Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 161726
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
126 PM EDT TUE SEP 16 2014

.SYNOPSIS...
A STRONG COLD FRONT WILL APPROACH FROM THE NORTHWEST TODAY AND
PUSH THROUGH THE REGION LATE WEDNESDAY. AN INLAND WEDGE OF HIGH
PRESSURE WILL THEN PREVAIL IN THE WAKE OF THE FRONT INTO THE
WEEKEND. ANOTHER COLD FRONT WILL APPROACH FROM THE WEST EARLY NEXT
WEEK.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A BROAD LARGE SCALE TROUGH WILL PREVAIL ACROSS THE EASTERN HALF OF
THE NATION TODAY AS SHORT WAVE ENERGY CONTINUES TO FEED INTO THE
TROUGH WITHIN THE WEST/NW FLOW ALOFT. OUR SURFACE PATTERN IS WEAK
BUT DEFINED BY DEVELOPING PRE-FRONTAL TROUGH ALIGNED NE-SW THAT
LOOKS TO STAY OFF TO OUR NW THROUGH DARK. THIS SCENARIO SETS THE
STAGE FOR MORE MORE FAIRLY ACTIVE CONVECTIVE AFTERNOON.

WATER VAPOR SATELLITE IMAGES SHOWING A COUPLE OF MID LEVEL
IMPULSES HEADING OUR WAY FROM UPSTREAM. ONE IS OVER SOUTHERN GA
AND THE SECOND IS A STRUNG OUT BUT PRONOUNCED VORT LOBE ENTERING
THE UPSTATE OF SC AND INTO NORTHERN GA. THESE FEATURES WILL IMPACT
THE AREA THIS AFTERNOON OR EVENING...AND THAT PLUS UPPER DIFLUENCE
ASSOCIATED WITH THE RIGHT REAR QUAD OF AN 250 MB JET WILL SUPPLY
THE NECESSARY FORCING FOR ASCENT. MEANWHILE THERE IS STILL AN
ABUNDANCE OF MOISTURE...UP ALMOST AT 2 STANDARD DEVIATIONS ABOVE
NORMAL. STEERING FLOW IS A LITTLE MORE NOTICEABLE THAN RECENT
DAYS...BUT STILL SLOW ENOUGH TO SUPPORT LIMITED MOVEMENT TO OUR
SHOWERS AND T-STORMS. PLUS THE CONVECTION WILL BE STEERED TOWARD
THE COAST WITH THE WEST/NW FLOW AND WILL BUMP UP AGAINST THE
PINNED SEA BREEZE /ONCE IT ACTUALLY DEVELOPS/...WHICH IMPLIES THAT A
PILING UP OF RAINFALL CAN OCCUR AS IT HAS NOWHERE FURTHER EAST OR
SE TO GO...STUCK AT THE SEA BREEZE BOUNDARY. WE ARE STILL HITTING
THE COASTAL ZONES OF SC AND AREAS SOUTH OF I-16 IN SE GA WITH THE
FLASH FLOOD WATCH...AND IT/S IN THESE AREAS THAT HAVE THE HIGHEST
POPS IN THE 60-70 PERCENT RANGE.

COVERAGE OF SHOWERS AND T-STORMS WILL CONTINUE TO INCREASE THIS
AFTERNOON OVER OUR NORTHERN AND EASTERN FIRST WHERE WE HAVE SEEN
SOME INSOLATION FROM EARLIER IN THE DAY. FURTHER SOUTH THE
INSOLATION HAS BEEN SLOWER IN ARRIVING AND CINH HAS BEEN HOLDING
ON...THUS DELAYING THE ONSET OF CONVECTION. BUT THAT AREA WILL
ALSO GET IN ON THE ACTION BY 3 OR 4 PM. RAINFALL IN OUR EASTERN
AND SOUTHERN ZONES WILL EXPERIENCE RAINFALL HEAVY AT TIMES. THE
INCREASE IN SHOWERS AND T-STORMS WILL BE THE RESULT OF THE
PERTURBATIONS ALOFT...THE PINNED SEA BREEZE...DIFFERENTIAL HEATING
BOUNDARIES FROM MORNING FOG/STRATUS AND OUTFLOW BOUNDARY
INTERACTIONS OCCUR. FURTHER WEST AND NW WE/LL RELY ON BOUNDARY
INTERACTIONS TO POP ANY CONVECTION...AND POPS ARE HELD TO 30-40
PERCENT DURING THE SECOND HALF OF THE AFTERNOON.

ALSO OF NOTE...THERE IS A TAD BIT DRIER AND COLDER AIR IN THE MID
LEVELS AND MORE MOVEMENT IN THE TROPOSPHERE THAN OF LATE.
MODIFIED 12Z SOUNDINGS AND RECENT SPC MESO-ANALYSIS HAS SOME
DECENT THERMODYNAMICS....PERHAPS SUPPORTIVE A FEW STRONGER OR
EVEN MARGINALLY SEVERE STORMS WHERE THE BOUNDARY
INTERACTIONS...MERGERS AND/OR COLLISIONS OCCUR. THIS IS A LOW END
PROBABILITY...BUT WORTHY OF TALKING ABOUT WITH FREQUENT LIGHTNING
AND STRONG DOWNBURSTS OF WIND POSSIBLE IN A COUPLE OF THE TALLER
STORMS.

DOWNSLOPE FLOW OFF THE MOUNTAINS AND WARM ADVECTION IN ADVANCE OF
THE SURFACE TROUGH AND UPSTREAM COLD FRONT WILL NEGATE THE EFFECTS
OF EXTENSIVE CLOUD COVER TO WARM TEMPS YET AGAIN ABOVE
CLIMATOLOGICAL NORMS. THERE IS EVEN SOME POCKETS OF DECENT
INSOLATION BETWEEN LARGE BREAKS IN THE CLOUD COVER THROWN INTO THE
MIX. WE MIGHT BE A LITTLE CONSERVATIVE ON ACTUAL HIGHS...BUT WE
HAVE TWEAKED TEMPS A DEGREE OR 2...MAINLY INTO THE UPPER 80S OR
NEAR 90 AWAY FROM THE IMMEDIATE COAST. PENDING TRENDS WE MIGHT
EVEN NEED TO GO A TAD HIGHER. KEEP IN MIND THAT ONCE CONVECTION
INITIATES AND COVERAGE SPREADS OUT THAT OUR DIURNAL TEMP CURVE
WILL NEED BIG ADJUSTMENTS. WE HAVE LEANED THIS WAY IN OUR HOURLY
TEMP FORECAST...BUT NO DOUBT FUTURE AMENDMENTS WILL BE REQUIRED.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...
TONIGHT...CONVECTION WILL QUICKLY WANE THIS EVENING WITH THE LOSS
OF INSOLATION. THE RISK FOR RAINFALL WILL GRADUALLY SHIFT OFFSHORE
CLOSER TO THE WESTERN WALL OF THE GULF STREAM AS A COLD FRONT
QUICKLY APPROACHES FROM THE NORTH. THE FRONT LOOKS TO REMAIN NORTH
AND NORTHWEST OF THE AREA THROUGH SUNRISE WEDNESDAY. SOME CLEARING
WILL LIKELY TAKE HOLD EARLY WEDNESDAY...ESPECIALLY ACROSS THE
WESTERN ZONES. WILL HAVE TO WATCH FOR POSSIBLE LOW STRATUS AND FOG
ONCE AGAIN GIVEN THE WET GROUNDS AND THE LIGHT WIND FIELDS THAT
WILL BE IN PLACE.

WEDNESDAY...THE SURFACE TROUGH WILL HAVE SHIFTED OFF THE SOUTHEAST
COASTLINE BY DAYBREAK...JUST DOWNSTREAM OF A SERIES OF SHORTWAVES
EMBEDDED WITHIN THE BASE OF A SHARPENING UPPER TROUGH AXIS.
PROGRESSION OF A STRONG COLD FRONT THROUGH THE FORECAST AREA
WILL LIKELY BE DELAYED UNTIL THE EVENING OR NIGHTTIME
PERIOD...WHEN A MORE DISTINCT AIR MASS CHANGE IS EVIDENT ALONG THE
LEADING EDGE OF STRONG HIGH PRESSURE BUILDING FROM THE
WEST/NORTHWEST. THE RIBBON OF DEEP LAYER MOISTURE RESPONSIBLE FOR
SEVERAL DAYS OF HEAVY RAIN EARLIER IN THE WEEK WILL HAVE MOVED TO
THE EAST/SOUTHEAST WITH THE SURFACE TROUGH. HOWEVER...THERE
APPEARS SUFFICIENT RESIDUAL MOISTURE TO SUPPORT SCATTERED
THUNDERSTORM DEVELOPMENT DURING THE AFTERNOON TIME
FRAME...ESPECIALLY AS SHORTWAVE ENERGY TRACKS THROUGH THE REGION
AND SEABREEZE DEVELOPMENT PROVIDES SOME FORCING/CONVERGENCE.
ISOLATED SHOWERS/THUNDERSTORMS COULD ACCOMPANY THE FRONT TOWARD
THE COASTLINE THROUGH MIDNIGHT...AND THEN EXPECT BEST RAIN CHANCES
TO SHIFT OVER THE MARINE ZONES DURING THE PRE-DAWN HOURS. HIGH
TEMPERATURES WILL ONCE AGAIN RISE ABOVE SEASONAL NORMALS WITHIN
DOWNSLOPE FLOW AND PRE-FRONTAL COMPRESSION...PEAKING IN THE UPPER
80S AWAY FROM THE LOCALLY COOLER COASTLINE. LOW TEMPERATURES WILL
STILL BE A DEGREE OR TWO ABOVE NORMAL...YET TREND COOLER THAN
PREVIOUS NIGHTS BY FALLING INTO THE MID TO UPPER 60S.

THURSDAY AND FRIDAY...NORTHEAST FLOW DEVELOPING IN THE WAKE OF THE
FRONT AND WITHIN BUILDING HIGH PRESSURE FROM THE NORTH WILL
SUPPORT A NOTABLE COOLING TREND LATE IN THE WEEK. HIGH
TEMPERATURES ON THURSDAY WILL ONLY PEAK IN THE LOW TO MID
80S...WHILE ONLY RISING TO THE LOWER 80S FOR FRIDAY AFTERNOON.
NIGHTTIME LOW TEMPERATURES WILL ALSO REFLECT THE AIR MASS
CHANGE...AS LOWER DEWPOINTS/DRIER AIR ALLOW TEMPERATURES TO FALL
INTO THE LOW TO MID 60S. AS THE CENTER OF THE SURFACE HIGH TRACKS
FROM THE GREAT LAKES REGION THURSDAY AND INTO THE NEW ENGLAND
STATES FRIDAY...THE INLAND WEDGE WILL AMPLIFY DOWN THE
APPALACHIANS AND A COASTAL TROUGH WILL TAKE SHAPE ON THE SOUTHERN
COASTLINE. THIS PATTERN COULD ALLOW A DISTINCT GRADIENT IN
PRECIPITATION POTENTIAL BETWEEN THE DRIER INLAND AREAS AND COASTAL
COUNTIES NEAR CONVERGENCE ALONG THE TROUGH. RAIN COVERAGE WILL
STILL BE CONSIDERABLY LOWER THAN EARLIER IN THE WEEK FOR ALL
LOCATIONS...AND WILL ADVERTISE JUST ISOLATED SHOWERS/THUNDERSTORMS
INLAND AND SCATTERED ALONG THE COAST...MAINLY DURING THE AFTERNOON
PERIOD.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
A WEDGE OF SURFACE HIGH PRESSURE INLAND AND A COASTAL TROUGH WILL
CONTROL THE PATTERN THROUGH SATURDAY...BEFORE BREAKING DOWN SUNDAY
AHEAD OF ANOTHER STRONG COLD FRONT APPROACHING EARLY NEXT WEEK.
EXPECT BEST RAIN CHANCES TO BE FOUND ALONG THE COAST AND OVER THE
MARINE ZONES THROUGH THE WEEKEND...AS THESE LOCATIONS WILL BE
CLOSEST TO MOISTURE AND CONVERGENCE ASSOCIATED WITH THE COASTAL
TROUGH. HIGH TEMPERATURES WILL ONCE AGAIN BE SUPPRESSED IN THE
LOWER 80S FOR SATURDAY AFTERNOON...BEFORE CONDITIONS BEGIN TO
MODERATE SUNDAY AS THE INLAND WEDGE PATTERN WEAKENS. MEDIUM RANGE
MODELS ARE SUGGESTING THAT THE NEXT COLD FRONT COULD MOVE THROUGH
THE REGION BY TUESDAY...YET ARE NOT INDICATING A SUBSTANTIAL
AMOUNT OF PRECIPITATION ASSOCIATED WITH ITS PROGRESSION. WILL THUS
REMAIN RATHER CONSERVATIVE WITH RAIN CHANCES TOWARD THE END OF THE
FORECAST PERIOD...CAPPING POPS IN THE SLIGHT CHANCE TO LOW CHANCE
RANGE FOR MONDAY AND TUESDAY.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
COVERAGE OF SHRA AND TSRA WILL BE ON THE RISE DURING THE MID AND
LATE AFTERNOON...ALLOWING FOR POSSIBLE DIRECT IMPACTS AT BOTH KCHS
AND KSAV. MOST LIKELY TIME FRAME IS FROM 20-23Z AT KCHS AND FROM
21-24Z AT KSAV. MVFR OR LOWER CONDITIONS ARE POSSIBLE AS A
RESULT...WITH HEAVY RAINS AND GUSTY WINDS A CONCERN.

ACTIVITY WILL WANE THIS EVENING...AND GIVEN WET GROUNDS...LITTLE
TO NO WINDS AND A DECENT NOCTURNAL INVERSION TO FORM WE LOOK FOR
SOME LOW STRATUS AND/OR FOG LATE TONIGHT INTO WEDNESDAY MORNING.
MVFR OR LOWER CONDITIONS ARE AGAIN A POSSIBILITY...BEFORE VFR
CONDITIONS RETURN AFTER 13-14Z WEDNESDAY.

EXTENDED AVIATION OUTLOOK...PERIODIC FLIGHT RESTRICTIONS POSSIBLE
IN OCCASIONAL SHRA AND/OR TSRA DURING THE AFTERNOON AND EARLY
NIGHT TIME PERIODS...AND IN LATE NIGHT/MORNING LOW STRATUS AND/OR
FOG.

&&

.MARINE...
TODAY...A FAIRLY NEBULOUS PRESSURE PATTERN SETS UP UNDERNEATH
WEAK HIGH PRESSURE AND IN ADVANCE OF OUR NEXT UPSTREAM COLD FRONT.
THIS WILL ALLOW FOR LIGHT WEST/NW TO GIVE WAY TO A RESULTANT
SOUTH/SW SEA BREEZE CIRCULATION DURING THE MID AND LATE AFTERNOON.
ALTHOUGH AT TIMES THE WIND FIELDS WILL BECOME SOMEWHAT MUDDLED IN
CHARLESTON HARBOR AND THE NEARBY COASTAL WATERS DUE TO CONVECTION.
WE/RE STARTING TO SEE A LITTLE SWELL ENERGY FROM DISTANT HURRICANE
EDOUARD APPROACHING OUR WATERS FROM THE EAST...BUT AVERAGE WAVE
HEIGHTS WILL BE HELD TO AROUND 2 FT THROUGHOUT THE DAY. MARINERS
ARE ALERTED TO A FEW STRONGER STORMS WHICH CAN PRODUCE FREQUENT
LIGHTNING...HEAVY RAINS AND GUSTY WINDS.

TONIGHT...LONG PERIOD SWELLS FROM HURRICANE EDOUARD WILL BEGIN TO
AFFECT THE WATERS WITH PERIODS OF 11-14 SECONDS. SEAS OF 1-2 FT
WILL BUILD TO 2-4 FT NEARSHORE WATERS AND 4-5 FT OFFSHORE WATERS.
ATTM IT APPEARS SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY
THRESHOLDS...ALTHOUGH IT WILL BE CLOSE FOR THE GEORGIA OFFSHORE
WATERS.

WEDNESDAY AND THURSDAY...A COLD FRONT WILL PROGRESS INTO AND
THROUGH THE WATERS DURING THE MID WEEK PERIOD. LIGHT WINDS IN
ADVANCE OF THE FRONT WEDNESDAY WILL BECOME NORTH/NORTHEAST AND
INCREASE IN ITS WAKE BY THURSDAY. SEAS WILL BE COMPRISED MAINLY OF
LONG PERIOD SWELLS FROM DISTANT TROPICAL CYCLONE EDOUARD...AROUND
2 TO 4 FT WITHIN 20 NM AND 3 TO 5 FT FARTHER OFFSHORE.

FRIDAY THROUGH SUNDAY...THE PRESSURE GRADIENT WILL BECOME QUITE
PINCHED AS THE COLD FRONT STALLS FAR OFFSHORE AND REMAINS FAIRLY
STATIONARY ALONG THE PERIPHERY OF AN INLAND WEDGE OF HIGH
PRESSURE. SMALL CRAFT ADVISORY CONDITIONS ARE POSSIBLE FOR
PORTIONS OF THE MARINE ZONES FRIDAY AND SATURDAY...BEFORE
CONDITIONS MODERATE WITHIN A TRANSITIONING PATTERN BY SUNDAY.

RIP CURRENTS...LONG PERIOD SWELL FROM HURRICANE EDOUARD WILL
ARRIVE AT THE BEACHES LATE THIS AFTERNOON AND CONTINUE INTO
THURSDAY. THERE DOES NOT APPEAR TO BE MUCH MORE THAN ABOUT 1 OR 2 FT
SWELL THAT MAKES IT TO THE BEACHES BY LATE AFTERNOON...SO THE RIP
CURRENT RISK WILL REMAIN IN THE LOW CATEGORY FOR TODAY. LARGER
SWELLS AROUND 3 FT WILL OCCUR WEDNESDAY INTO THURSDAY AND THIS
WILL LEAD TO AN ENHANCED RISK OF RIP CURRENTS. FOR FRIDAY THROUGH
SUNDAY SWELL FROM EDOUARD WILL DIMINISH...BUT STRONGER NE WINDS
WILL OCCUR AND THIS MAY ALLOW FOR AN ELEVATED RISK OF RIP CURRENTS
TO AGAIN PREVAIL AT AREA BEACHES.

&&

.HYDROLOGY...
ONE MORE DAY FOR HEAVY RAIN AND POSSIBLE FLASH FLOODING IN PARTS
OF THE AREA. HIGH RESOLUTION MODELS ARE SHOWING A DISTINCT SIGNAL
THAT AN AREA OF HEAVY RAIN WILL SETUP IN THE AREA FROM SAVANNAH
SOUTH TO DARIEN AND OVER TO SPRINGFIELD AND GLENNVILLE FOR SEVERAL
HOURS THIS AFTERNOON ALONG A PINNED SEA BREEZE CIRCULATION.
ALTHOUGH STORM MOTIONS WILL BE FASTER THAN PREVIOUS DAYS...THE
POTENTIAL FOR NUMEROUS MESOSCALE BOUNDARY INTERACTIONS...STRONG
UPPER DIVERGENCE AND HIGH PWAT VALUES APPROACHING 2 STANDARD
DEVIATIONS ABOVE NORMAL FOR MID SEPTEMBER SUGGEST THERE IS A HIGH
POTENTIAL FOR EXCESSIVE HOURLY RAINFALL RATES EXCEEDING 2-3
IN/HR...WHICH IS NEAR OR HIGHER THAN THE LATEST 1-HR FLASH FLOOD
GUIDANCE. IN ADDITION...SOME AREAS HAVE ALREADY SEEN IN EXCESS OF
6 INCHES OVER THE PAST FEW DAYS.

A FLASH FLOOD WATCH HAS BEEN MAINTAINED FOR AREAS SOUTH OF I-16
IN SOUTHEAST GEORGIA AND THE COASTAL ZONES OF SOUTHERN SOUTH
CAROLINA THROUGH 8 PM EDT...AS THESE AREAS HAVE THE HIGHEST
POTENTIAL FOR FLASH FLOODING TODAY. THE COASTAL SOUTH CAROLINA
ZONES HAVE BEEN INCLUDED IN THE WATCH MAINLY TO ACCOUNT FOR
POSSIBLE HEAVY RAIN FALLING AROUND HIGH TIDE THIS AFTERNOON.
SIMILAR TO THE PAST FEW DAYS...ISOLATED RAIN AMOUNTS IN EXCESS OF
4 OR EVEN 5 INCHES WILL BE POSSIBLE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...FLASH FLOOD WATCH UNTIL 8 PM EDT THIS EVENING FOR GAZ114>119-
     137>141.
SC...FLASH FLOOD WATCH UNTIL 8 PM EDT THIS EVENING FOR SCZ048>052.
MARINE...NONE.

&&

$$

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
HYDROLOGY...





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