Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 162115 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 515 PM EDT TUE SEP 16 2014 .SYNOPSIS... A TROUGH WILL MOVE INTO AND THROUGH THE AREA TONIGHT....FOLLOWED BY A STRONG COLD FRONT WILL APPROACH FROM THE NORTHWEST LATE TONIGHT AND PUSHES 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 AM WEDNESDAY MORNING/... THROUGH THIS EVENING...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 INTO THIS 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...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 THROUGH 8 PM...AND IT/S IN THESE AREAS THAT HAVE THE HIGHEST POPS IN THE 60-80 PERCENT RANGE. 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. FOR TONIGHT...CONVECTION WILL BEGIN TO WANE DURING THE LATE EVENING AS CINH REDEVELOPS AND THE TYPICAL NOCTURNAL COOLING/STABILIZATION TAKES PLACE. HOWEVER...GIVEN NUMEROUS BOUNDARY INTERACTIONS THAT WILL CONTINUE WE HAVE HELD ONTO ISOLATED POPS FAR INLAND AND SCATTERED POPS FURTHER SOUTH AND EAST THROUGH AT LEAST 03-05Z. THE HIGH RESOLUTION MODEL GUIDANCE IS INDICATING THAT CONVECTION WILL ATTEMPT TO MERGE FROM NEAR BEAUFORT COUNTY SOUTH TO THE ALTAMAHA LATE THIS EVENING AND WE MAY HAVE TO EXTEND THE FLASH FLOOD WATCH FOR THESE COUNTIES IF THIS COMES TO FRUITION. BY DAYBREAK WEDNESDAY WE LOOK FOR ALL CONVECTION TO HAVE SHIFTED OFFSHORE. OF ADDITIONAL CONCERN TONIGHT IS THE POTENTIAL FOR A RETURN OF FOG AND LOW STRATUS WITH A FAVORABLE 1-2C INVERSION TO SET UP OFF THE DECK. GIVEN THE COPIOUS AMOUNTS OF RAINFALL IN RECENT DAYS AND THAT FROM EARLIER TODAY...WITH LITTLE TO NO WINDS AND A SMIDGEN OF DRIER AIR ALOFT SHIFTING IN...CONDITIONS AGAIN APPEAR FAVORABLE FOR AREAS OF FOG TO OCCUR AFTER 2 OR 3 AM...EVEN DOWN TO THE COAST WITH AN OFFSHORE FLOW DEVELOPING AND CONDENSATION PRESSURE DEFICITS FALL TO UNDER 10 OR 20 MB. OUTSIDE RISK OF DENSE FOG IN LOCATIONS WHERE THE HEAVIEST RAINS HAVE OCCURRED. THE TEMP CURVE HAS BECOME INTERRUPTED IN AND NEAR CONVECTION AND WE HAVE ATTEMPTED OUR BEST TO SHOW THIS IN THE HOURLY GRIDS. AS COVERAGE/INTENSITY OF CONVECTION WINDS DOWN LATER THIS EVENING THE TREND WILL BE FOR A SLOW BUT STEADY FALL TO TEMPS. DUE TO THE PASSAGE OF A WEAK SURFACE TROUGH OVERNIGHT...A TAD COOLER AIR WILL FILTER IN AS THE PLANETARY BOUNDARY LAYER FLOW VEERS AROUND TO THE NW AND NORTH. THUS WE SHOULD BE ABLE TO ACHIEVE UPPER 60S INTO OUR NW HALF OF THE CWFA BY MORNING...WITH LOWER 70S CLOSER TO THE COAST AND A FEW MID 70S ON THE BARRIER ISLANDS. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/... WEDNESDAY...A MID LVL TROUGH OF LOW PRESSURE WILL BEGIN SLIDING OFF THE EASTERN SEABOARD BY DAYBREAK...SPAWNING AN AREA OF LOW PRESSURE ALONG AN EAST-WEST EXTENDING SFC COLD FRONT THAT GRADUALLY SHIFTS THROUGH THE ENTIRE AREA BY WEDNESDAY EVENING. THE FRONT WILL LIKELY SLOW IN PROGRESSION WHILE ENCOUNTERING AN AFTERNOON SEABREEZE IN SOUTHEAST GEORGIA...BEFORE SHIFTING FURTHER SOUTH AND/OR OFFSHORE THROUGH WEDNESDAY EVENING. AMPLE MOISTURE ALONG AND AHEAD OF THE FRONT SHOULD SUPPORT SCATTERED SHOWERS AND THUNDERSTORMS THROUGH WEDNESDAY AFTERNOON...WITH BEST CHANCES OCCURRING WHERE THE FRONT AND SEABREEZE COLLIDE. A FEW SHOWERS AND/OR ISOLATED THUNDERSTORM WILL REMAIN POSSIBLE INTO LATE EVENING AND EARLY OVERNIGHT HOURS AS SOME MOISTURE CONTINUES TO LINGER BEHIND THE FRONT. ACTIVITY SHOULD THEN DIMINISH THEREAFTER AS DRYER AIR ASSOCIATED WITH HIGH PRESSURE BUILDS FROM THE NORTH/NORTHWEST. LATEST 1000-850 THICKNESS VALUES SUPPORT ALONG WITH A DOWNSLOPING WIND COMPONENT OFF THE SFC SUPPORT OVERALL HIGH TEMPS IN THE MID/UPPER 80S WEDNESDAY AFTERNOON. OVERNIGHT LOW TEMPS SHOULD BE SLIGHTLY COOLER THAN PREVIOUS DAYS AS HIGH PRESSURE BUILDS FROM THE NORTH LATE...IN THE MID 60S AWAY FROM THE COAST. THURSDAY AND FRIDAY...HIGH PRESSURE WILL BUILD FROM THE NORTH AND NORTHWEST THROUGH EARLY THIS WEEKEND...RESULTING IN DRYER CONDITIONS ON THURSDAY AS THE FLOW REMAINS MAINLY OFFSHORE. WE COULD SEE AN ISOLATED SHOWER AND/OR THUNDERSTORMS DEVELOP ALONG THE COAST LATE AS SFC FLOW BECOMES MORE NORTH/NORTHEAST IN RESPONSE TO THE CENTER OF HIGH PRESSURE SHIFTING OVER THE NORTHEASTERN UNITED STATES. HOWEVER...ACTIVITY WILL LIKELY REMAIN CONFINED TO COASTAL LOCATIONS. BY FRIDAY...A STRONGER NORTHEAST FLOW WILL SET UP ALONG THE SOUTHEAST COAST AS HIGH PRESSURE BECOMES CENTERED OFF THE NORTHEAST COAST. THIS SHOULD SUPPORT SLIGHTLY HIGHER CHANCES OF PRECIP AS MOISTURE BEGINS TO RETURN OFF THE ATLANTIC. EXPECT MOST ACTIVITY TO REMAIN ALONG THE COAST INITIALLY BEFORE GRADUALLY SHIFTING INLAND WITH AN AFTERNOON SEABREEZE. HAVE MAINTAINED LOW-END CHANCE POPS FRIDAY ALONG THE COAST AND SLIGHT CHANCE POPS FURTHER INLAND WITH THE AFTERNOON SEABREEZE. OVERALL HIGH TEMPS WILL BE COOLER AS HIGH PRESSURE EXTENDS OVER THE AREA FROM THE NORTH. EXPECT AFTERNOON HIGHS IN THE LOW/MID 80S ON THURSDAY AND LOW 80S ON FRIDAY. THURSDAY NIGHT TEMPS WILL ALSO REFLECT COOLER CONDITIONS...WITH LOWS IN THE LOW/MID 60S AWAY FROM THE COAST. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... 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 /21Z TUESDAY THROUGH SUNDAY/... SCATTERED TO NUMEROUS SHRA AND TSRA WILL CONTINUE INTO THIS EVENING...ALLOWING FOR DIRECT IMPACTS AT BOTH KCHS AND KSAV. MVFR OR LOWER CONDITIONS ARE LIKELY AS A RESULT...WITH HEAVY RAINS AND GUSTY WINDS ALSO A CONCERN. ACTIVITY WILL WANE LATE 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...OUTSIDE OF SHRA AND/OR TSRA...FLIGHT RESTRICTIONS WILL BE POSSIBLE EACH MORNING WITH FOG AND/OR LOW STRATUS SHIFTING ONSHORE. && .MARINE... TONIGHT...WE HAVE SEEN A LITTLE SWELL ENERGY FROM DISTANT MAJOR HURRICANE EDOUARD TRICKLING INTO OUR COASTAL WATERS...AND ADDITIONAL SWELL ENERGY IS TO ARRIVE AS TIME PROGRESSES. THE BULK OF THE WAVE SPECTRUM WILL BE COMPRISED OF THESE CONDITIONS...WHICH AVERAGES 2 OR 3 FT EARLY TONIGHT...BUILDING ANOTHER FOOT LATE...EXCEPT CLIMBING UP TO SOME 5 FOOTERS ON THE OUTER GA WATERS BY SUNRISE WEDNESDAY. MEANWHILE...WITH A WEAK PRESSURE PATTERN OUR WIND FIELDS WILL BE DRIVEN BY A LIGHT SE BREEZE CIRCULATION AND SCATTERED TO NUMEROUS CONVECTION INTO THIS EVENING. THEN AS THIS ACTIVITY STARTS TO FADE DURING THE LATE EVENING AND OVERNIGHT...AND A WEAK SURFACE TROUGH DROPS THROUGH FROM THE NORTH/NW...LAND BREEZE CIRCULATIONS WILL FORM. THIS ALLOWS FOR VEERING WINDS TO THE SW AND WEST BY MIDNIGHT...THEN AROUND TO THE NW OR NORTH LATE. OUTSIDE OF THE STRONGER CONVECTION WINDS WILL OBTAIN NO MORE THAN ABOUT 10 KT. HOWEVER...MARINERS ARE ADVISED THAT THE STRONGER STORMS EARLY TONIGHT WILL PRODUCE GUSTY WINDS UP NEAR 30 OR 35 KT...HEAVY RAINS LOWERING VISIBILITIES TO UNDER 1 NM AND FREQUENT LIGHTNING STRIKES. BE SAFE ON THE COASTAL WATERS. WEDNESDAY THROUGH SUNDAY...LIGHT WINDS WILL INCREASE AND BECOME NORTH/NORTHEAST BEHIND A COLD FRONT THAT SHIFTS OVER THE WATERS WEDNESDAY THROUGH 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. A STRONGER NORTHEAST FLOW WILL THEN DEVELOP OVER THE WATERS ON FRIDAY AND PERSIST INTO SATURDAY AS THE PRESSURE GRADIENT BECOMES ENHANCED ALONG THE SOUTHERN EDGE OF HIGH PRESSURE CENTERED ALONG THE NORTHEAST COAST. HOWEVER...WINDS/SEAS ARE EXPECTED TO REMAIN JUST BELOW SMALL CRAFT ADVISORY CRITERIA AT THIS TIME. CONDITIONS SHOULD MODERATE OVER THE WATERS WITHIN A TRANSITIONING PATTERN BY SUNDAY. RIP CURRENTS...LONG PERIOD SWELL FROM HURRICANE EDOUARD WILL OCCUR ALONG THE BEACHES WEDNESDAY INTO THURSDAY. DESPITE ONLY LIGHT ONSHORE WINDS...2-3 FT SWELLS WITH PERIODS NEAR 14 SECONDS SUPPORT AN ENHANCED PERIOD OF RIP CURRENTS ALONG THE SOUTH CAROLINA AND GEORGIA BEACHES. A MODERATE RISK OF RIP CURRENTS IS THEREFORE EXPECTED ALONG ALL BEACHES ON WEDNESDAY...AND POSSIBLY THURSDAY. && .HYDROLOGY... 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 AS THE CONVECTION RUNS INTO THE SEA BREEZE BOUNDARY THAT IS STRUGGLING TO MOVE INLAND. WITH FAVORABLE UPPER JET DYNAMICS AND FORCING FROM SHORT WAVES...ABUNDANT PWATS APPROACHING 2 STANDARD DEVIATIONS ABOVE NORMAL FOR MID-SEPTEMBER AND STORM MOTIONS LESS THAN 10 KT WE LOOK FOR HOURLY RAINFALL RATES OF 2-3 INCHES...AND 6 HOUR AMOUNTS OF AS MUCH AS 4 OR 5 INCHES. THESE ARE NEAR OR GREATER THAN THE FLASH FLOOD GUIDANCE FROM THE SERFC...SO NO REASON TO DEVIATE FROM THE EARLIER ISSUANCE OF THE FLASH FLOOD WATCH. && .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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