FXUS62 KCHS 120230
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
930 PM EST FRI DEC 11 2009
.SYNOPSIS...
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
SHIFT THROUGH THE AREA TUESDAY NIGHT. HIGH PRESSURE WILL THEN
BUILD BACK INTO THE AREA WEDNESDAY THROUGH FRIDAY.
&&
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
UPPER LEVEL FEATURES CONTINUE TO SHOW A LARGE SCALE SPLIT FLOW
ACROSS THE WESTERN PART OF THE NORTH AMERICAN CONTINENT...BUT WITH
CONFLUENT FLOW EMERGING DOWNSTREAM OVER THE EASTERN STATES WHERE
WE HAVE A VERY STRONG 180-190 KT JET OVER THE CENTRAL
APPALACHIANS. OUR AREA WILL CONTINUE TO EXPERIENCE WINDS ALOFT AS
HIGH AS 100-130 KT IN ASSOCIATION WITH THIS JET...WHILE MID LEVEL
WINDS ARE AS HIGH AS 50-80 KT. THIS WILL CONTINUE TO TRANSPORT
CONSIDERABLE MID AND HIGH LEVEL MOISTURE ACROSS THE AREA THROUGH
THE NIGHT...BUT IT WILL TAKE SOME TIME FOR THE LOWER LEVELS TO
MOISTEN WHERE DEW POINT DEPRESSIONS ARE AS MUCH AS 25-28 DEGREES C
AND PWATS ARE STILL DOWN NEAR 1/2 INCH AS PER 00Z SOUNDING FROM
KCHS.
SURFACE HIGH PRESSURE OVER THE CAROLINAS WILL STRENGTHEN A COUPLE
OF MILLIBARS AS IT MOVES EASTWARD TONIGHT...WHILE A COASTAL TROUGH
GRADUALLY BEGINS TO TAKE SHAPE OFF THE SE COAST LATE. THUS THE
PROSPECTS FOR PRECIP TONIGHT WILL BE MAINLY DRIVEN BY ISENTROPIC
LIFTING AND WEAK COASTAL CONVERGENCE AS THE ATLANTIC TROUGH BEGINS
TO TAKE SHAPE.
NONE OF THE FORECAST MODELS ARE INDICATING PRECIP THIS EVENING AS
FAR NORTH AS WE HAVE SEEN. IN FACT...THERE WERE SOME REPORTS OF A
LITTLE SLEET ASSOCIATED WITH A SHORT WAVE MOVING THROUGH SC THIS
AFTERNOON...AND RADAR AND SURFACE OBSERVATIONS CONTINUE TO SHOW A
LITTLE LIGHT RAIN/SPRINKLES IMPACTING SE GA. WE WILL CONTINUE TO
SHOW BEST RAIN PROBABILITIES TONIGHT SOUTH OF I-16...WHERE
CONDENSATION PRESSURE DEFICITS ARE MOST CONDUCIVE. BUT GIVEN RADAR
TRENDS WE WILL INDICATE SOME LIGHT PRECIP EVEN BEFORE MIDNIGHT...AND
RAISE POPS INTO THE CHANCE RANGE OVER THESE SOUTHERN AREAS. LITTLE
TO NO POPS ARE NECESSARY IN SC.
PRECIP TONIGHT LOOKS TO BE MAINLY IN THE FORM OF LIGHT RAIN OR
SPRINKLES...SO NO NEED TO ADD QPF TO THE GRIDS. BUT WITH WET BULB TEMPS
IN THE LOWEST FEW THOUSAND FEET STAYING BELOW FREEZING MUCH OF THE
NIGHT...IT IS NOT OUT OF THE QUESTION THAT A LITTLE SLEET COULD
ALSO OCCUR.
THERE HAVE BEEN SOME LARGE BREAKS IN THE EXTENSIVE CLOUDINESS THIS
EVENING AND THIS HAS ALLOWED FOR SOME TEMPS OVER WESTERN SECTIONS
TO DROP CLOSE TO FORECASTED MIN TEMPS. WE WILL MAKE A FEW
ADJUSTMENTS TO TWEAK SOME INTERIOR LOCATIONS DOWN...SHOWING BELOW
FREEZING ACROSS PARTS OF THE NW TIER. FURTHER SE...TEMPS WILL BE
MODERATED BY NE AND EAST LOW LEVEL FLOW...WITH LOWEST TEMPS
RANGING FROM THE MID 30S WEST OF US-17 TO LOWER 40S AT THE COAST.
&&
.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
A CLASSIC COLD AIR DAMMING PATTERN WILL RAPIDLY STRENGTHEN SATURDAY
AS INCREASING DIABATIC INFLUENCES ACT TO REINFORCE THE INLAND
WEDGE. INCREASING ISENTROPIC ASCENT ALONG THE NOSE OF A 40 KT
925-700MB JET COUPLED WITH THE BROAD RIGHT ENTRANCE REGION OF THE
UPPER JET WILL SUPPORT A LARGE AREA OF MODERATE TO OCCASIONALLY
HEAVY RAIN ACROSS MUCH OF THE SOUTHEAST STATES. RAINS WILL BEGIN
AS EARLY AS SUNRISE ACROSS THE FAR SOUTH ZONES THEN QUICKLY SPREAD
NORTH THROUGH THE MORNING AS DYNAMICS INCREASE. SHOULD SEE A SOLID
RAIN SHIELD EVOLVE ACROSS THE ENTIRE FORECAST AREA BY LATE MORNING
OR EARLY AFTERNOON WITH RAIN POTENTIALLY BECOME LOCALLY HEAVY A
TIMES. RAINFALL AMOUNTS THROUGH SUNSET WILL AVERAGE 0.25-0.75 INCH
WITH LOCALLY HIGHER AMOUNTS...ESPECIALLY ACROSS INTERIOR SOUTHEAST
GEORGIA.
THERE COULD BE A POTENTIALLY LARGE RANGE IN HIGHS SATURDAY AS
DIABATIC PROCESSES WILL KEEP THINGS RATHER CHILLY ESPECIALLY
ACROSS THE INTERIOR WHERE MODEL WET-BULB TEMPERATURES ONLY WARM
INTO THE UPR 30S. WE CONTINUE TO FAVOR A MUCH COLDER SOLUTION THAN
WHAT THE VARIOUS MODELS ARE DEPICTING AS THEY OFTEN EXHIBIT A
SIGNIFICANT WARM BIAS IN STRONG COLD AIR DAMMING PATTERNS. PLAN
TO LOWER HIGHS INTO THE LWR 40S WELL INLAND TO CLOSER MATCH MODEL
WET-BULB PROGS WITH HIGHS WARMING INTO THE UPR 50S/LWR 60S AT THE
COAST. DEPENDING ON HOW HEAVY AND WIDESPREAD THE RAIN SHIELD
BECOMES...HIGHS MAY VERY WELL STRUGGLE TO REACH THE UPR 30S WELL
INLAND. CONVERSELY...SHOULD THE COASTAL TROUGH NUDGE CLOSER TO THE
COAST THAN WHAT IS CURRENTLY ANTICIPATED...HIGHS ALONG THE BEACHES
COULD BE CONSIDERABLY WARMER. THERE IS OBVIOUSLY A HUGE
TEMPERATURE BUST POTENTIAL.
&&
.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
MEDIUM RANGE MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH THE
EXTENDED PERIODS THIS AFTERNOON...BUT FINE TUNING MESOSCALE DETAILS
WILL RESULT IN A RATHER CHALLENGING FORECAST FOR THE TAIL END OF THE
WEEKEND INTO EARLY NEXT WEEK.
MODEL SOLUTIONS ALL INDICATE INCREASING ISENTROPIC ASCENT AND DEEP
MOISTURE ALONG WITH GOOD UPPER LEVEL FORCING FOR ASCENT
OVERSPREADING THE FORECAST AREA SATURDAY NIGHT AS A WAVE OF LOW
PRESSURE LIFTS NORTHWARD ALONG A SHARP COASTAL TROUGH OFF THE SOUTH
CAROLINA AND GEORGIA COAST. AT THE SAME TIME...GUIDANCE ALSO SHOWS A
HIGH PRESSURE WEDGE HANGING TOUGH IN THE LEE OF THE APPALACHIANS.
LATEST MODEL TRENDS INDICATE THAT THE WAVE OF LOW PRESSURE MAY ALLOW
THE COASTAL TROUGH TO SHIFT INLAND AS IT LIFTS NORTHWARD INTO
COASTAL SOUTH CAROLINA. NOT ENTIRELY CONFIDENT THAT THE HIGH
PRESSURE WEDGE WILL BREAK DOWN...BUT GIVEN STRONG CONTINUITY AMONGST
THE OPERATIONAL MODELS INDICATING THAT IT WILL ACROSS AT LEAST
COASTAL SECTIONS OF THE FORECAST AREA...HAVE TRENDED TEMPERATURES
UPWARD THROUGH THE NIGHT. AFTER LOWS AT OR CLOSE TO THE DAYTIME
MAXES SATURDAY...EXPECT TEMPERATURES TO RISE INTO THE UPPER 40S
ACROSS EXTREME WESTERN ZONES AND INTO THE LOWER 60S ALONG THE COAST
PRIOR TO DAYBREAK SUNDAY. AS FAR AS SENSIBLE WEATHER DETAILS ARE
CONCERNED...RAIN LOOKS LIKE A NEAR CERTAINTY THROUGH THE NIGHT...SO
WILL CONTINUE TO CARRY CATEGORICAL POPS.
THE WAVE OF LOW PRESSURE WILL CONTINUE TO LIFT NORTHWARD THROUGH
SOUTHERN SOUTH CAROLINA SUNDAY MORNING AND INTO THE NORTH CAROLINA
OUTER BANKS BY LATE SUNDAY AFTERNOON. BEST ISENTROPIC ASCENT AND
DEEP MOISTURE WILL TRANSLATE ACROSS THE AREA IN THE MORNING...SO
WILL CARRY LIKELY CATEGORICAL POPS THROUGH 18Z...THEN TREND POPS
INTO THE CHANCE RANGE FOR THE 18Z SUNDAY THROUGH 00Z MONDAY TIME
FRAME. TEMPERATURES ONCE AGAIN WILL BE VERY TRICKY...AS THE HIGH
PRESSURE WEDGE WILL LIKELY HOLD ACROSS FAR INTERIOR ZONES...WHILE
AREAS WHICH WARM SECTORED SATURDAY NIGHT MAY FALL BACK UNDER THE
INFLUENCE OF THE WEDGE BY SUNDAY AFTERNOON AS WINDS SWING BACK
AROUND TO THE WEST AND SOUTHWEST AS LOW PRESSURE LIFTS NORTHEASTWARD
AND AWAY FROM THE REGION. WILL FORECAST HIGHS IN THE MID TO UPPER
60S ALONG THE COAST...WITH HIGHS HOLDING IN THE MID 50S ACROSS FAR
NORTHWESTERN ZONES...BUT NEEDLESS TO SAY CONFIDENCE IN HOW THINGS
WILL PLAY OUT REMAINS LOW.
WEAK HIGH PRESSURE OR THE LINGERING WEDGE WILL REMAIN OVER THE
REGION SUNDAY NIGHT AND MONDAY. MODEL SOLUTIONS DEPICT SOME LIGHT
QPF OVER THE REGION THROUGH THE PERIOD...LIKELY ACCOUNTING FOR
DRIZZLE GIVEN THE WEAK GRADIENT...SO HAVE CARRIED SOME SLIGHT CHANCE
POPS THROUGH THE PERIOD. GUIDANCE CONTINUES TO ADVERTISE
TEMPERATURES IN THE LOWER 70S...BUT GIVEN THE SIGNALS IN THE MODEL
FIELDS INDICATING THAT THE WEDGE WILL HOLD IN PLACE ALONG WITH
PLENTY OF LOW OVERCAST AS POSSIBLY FOG...WILL KEEP HIGHS GENERALLY
IN THE LOWER AND MIDDLE 60S.
THE HIGH PRESSURE WEDGE IS FINALLY EXPECTED TO BREAK DOWN MONDAY
NIGHT AND TUESDAY AS A COLD FRONT APPROACHES THE REGION FROM THE
WEST. THIS COLD FRONT SHOULD SWING THROUGH THE AREA TUESDAY
NIGHT...FOLLOWED BY HIGH PRESSURE WEDNESDAY THROUGH FRIDAY. HAVE
CARRIED CHANCE POPS MONDAY NIGHT THROUGH TUESDAY NIGHT TO ACCOUNT
FOR WARM ADVECTION SHOWERS AS WELL AS RAIN ASSOCIATED WITH THE COLD
FRONTAL PASSAGE. AT THIS TIME IT APPEARS THAT WEDNESDAY THROUGH
FRIDAY WILL BE RAIN FREE. TEMPERATURES ARE EXPECTED TO RUN WELL
ABOVE NORMAL TUESDAY...THEN TREND SEVERAL DEGREES BELOW SEASONAL
NORMS WEDNESDAY THROUGH FRIDAY.
&&
.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
STRONG JET IN THE MID AND UPPER LEVELS WILL CONTINUE TO TRANSPORT
CONSIDERABLE MID AND HIGH CLOUDINESS TO THE TERMINALS TONIGHT...BUT
WITH CIGS REMAINING AT 10K FT OR GREATER. BY LATE TONIGHT AND
EARLY SATURDAY MORNING SOME MOISTENING OF THE LOWER LEVELS WILL
DEVELOP AS A COASTAL TROUGH FORMS OFF THE SE COAST OF THE
COUNTRY. THIS WILL RESULT IN SCT STRATOCUMULUS DEVELOPING...IN
ADDITION TO THE MID AND HIGH CLOUD COVER. WITH INCREASING
ISENTROPIC LIFT TO FORM THROUGH THE DAY...AIDED BY SHORT WAVES
ALOFT AND STRONG WIND FIELDS THROUGHOUT THE VERTICAL...EVENTUALLY
LIGHT TO MODERATE RAINS AND MVFR CONDITIONS WILL FORM. THE
DETERIORATING CONDITIONS WILL OCCUR EARLIER AT KSAV THAN AT
KCHS...WITH MVFR CIGS AND VSBYS TO SET IN AROUND 19Z AT KSAV AND
AROUND 21Z AT KCHS. LATER TAF ISSUANCES WILL NEED TO CONSIDER THE
POSSIBILITY OF IFR WEATHER LATE SATURDAY AND/OR SATURDAY NIGHT.
GIVEN WINDS AS HIGH AS 20-30 KT WITHIN THE FIRST FEW THOUSAND FEET
ON SATURDAY...OUR WIND FORECAST WHICH PRESENTLY SHOWS AROUND
12-13 KT...MIGHT BE A LITTLE UNDERDONE. BUT THE EXTENSIVE CLOUD
COVER WILL LIMIT THE MIXING POTENTIAL AND THUS THE MORE CONSERVATIVE
FORECAST AT THIS TIME.
EXTENDED AVIATION OUTLOOK...PERIODS OF IFR OR LOWER CONDITIONS
POSSIBLE SATURDAY NIGHT THROUGH MONDAY. MVFR OR LOWER CONDITIONS ARE
POSSIBLE MONDAY NIGHT THROUGH TUESDAY NIGHT. VFR CONDITIONS MAY
RETURN ON WEDNESDAY.
&&
.MARINE...
THE PRESSURE GRADIENT CONTINUES FAIRLY TIGHT ACROSS OUR SOUTHERN
WATERS THIS EVENING...BETWEEN A 1033 MILLIBAR HIGH CENTERED OVER
THE CAROLINAS AND THE OLD COLD FRONT ACROSS THE SE GULF AND THE
FLORIDA STRAITS. THE PINCHING WILL ONLY STRENGTHEN THROUGH THE
NIGHT AS THE HIGH TO THE NORTH INCREASES ANOTHER MILLIBAR OR
TWO...AND A COASTAL TROUGH BEGINS TO TAKE SHAPE NEAR THE LOCAL
WATERS. WINDS HAVE BEEN AVERAGING AROUND 15 KT ACROSS OUR SC
WATERS...CLOSE TO 20 KT OVER AMZ354 AND GREATER THAN 20 KT ALONG
THE OUTER GA WATERS. SEAS HAVE BEEN AROUND 3-4 FT WITHIN 20 NM OF
THE COAST...BUT 5 FT OR GREATER FURTHER OFFSHORE. WIND SPEEDS
WILL INCREASE AT LEAST ANOTHER 5 KT OR SO...WHICH IN TURN WILL
ALLOW FOR SEAS ANOTHER FOOT OR TWO. WE WILL MAINTAIN OUR SCA FOR
AMZ374...AND INITIATE THE START TIME FOR AMZ354 THIS EVENING SINCE
THEY ARE ALREADY CLOSE TO SCA CRITERIA WITH FREQUENT WIND GUSTS AND
CONDITIONS WILL ONLY GET WORSE. ACROSS OUR SC WATERS WE BELIEVE
THAT GIVEN A TIGHTENING OF THE GRADIENT THAT WINDS AND/OR SEAS
WILL BE ABLE TO REACH SCA CONDITIONS BY LATE TONIGHT. SO IN ORDER
TO GIVE MORE OF A HEADS UP TO MARINERS WE WILL START THE SCA WITH
AN EVENING UPDATE. CHARLESTON HARBOR WILL BE BORDERLINE FOR ANY
SCA CONDITIONS TONIGHT...BUT WILL BE ABLE TO REACH 15-20 KT BY
MORNING WITH THE TIGHTENING OF THE GRADIENT.
CONDITIONS WILL CONTINUE TO DETERIORATE SATURDAY AS THE INLAND
WEDGE STRENGTHENS. SHOULD SEE A SOLID 20-25 KT ACROSS ALL WATERS
DURING THE MORNING HOURS WHICH WILL PERSIST INTO THE AFTERNOON AS
A COASTAL TROUGH TRIES TO MAKE WESTWARD PROGRESS. THE GEORGIA
OFFSHORE WATERS BEYOND 40 NM MAY PUNCH IN THE WARM SECTOR BEHIND
THE COASTAL TROUGH AND WITH WATER TEMPERATURES STILL HOLDING IN
THE UPR 60S/LWR 70S IN THAT AREA...COULD SEE WINDS GUST AS HIGH AS
30-32 KT IN THAT AREA AT TIMES. GUIDANCE AT FBIS1 HAS BEEN SHOWING
A POTENTIAL FOR AT LEAST 20 KT FOR SEVERAL RUNS NOW...SO A SMALL
CRAFT ADVISORY WILL BE ISSUED FOR THE CHARLESTON HARBOR.
SATURDAY NIGHT THROUGH WEDNESDAY...A WAVE OF LOW PRESSURE IS
EXPECTED TO TRACK NORTHWARD THROUGH THE COASTAL WATERS SATURDAY
NIGHT AND SUNDAY. HIGH PRESSURE IS THEN EXPECTED TO BUILD BACK
INTO THE REGION SUNDAY NIGHT INTO TUESDAY BEFORE A COLD FRONT
SWEEPS THROUGH THE WATERS TUESDAY NIGHT. HIGH PRESSURE WILL BUILD
FROM THE NORTH ON WEDNESDAY. EXPECT A RATHER TIGHT PRESSURE
GRADIENT TO PERSIST ACROSS THE WATERS SATURDAY NIGHT...WITH SMALL
CRAFT CONDITIONS EXPECTED ACROSS WATERS. SMALL CRAFT CONDITIONS
MAY LINGER ACROSS PORTIONS OF THE WATERS THROUGH SUNDAY NIGHT.
WINDS AND SEAS SHOULD SUBSIDE BELOW SMALL CRAFT LEVELS MONDAY
THROUGH TUESDAY. COULD SEE SMALL CRAFT CONDITIONS ONCE AGAIN
TUESDAY NIGHT AND WEDNESDAY FOLLOWING A STRONG COLD FRONT PASSAGE.
&&
.HYDROLOGY...
WIDESPREAD RAIN TOTALS OF 2-3 INCHES WITH LOCALLY HIGHER AMOUNTS
ARE EXPECTED TO OCCUR ACROSS MUCH OF THE FORECAST AREA THROUGH
SUNDAY. FLASH FLOOD GUIDANCE VALUES REMAIN FAIRLY ELEVATED
CONSIDERING THE AREA HAS SEEN SOME FAIRLY SIGNIFICANT RAINS OVER
THE PAST FEW WEEKS. WE DO NOT BELIEVE A FLASH FLOOD WATCH IS
NEEDED FOR ANY PORTION OF THE FORECAST AREA ATTM SINCE MUCH OF
THE AREA WILL LIKELY BE ABLE TO HANDLE 2-3 INCHES OF RAIN OVER A
24-48 HOUR PERIOD. THE ONLY EXCEPTION COULD BE AT THE COAST SHOULD
MODERATE TO HEAVY RAINS FALL DURING HIGH TIDE.
PORTIONS OF THE SAVANNAH AND SOUTH SANTEE RIVERS REMAIN ABOVE
FLOOD STAGE. ADDITIONAL RISES ARE LIKELY ON AREA RIVERS...WHICH
MAY HELP PUSH ADDITIONAL POINTS INTO FLOOD NEXT WEEK.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 11 AM SATURDAY TO 5 AM EST SUNDAY
FOR AMZ330.
SMALL CRAFT ADVISORY UNTIL 5 PM EST SUNDAY FOR AMZ350-352.
SMALL CRAFT ADVISORY UNTIL 5 AM EST MONDAY FOR AMZ374.
SMALL CRAFT ADVISORY UNTIL 11 AM EST SUNDAY FOR AMZ354.
&&
$$
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