAST BY LATE
MONDAY/EARLY TUESDAY...ALLOWING FOR A LOW AMPLITUDE UPPER RIDGE TO
DEVELOP OVER THE SE U.S. TUESDAY THROUGH EARLY WEDNESDAY. IN
GENERAL...A BROAD/FULL LATITUDE UPPER TROF BEGINS TO CARVE OUT OVER
THE CENTRAL U.S. BY THURSDAY AND INTO FRIDAY WITH SW UPPER FLOW OVER
THE SE U.S.. THE MAIN DIFFICULTY WITH THIS FORECAST WILL BE TRYING
TO TIME UPPER IMPULSES THAT APPEAR TO MOVE RAPIDLY FROM
MEXICO...EASTWARD TOWARD THE NE GULF AND SE U.S. WITHIN A STRONG
SUBTROPICAL FLOW. MODELS HISTORICALLY HAVE TROUBLE WITH FEATURES
ACROSS THIS REGION DUE TO SPARSE OBSERVATIONAL DATA. THE FIRST WEAK
IMPULSE LOOKS LIKE IT COULD AFFECT THE AREA ON SUNDAY. LATEST RUNS
OF ALL MODELS APPEAR TO BE BACKING OFF ON THE STRENGTH AND NORTHWARD
EXTENT OF DEEP LAYER MOISTURE WITH THIS FEATURE. IN RESPONSE TO THIS
TREND...HAVE TRIMMED BACK POPS TO SLIGHT CHC FOR JUST THE S 1/3 OF
THE AREA. BEHIND THIS FEATURE...A MODERATELY STRONG SFC RIDGE
/1033MB/ BUILDS FROM THE WEST...USHERING IN COLDER AND DRIER AIR FOR
MONDAY AND TUESDAY. HIGHS BOTH DAYS ONLY IN THE 50S. HAVE KEPT LOWS
JUST ABOVE FREEZING MOST AREAS EXCEPT WELL INLAND FOR MONDAY
MORNING. TUESDAY MORNING LOOKS TO BE THE COLDEST AT THIS TIME.
MODELS STILL SHOWING THAT UPPER 20S ARE POSSIBLE AS FAR EAST AS CHS
AND SAV. ANOTHER COLD NIGHT TUESDAY NIGHT/WEDNESDAY...DESPITE THE
SFC RIDGE STARTING TO SHIFT OFFSHORE. HAVE KEPT MINS IN THE UPPER
20S TO LOWER 30S MOST AREA EXCEPT IMMEDIATE COAST.
WEDNESDAY THROUGH FRIDAY...CONFIDENCE IN THE MODEL FORECASTS FOR
THIS PERIOD IS QUITE LOW. IN GENERAL...THE MODELS SHOW AN
INCREASE IN DEEP LAYER MOISTURE AS ANOTHER SOUTHERN STREAM WAVE
MOVES ENE ACROSS THE GULF OF MEXICO. IT SEEMS NEARLY IMPOSSIBLE AT
THIS POINT TO PIN POINT THE EXACT TIMING AND STRENGTH OF THIS
POTENTIAL SYSTEM. HAVE NUDGED POPS UP A TAD...BUT KEPT THE TREND OF
THE PREVIOUS FORECAST WITH SLIGHT CHANCE POPS LATER WEDNESDAY AND
WEDNESDAY NIGHT...THEN LOW END CHANCE POPS FOR THURSDAY THROUGH
FRIDAY. GIVEN THE MODELS SHOW THE MAIN UPPER ENERGY AND SFC WAVE
REMAINING SOUTH OF THE REGION...KEPT OUT MENTION OF ANY CONVECTIVE
PRECIP. TEMPS ARE EXPECTED TO REMAIN BELOW NORMAL WITH HIGHS IN THE
LOWER TO MID 50S AND LOWS IN THE MID TO UPPER 30S.
&&
.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
POSSIBLE MVFR VSBYS AT THE SAV EARLY TONIGHT...OTHERWISE VFR
CONDITIONS ARE EXPECTED AT BOTH CHS AND SAV TERMINALS FOR THE NEXT
24 HOURS.
SOME UNCERTAINTY REMAINS IN MVFR VSBYS AT THE SAV TERMINAL EARLY
TONIGHT. RECENT RAINS ALONG WITH CALM WINDS WILL CREATE FAVORABLE
CONDITIONS FOR LOWERING VSBYS...BUT WIDESPREAD MVFR CONDITIONS ARE
NOT ANTICIPATED AS LIGHT DOWNSLOPING WINDS DEVELOP OVERNIGHT.
HAVE INTRODUCED TEMPO MVFR VSBYS UNTIL 02Z WHILE WINDS REMAIN
LIGHT/CALM. FURTHER LOWERING OF VSBYS IS POSSIBLE IF WINDS REMAIN
LIGHT FOR A LONGER PERIOD...BUT CONFIDENCE IS TOO LOW TO INCLUDE
IN TAF AT THIS TIME.
LIGHT WEST WINDS WILL BECOME NORTHWEST SHORTLY AFTER DAYBREAK
SATURDAY. VFR CONDITIONS WILL PREVAIL SATURDAY MORNING THROUGH
SATURDAY AFTERNOON AS DRYER AIR ENCOUNTERS INTO THE REGION.
EXTENDED AVIATION OUTLOOK...GENERALLY VFR CONDITIONS EXPECTED
SUNDAY THROUGH TUESDAY. NEXT CHANCE FOR SUB-VFR CONDITIONS WILL
BE WEDNESDAY AS ANOTHER POTENTIAL SYSTEM MOVES WELL TO THE SOUTH
OF THE REGION. CONFIDENCE IN THE TIMING AND STRENGTH OF THIS
FEATURE IS VERY LOW.
&&
.MARINE...
THE WEATHER MAPS LATE CHRISTMAS EVENING SHOW A DEPARTING 1009 MB
LOW OVER NC CONTINUING TO MOVE NE...WHILE A 1019 MB HIGH OVER SW
LA IS PROGRESSING EASTWARD IN WAKE OF THE LOW. WINDS HAVE
TEMPORARILY DROPPED OFF IN WAKE OF A COLD FRONT THIS EVENING AND
WITHIN THE BAGGINESS TO THE PRESSURE PATTERN AROUND THE LOW. BUT
AS COLD AIR ADVECTION INCREASES THROUGH THE NIGHT AND SMALL BUT
STEADY PRESSURE RISES DEVELOP...WINDS WILL INCREASE AGAIN AS THEY
VEER AROUND TO THE NW. HOWEVER...WINDS WILL NOT REACH ANY HIGHER
THAN AROUND 20 KT...WITH OCCASIONAL GUSTS BETWEEN 20 AND 25 KT
ACROSS THE OUTER GA WATERS. SEAS WILL REMAIN ELEVATED DUE TO THE
LARGE SWELLS THAT HAVE BEEN IMPACTING THE REGION OF LATE. AND
THESE SWELLS FROM THE EAST AND SE...THAT HAVE BEEN AS 13 FEET EVERY
10-11 SECONDS AT 41004...AND 4-5 FEET EVERY 8-10 SECONDS AT 41008
WILL TAKE SOME TIME IN COMING DOWN. ALTHOUGH WIND WAVES WILL BE
LIMITED DUE TO THE OFFSHORE FETCH...THE COMBINATION OF THE WIND
WAVES AND SWELLS WILL BE HIGH ENOUGH TO ALLOW FOR A CONTINUATION
OF THE SCA/S ACROSS THE SC ATLANTIC WATERS AND THE OUTER GA
WATERS. WE WILL DROP THE SCA FOR AMZ354 AND THE SCA FOR AMZ330 WAS
DISCONTINUED EARLIER.
THERE HAVE BEEN REPORTS OF LIGHT FOG ALONG SEVERAL COASTAL
SITES...DUE TO THE LIGHT WINDS AND PARTIAL CLEARING SKIES. AS A
RESULT WE HAVE ISSUED A MARINE WEATHER STATEMENT ADDRESSING
VISIBILITY DOWN TO 3/4 OR 1 NM THROUGH MIDNIGHT. WITH INCREASING
WINDS THROUGH THE NIGHT AND INCREASING DEW POINT DEPRESSIONS THE
RISK OF ANY REDUCED VISIBILITY IN FOG WILL END BY MIDNIGHT.
GREATLY IMPROVING CONDITIONS WILL CONTINUE THROUGH SATURDAY
AFTERNOON AS THE GRADIENT DECREASES.
NO HIGHLIGHTS EXPECTED THROUGH EARLY MONDAY WITH HIGH PRESSURE
SITTING OVER THE REGION. BY LATER MONDAY...STRONGER HIGH PRESSURE
WILL BUILD FROM THE WEST...WITH STRONGER COLD ADVECTION OVER THE
ATLANTIC WATERS. THIS WILL HELP TO INCREASE WINDS TO AT LEAST LOW
END SCA LEVELS OVER THE GA OFFSHORE WATERS LATER MONDAY AND MONDAY
NIGHT. THE WINDS AND SEAS WILL DECREASE AGAIN TUESDAY AS HIGH
PRESSURE BUILDS BACK OVER THE AREA. HIGH PRESSURE WILL WEAKEN AND
MOVE EAST OF THE AREA ON WEDNESDAY AS ANOTHER WEAK AREA OF LOW
PRESSURE TRIES TO MOVE TOWARD THE REGION FROM THE GULF OF MEXICO.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...HIGH SURF ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR SCZ048>051.
MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EST SATURDAY FOR AMZ352.
SMALL CRAFT ADVISORY UNTIL NOON EST SATURDAY FOR AMZ350.
SMALL CRAFT ADVISORY UNTIL 5 PM EST SATURDAY FOR AMZ374.
&&
$$
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