FXUS62 KCHS 082050
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
350 PM EST TUE DEC 8 2009
.SYNOPSIS...
A WARM FRONT WILL LIFT NORTH THROUGH THE AREA TONIGHT...BEFORE A
COLD FRONT SHIFTS ACROSS THE SOUTHEAST WEDNESDAY. SURFACE HIGH
PRESSURE WILL BUILD INTO THE REGION WEDNESDAY NIGHT THROUGH
FRIDAY. LOW PRESSURE WILL ADVANCE NORTHEAST NEAR THE SOUTHEAST
COAST SATURDAY INTO SUNDAY...FOLLOWED BY HIGH PRESSURE INTO
MONDAY. A COMPLEX STORM SYSTEM WILL THEN IMPACT THE REGION LATE
MONDAY THROUGH TUESDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
LATEST MSAS ANALYSIS SHOWS A NEARLY 3.5 MB DROP IN SURFACE
PRESSURE OVER THE LAST THREE HOURS...AS THE INLAND WEDGE BEGINS TO
BREAK DOWN NORTH OF A DEVELOPING WARM FRONT AND AHEAD OF A STRONG
LOW PRESSURE SYSTEM TRACKING TOWARD THE OHIO VALLEY. A DISTINCTIVE
SHORTWAVE IS JUST OVER THE FORECAST AREA AND HAS ENHANCED SHOWER
COVERAGE ACROSS THE SOUTH CAROLINA MIDLANDS.
MODELS ARE IN FAIRLY GOOD AGREEMENT THAT PRECIP COVERAGE WILL
GRADUALLY INCREASE ACROSS SOUTHEAST SOUTH CAROLINA AND SOUTHEAST
GEORGIA DURING THE NEXT FEW HOURS...WITH DEEP MOISTURE AND ASCENT
STRENGTHENING AS THE WARM FRONT LIFTS NORTH INTO THE AREA. HAVE
THEREFORE INCREASED POPS TO CATEGORICAL BY 21Z.
ALTHOUGH DYNAMICS ARE IMPRESSIVE WITH THE LOW SYSTEM TO THE
WEST...INSTABILITY TONIGHT LOOKS CONFINED TO THE MARINE ZONES.
HAVE THUS LIMITED MENTION OF THUNDER TO THE COASTAL WATERS THROUGH
TONIGHT.
THE BEST RAIN CHANCES WILL SHIFT NORTHWARD OVERNIGHT...COINCIDENT
WITH THE NORTHWARD MOVEMENT OF THE WARM FRONT. CONFIDENCE IS
FAIRLY HIGH...AND MODELS REMAIN IN DECENT AGREEMENT...THAT
LOCATIONS TO THE SOUTH OF THE WARM FRONT WILL SEE A DISTINCT
REDUCTION IN RAIN CHANCES AS DEEP MOISTURE REMAINS WITH THE
ASCENDING FRONT. HAVE GRADUALLY REDUCED RAIN CHANCES FROM SOUTH TO
NORTH AROUND MIDNIGHT...REMOVING MENTION OF PRECIPITATION FROM THE
EASTERN HALF OF THE GEORGIA AREA AND ALONG THE SOUTH CAROLINA
COAST GENERALLY SOUTH OF THE CHARLESTON HARBOR.
&&
.SHORT TERM /WEDNESDAY/...
THE FORECAST AREA WILL LAY SOLIDLY WITHIN THE WARM SECTOR AHEAD OF
AN APPROACHING COLD FRONT TO THE WEST ON WEDNESDAY. AS A
RESULT...TEMPERATURES WILL BE UNSEASONABLY WARM IN THE MID 70S.
SOME MIXING OF THE VERY STRONG LOW TO MID LEVEL 50 KT JET IS ALSO
POSSIBLE...WITH BREEZY CONDITIONS EXPECTED BY THE AFTERNOON HOURS.
HAVE ISSUED A LAKE WIND ADVISORY FOR LAKE MOULTRIE TO ACCOUNT FOR
ENHANCED WINDS OVER THE OPEN LAKE WATERS.
THE COLD FRONT WILL CROSS THE AREA WEDNESDAY AFTERNOON.
INTERESTINGLY...THE FRONT APPEARS TO LACK A DECENT TAP INTO DEEP
MOISTURE...AND MODEL QPF IS FAIRLY LIMITED DURING ITS PASSAGE. HAVE
THUS CAPPED POPS ON WEDNESDAY AT OR BELOW 30 PERCENT.
INSTABILITY IS AGAIN CONCENTRATED OVER THE COASTAL WATERS AHEAD OF
THE FRONT. STORM PREDICTION CENTER HAS KEPT THE FORECAST AREA WITHIN
A SEE TEXT/5 PERCENT ZONE FOR DAMAGING WINDS ON WEDNESDAY. CONSIDERING
THAT TEMPERATURES WILL BE QUITE WARM...DEWPOINTS WILL SURGE INTO
THE LOW TO MID 60S DURING THE AFTERNOON...AND WIND FIELDS ARE VERY
IMPRESSIVE...THERE REMAINS A SMALL CHANCE THAT THUNDERSTORMS COULD
FORM AND POTENTIALLY PRODUCE DAMAGING WINDS. THE HIGHEST RISK FOR
CONVECTIVE DEVELOPMENT WOULD BE ALONG COASTAL COUNTIES...IF THE
AREA OF INSTABILITY OFFSHORE CAN SCRAPE THE COASTLINE. PREFER TO
KEEP MENTION OF THUNDER OVER THE WATERS AT THIS TIME DUE TO
LIMITED INSTABILITY AND WEAK LAPSE RATES.
&&
.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
DEEP LAYER DRY SLOTTING WILL ENSURE RAIN FREE CONDITIONS EXCEPT
PERHAPS SOUTHERN COASTAL WATERS WEDNESDAY EVENING. THEN...12Z
NAM/GFS GUIDANCE DEPICTS THE SURFACE BOUNDARY/ASSOCIATED BAROCLINIC
ZONE STALLING JUST S/SE OF THE FORECAST AREA. A WEAK WAVE WILL
RIPPLE ENE THROUGH THE BAROCLINIC ZONE...PUSHING A NARROW BAND OF
DEEP LAYER MOISTURE TOWARD S/E COUNTIES OF THE FORECAST AREA.
INITIALLY...LOW LEVEL DRY WILL PREVENT MOST/ALL PRECIPITATION
FROM REACHING THE GROUND...BUT A PERIOD OF LIGHT RAIN WILL PROBABLY
IMPACT AT LEAST FAR SOUTHERN COUNTIES THURSDAY MORNING.
THUS...INTRODUCED CHANCE POPS FAR SOUTH LATE WEDNESDAY NIGHT THROUGH
THURSDAY MORNING.
AS THE WAVE EXITS STAGE EAST AND HIGH PRESSURE EXPENDS INTO THE
REGION FROM THE N/NW...DEEP LAYER MOISTURE WILL RETREAT FROM S
SECTIONS OF THE FORECAST AREA...AND COLD ADVECTION WILL ENSURE A
COLD NIGHT THURSDAY NIGHT WITH FREEZING TEMPERATURES AT MANY
LOCATIONS...PERHAPS EVEN TO THE SOUTH CAROLINA COAST. MID-HIGH LEVEL
CLOUDS COULD SPREAD QUICKLY INTO S/W COUNTIES AHEAD OF THE NEXT
SYSTEM BY EARLY FRIDAY MORNING...PERHAPS SUPPORTING STEADY OR SLOWLY
RISING TEMPERATURES OVERNIGHT. NEVERTHELESS...THE POTENTIAL FOR A
FREEZE APPEARS SUFFICIENT TO JUSTIFY A MENTION WITHIN THE HAZARDOUS
WEATHER OUTLOOK FOR COUNTIES WHERE THE GROWING SEASON HAS NOT YET
ENDED.
AS SURFACE HIGH PRESSURE REMAINS WEDGED OVER THE REGION...ISENTROPIC
ASCENT WILL SUPPORT AN EXPANDING AREA OF DEEP LAYER MOISTURE
FRIDAY...AND A CHILLY RAIN SHOULD OVERSPREAD THE REGION LATE FRIDAY
AND FRIDAY NIGHT...CONTINUING INTO SATURDAY AS A SURFACE LOW
PRESSURE TAKES SHAPE WITHIN THE BAROCLINIC ZONE FOCUSED JUST OFF THE
COAST. RAISED POPS TO LIKELY MOST AREAS FRIDAY NIGHT INTO
SATURDAY.
AT FIRST GLANCE...SURFACE TEMPERATURES IN THE MID-UPPER 30S FRIDAY
NIGHT MIGHT RAISE SOME PRECIPITATION TYPE CONCERNS...AND THIS
SCENARIO WARRANTS BRIEF CLARIFICATION. 12Z GUIDANCE DEPICTS STRONG
WARM ADVECTION JUST ABOVE THE SURFACE WEDGE OF COLD AIR...PUSHING
925-850 MB TEMPS INTO THE 8C-13C RANGE. THUS...EVEN IF MESOSCALE
ASCENT/PRECIP RATES BECOME ENHANCED BY STRONG/DEEP LAYER
FRONTOGENESIS AND ELEVATED DESTABILIZATION APPARENT WITHIN GFS CROSS
SECTIONS...COOLING THE ENVIRONMENT TO A GREATER EXTENT THAN
INDICATED WITHIN MODEL SOUNDINGS...THE DEGREE OF COOLING WILL REMAIN
INSUFFICIENT TO SUPPORT ANYTHING OTHER THAN LIQUID PRECIPITATION,
FURTHER...DUE TO THE ABSENCE OF ANY INFLUX OF COLD/DRY SURFACE
AIR...RAIN FALLING THROUGH THE ELEVATED WARM LAYER WILL MAINTAIN
TEMPERATURES ABOVE FREEZING AT THE SURFACE...NEGATING ANY THREAT
FOR FREEING RAIN.
AS LOW PRESSURE DEPARTS...HIGH PRESSURE WILL EXPAND OVER THE
REGION...TEMPORARILY ENDING THE THREAT FOR RAIN SUNDAY EXCEPT
PERHAPS ACROSS FAR SOUTHERN COUNTIES WHERE THE PRIMARY BAROCLINIC
ZONE/AXIS OF DEEPER MOISTURE COULD PERSIST. THEN...THE NEXT
SYSTEM...PERHAPS A STRONGER LOW PRESSURE SYSTEM...WILL TAKE SHAPE
OVER THE NORTHERN GULF OF MEXICO AND WILL SPREAD THE NEXT ROUND OF
OVERRUNNING RAIN INTO THE REGION MONDAY. THIS DEEPENING LOW WILL
FOLLOW AN UNCERTAIN TRACK TOWARD THE NORTHEAST TUESDAY...MAINTAINING
THE THREAT FOR RAIN/SHOWERS. IF THE LOW TRACKS INLAND AND ALLOWS THE
WARM SECTOR TO SURGE INTO THE REGION...THUNDERSTORMS COULD IMPACT
THE REGION AHEAD OF THE TRAILING COLD FRONT. HOWEVER...THERE REMAINS
FAR TOO MUCH FORECAST UNCERTAINTY TO JUSTIFY SUCH DETAILS AT THIS
EARLY STAGE.
&&
.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
EXPECT CONDITIONS TO CONTINUE TO DETERIORATE TO IFR LEVELS OR LOWER
THIS AFTERNOON AND EVENING FOR BOTH KCHS AND KSAV. INCLUDED A TEMPO
GROUP OF -SHRA THROUGH 22Z FOR BOTH SITES...AS PATCHY LIGHT RAIN
AFFECTS THE TERMINALS. EXPECT RAIN COVERAGE AND INTENSITY TO
INCREASE THIS EVENING...AS A LOW PRESSURE TO THE WEST AND A WARM
FRONT TO THE SOUTH APPROACHES THE AREA. THE MAJORITY OF THE
MOISTURE WILL CONTINUE TO PUSH NORTHWARD OVERNIGHT. THEREFORE ENDED
PRECIP EARLIER IN KSAV THAN KCHS.
MODEL SOUNDINGS CONTINUE TO SHOW 2K FT WINDS APPROACHING 50 KT
THIS EVENING AND INTO EARLY WEDNESDAY MORNING...THUS MAINTAINED
LLWS FOR BOTH SITES. EXPECT WINDS TO MIX DOWN TO THE SURFACE
AROUND 07Z AT KCHS...RESULTING IN GUSTY CONDITIONS THROUGH THE
WEDNESDAY MORNING HOURS. INCLUDED GUSTS UP TO 20 TO 25 KNOTS BY
13Z FOR BOTH SITES...WITH MVFR CEILINGS MORE LIKELY TO LINGER AT
KCHS. EXPECT BROKEN SKIES AT KSAV WEDNESDAY BUT CEILINGS TO REMAIN
JUST ABOVE MVFR LEVELS.
EXTENDED AVIATION OUTLOOK...VFR CONDITIONS LIKELY WEDNESDAY NIGHT
THROUGH THURSDAY NIGHT. PERIODS OF IFR OR LOWER CONDITIONS
POSSIBLE FRIDAY THROUGH SATURDAY.
&&
.MARINE...
LATEST COASTAL OBSERVATIONS INDICATE A GENERAL NORTHEAST FLOW OF
15 TO 20 KT. THE INLAND WEDGE IS CURRENTLY DIMINISHING AS A WARM
FRONT HAS DEVELOPED ACROSS NORTHERN FLORIDA. THE FRONT WILL LIFT
NORTH THROUGH THE AREA OVERNIGHT...ALLOWING NORTHEAST FLOW TO
STEADILY VEER TO THE SOUTH BY MIDNIGHT. WIND SPEEDS COULD DIMINISH
A BIT THIS EVENING...BEFORE SURGING FROM THE SOUTH WITHIN A TIGHTENED
PRESSURE GRADIENT BELOW THE ASCENDING FRONT AND A STRENGTHENING
LOW LEVEL JET.
EXPECT SMALL CRAFT ADVISORY CONDITIONS TO BEGIN ACROSS THE
OFFSHORE GEORGIA WATERS AND NEARSHORE SOUTH CAROLINA WATERS AFTER
MIDNIGHT...AND WILL THUS MAINTAIN THE SMALL CRAFT ADVISORY FOR
THOSE AREAS. GUSTY SOUTHERLY WINDS WILL CONTINUE ON
WEDNESDAY...ALLOWING THE HARBOR TO SURGE INTO SMALL CRAFT
ADVISORY CONDITIONS DURING THE DAYTIME HOURS.
SMALL CRAFT ADVISORY CONDITIONS SHOULD GRADUALLY WIND DOWN WEDNESDAY
NIGHT ACROSS NEARSHORE WATERS...THEN CONDITIONS SHOULD IMPROVE AT
LEAST TEMPORARILY ACROSS THE OUTER GA WATERS BY THURSDAY EVENING.
THEN...AS THE SURFACE PRESSURE GRADIENT TIGHTENS BETWEEN A
STATIONARY/WARM FRONT OVER OR JUST S/E OF THE WATERS AND AN INLAND
WEDGE OF HIGH PRESSURE...AT LEAST SMALL CRAFT ADVISORY WINDS WILL
LIKELY DEVELOP BY LATE FRIDAY AND WILL CONTINUE THROUGH SATURDAY AS
LOW PRESSURE DEVELOP ALONG THE SURFACE FRONT AND TRACKS NE. ELEVATED
WINDS/SEAS WILL THEN LIKELY PERSIST THROUGH SUNDAY AS WINDS SHIFT TO
THE N/NW IN THE WAKE OF THE DEPARTING LOW PRESSURE. WINDS/SEAS WILL
DEPEND GREATLY ON THE POSITION OF THE SURFACE BOUNDARY AND
TRACK/STRENGTH OF THE SURFACE LOW...WITH EVEN MINOR DEVIATIONS
TRANSLATING TO SIGNIFICANT CHANGES ACROSS MARINE ZONES...AND
FORECAST UNCERTAINTY REMAINS HIGH LATE THIS WEEK THROUGH THIS
WEEKEND.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...LAKE WIND ADVISORY FROM 6 AM WEDNESDAY TO MIDNIGHT EST
WEDNESDAY NIGHT FOR SCZ045.
MARINE...SMALL CRAFT ADVISORY FROM 6 AM TO 9 PM EST WEDNESDAY FOR
AMZ330.
SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO MIDNIGHT EST
WEDNESDAY NIGHT FOR AMZ352.
SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 5 AM EST
THURSDAY FOR AMZ350.
SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 7 PM EST
THURSDAY FOR AMZ374.
&&
$$
