025  
FXUS62 KCHS 031534  
AFDCHS  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHARLESTON SC  
1134 AM EDT MON AUG 3 2020  
   
SYNOPSIS
 
 
ISAIAS WILL BRING SIGNIFICANT IMPACTS TO OUR AREA THROUGH  
TONIGHT. AFTER THE STORM MOVES AWAY, A STATIONARY FRONT WILL  
THEN HOVER OVER OR NEAR THE AREA MID TO LATE IN THE WEEK.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
LATE THIS MORNING: THE CIRCULATION OF TROPICAL STORM ISAIAS  
CONTINUES TO MOVE NORTHWARD TOWARD THE SOUTHEAST SOUTH CAROLINA  
COAST AND THE PRECIPITATION SHIELD ASSOCIATED WITH IT IS ALSO  
EXPANDING NORTHWARD. THE OFFICIAL NHC TRACK FORECAST HAS TRENDED  
A BIT TO THE EAST IN THE LAST FEW ADVISORIES, AS WELL AS A BIT  
FASTER. THIS WILL HAVE NOTABLE IMPLICATIONS FOR RAINFALL AND  
STORM SURGE RELATED IMPACTS. OVERALL THE WIND FORECAST HAS NOT  
CHANGED SIGNIFICANTLY AS THE SLIGHTLY MORE OFFSHORE FORECAST IS  
BALANCED A BIT BY THE FORECAST STRENGTHENING TO A HURRICANE IN  
THE NEXT 6-12 HOURS. STRONGEST WINDS ARE STILL EXPECTED TO  
IMPACT THE AREA FROM BEAUFORT COUNTY UP THROUGH THE TRI-COUNTY  
REGION, PRIMARILY IN LOCATIONS CLOSER TO THE COAST.  
 
RAINFALL REMAINS A RATHER CHALLENGING FORECAST. THERE WILL BE A  
VERY TIGHT GRADIENT ON THE WEST SIDE OF THE PRECIPITATION  
SHIELD, MEANING THAT OVER A SHORT DISTANCE WE WILL GO FROM  
RATHER LIGHT RAINFALL AMOUNTS TO RATHER SIGNIFICANT RAINFALL  
AMOUNTS THAT COULD RESULT IN FLOODING IMPACTS. BASED ON THE  
SUITE OF MODELS AND THE OFFICIAL TRACK FORECAST, WE STILL LOOK  
ON TRACK TO HAVE THE HIGHEST AMOUNTS OCCUR ACROSS THE CHARLESTON  
TRI-COUNTY REGION. EXPECTED AMOUNTS HAVEN'T CHANGED AT THIS  
POINT, WITH 3-6 INCHES WITH LOCALLY HIGHER AMOUNTS WHERE THE  
FLASH FLOOD WATCH IS IN PLACE. WEST OF THIS (AND GENERALLY WEST  
OF THE I-95 CORRIDOR) AMOUNTS ARE STILL MUCH LESS, WITH MUCH OF  
SOUTHEAST GEORGIA WELL LESS THAN AN INCH. THE FLASH FLOOD WATCH  
STILL HIGHLIGHTS THE AREA OF HEAVIEST RAIN, AND REMAINS IN  
PLACE. THE LATE AFTERNOON INTO THE EVENING TIME PERIOD IS STILL  
THE PERIOD OF GREATEST CONCERN. ONE BIT OF GOOD NEWS IS THAT  
BASED ON THE CURRENT FORWARD SPEED OF THE STORM, WE MIGHT AVOID  
SEEING THE PEAK SURGE PERFECTLY COINCIDE WITH HIGH TIDE. THIS  
WOULD LESSEN THE PEAK MAGNITUDE OF POTENTIAL FLOODING, BUT TIDE  
LEVELS WILL STILL BE ELEVATED WITH HEAVY RAIN SO AT LEAST SOME  
FLOODING IS EXPECTED IN AND AROUND DOWNTOWN CHARLESTON.  
 
WE CONTINUE TO MONITOR THE TORNADO THREAT, BUT IT REMAINS  
LIMITED TO COASTAL AREAS IN AND AROUND THE TRI-COUNTY REGION.  
THE LACK OF INSTABILITY AND SLIGHTLY MORE EASTWARD TRACK SHOULD  
KEEP THE MAIN THREAT ZONE OVER THE COASTAL WATERS, BUT WE COULD  
STILL SEE ROTATING STORMS APPROACH AND PERHAPS MOVE INTO THE  
COAST.  
 
CONDITIONS WILL QUICKLY IMPROVE FROM SOUTH TO NORTH TONIGHT AS  
ISAIAS MOVES AWAY TO THE NORTH WITH THE RAIN ENDING AND WIND  
SUBSIDING. LOWS TONIGHT SHOULD MAINLY BE IN THE LOWER TO MID  
70S.  
 

 
   
SHORT TERM /TUESDAY THROUGH THURSDAY/
 
 
TUESDAY: ISAIAS WILL BE FAR TO THE NORTH OF OUR AREA IN THE  
MORNING AND CONTINUING TO QUICKLY MOVE AWAY AS IT GETS ABSORBED  
INTO A MID- LEVEL TROUGH ALONG THE EAST COAST. MEANWHILE,  
RELATIVELY DRIER AIR BE IN PLACE ACROSS OUR AREA IN THE MORNING.  
THOUGH, A COLD FRONT SHOULD APPROACH OUR AREA FROM THE WEST IN  
THE AFTERNOON, THEN TRANSITION TO A STATIONARY FRONT DURING THE  
EVENING AND OVERNIGHT HOURS. MODELS INDICATE SOME SEA BREEZE  
CONVECTION FORMING IN THE AFTERNOON, WITH THE HIGHEST  
PROBABILITIES ACROSS OUR GA COUNTIES. THERE DOESN'T APPEAR TO BE  
MUCH INSTABILITY IN PLACE, SO THE SEVERE RISK IS LOW. THE  
CONVECTION SHOULD DISSIPATE IN THE EVENING, THEN TRANSITION TO  
THE COASTAL WATERS OVERNIGHT. AFTERNOON TEMPERATURES SHOULD PEAK  
IN THE 90S WHILE OVERNIGHT LOWS WILL GENERALLY BE IN THE 70S.  
 
WEDNESDAY AND THURSDAY: A MID-LEVEL TROUGH WILL STRETCH FROM  
THE GREAT LAKES REGION DOWN INTO THE DEEP SOUTH. MEANWHILE, A  
STATIONARY FRONT WILL GENERALLY BE JUST WEST OF OUR AREA. MODELS  
ARE IN GOOD AGREEMENT SHOWING A RETURN TO OUR TYPICAL  
SUMMERTIME PATTERN. HIGH TEMPERATURES SHOULD BE IN THE LOW TO  
MID 90S. INSTABILITY AND LIFT FROM THE AFTERNOON SEA BREEZE ARE  
EXPECTED TO FIRE OFF SHOWERS/THUNDERSTORMS. THOUGH, THE DETAILS  
WILL NEED TO BE REFINED AS THIS TIME PERIOD GETS CLOSER. SOME  
CONVECTION COULD PERSIST INTO THE OVERNIGHT HOURS. LOWS ARE  
FORECASTED TO BE IN THE 70S.  
 

 
   
LONG TERM /THURSDAY NIGHT THROUGH MONDAY/
 
 
A MID-LEVEL TROUGH WILL PERSIST OVER THE EAST COAST WHILE A  
STATIONARY FRONT HOVERS OVER OR NEAR AREA. THE TYPICAL AFTERNOON  
AND EVENING SHOWERS AND THUNDERSTORMS ARE EXPECTED.  
TEMPERATURES SHOULD BE NEAR NORMAL.  
 

 
   
AVIATION /15Z MONDAY THROUGH FRIDAY/
 
 
CONDITIONS ARE EXPECTED TO CONTINUE TO DETERIORATE TO AT LEAST  
MVFR BUT LIKELY IFR OR WORSE AT TIMES AS BANDS OF SQUALLY  
SHOWERS AND ISOLATED STORMS SHIFT WEST THROUGH THE AREA AHEAD OF  
ISAIAS. WINDS WILL ALSO STEADILY INCREASE EVEN OUTSIDE OF THE  
RAIN AS ISAIAS NEARS THE AREA FROM THE SOUTHEAST WITH GUSTS UP  
TO 20-25 KT AT KSAV THROUGH ABOUT 00Z TUESDAY AND GUSTS TO  
AROUND 40 KT AT KCHS FROM AROUND 21Z MONDAY THROUGH ABOUT 03Z  
TUESDAY. STRONGER WINDS GUSTS WILL BE POSSIBLE HOWEVER IN THE  
HEAVIEST SHOWERS/STORMS. CONDITIONS COULD IMPROVE TO VFR LATE IN  
THE 06Z TAF PERIOD AT KSAV.  
 
EXTENDED AVIATION OUTLOOK: BRIEF FLIGHT RESTRICTIONS ARE  
POSSIBLE WITHIN SHOWERS/THUNDERSTORMS, MAINLY EACH AFTERNOON  
THROUGH FRIDAY.  
 

 
   
MARINE
 
 
TODAY AND TONIGHT: TROPICAL STORM ISAIAS WILL MOVE NORTH  
THROUGH THE LOCAL WATERS BRINGING DANGEROUS MARINE CONDITIONS,  
WHILE POSSIBLY STRENGTHENING TO A MINIMAL HURRICANE. TROPICAL  
STORM WARNINGS REMAIN IN EFFECT FOR THE ENTIRE AREA INCLUDING  
THE CHARLESTON HARBOR, ALTHOUGH HURRICANE WARNINGS COULD BE  
NEEDED, ESPECIALLY FOR THE OFFSHORE GA WATERS AND CHARLESTON  
COUNTY NEARSHORE WATERS. CONDITIONS WILL ALSO BE FAVORABLE FOR  
WATERSPOUTS ESPECIALLY FROM LATE MORNING THROUGH EARLY EVENING.  
EXPECT QUICKLY IMPROVING CONDITIONS FROM SOUTH TO NORTH TONIGHT  
AS ISAIAS MOVES AWAY TO THE NORTH. WE COULD NEED SMALL CRAFT  
ADVISORIES IN THE IMMEDIATE WAKE TO AT LEAST ACCOUNT FOR  
RESIDUAL ELEVATED SEAS.  
 
TUESDAY THROUGH SATURDAY: ANY MARINE HEADLINES IN EFFECT  
TUESDAY MORNING SHOULD BE DUE TO ELEVATED SEAS FROM THE STORM.  
THOUGH, THEY WILL QUICKLY SUBSIDE INTO TUESDAY AFTERNOON AS THE  
STORM MOVES AWAY. A STATIONARY FRONT IS THEN FORECASTED TO STAY  
ALONG THE SOUTHEAST COAST MID TO LATE THIS WEEK, LEADING TO A  
RETURN OF THE TYPICAL SUMMER CONDITIONS. WINDS AND SEAS ARE  
EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY CRITERIA DURING  
THIS TIME PERIOD.  
 
RIP CURRENTS: DANGEROUS SURF CONDITIONS ARE EXPECTED THROUGH  
TONIGHT AS ISAIAS MOVES NORTH JUST OFF THE SOUTHEAST GEORGIA AND  
SOUTHEAST SOUTH CAROLINA COASTS. NOBODY SHOULD ENTER THE WATER  
DURING THIS TIME.  
 
ON TUESDAY, REMNANT SWELL FROM THE DEPARTING STORM WILL BE  
MOVING THROUGH THE COASTAL WATERS. THE COMBINATION OF THIS  
SWELL, WINDS GRADUALLY TURNING ONSHORE, AND NEW COASTAL  
BATHYMETRY IN SOME AREAS POST STORM WILL MAKE IT EASY FOR RIP  
CURRENTS TO FORM. A MODERATE RISK IS NOW IN EFFECT.  
 

 
   
HYDROLOGY
 
 
PERIODS OF HEAVY RAINFALL ARE EXPECTED TODAY AS OUTER BANDS OF  
ISAIAS MOVE INLAND ACROSS THE AREA. MORE STEADY HEAVIER RAIN IS  
THEN LIKELY TO PUSH INTO THE SC COAST LATER TODAY INTO THIS  
EVENING AND THE HIGHEST TOTAL RAINFALL AMOUNTS ARE EXPECTED  
GENERALLY AROUND THE CHARLESTON METRO WHERE 6 OR MORE INCHES  
COULD FALL. OUR BIGGEST CONCERN IS THE THREAT FOR HEAVY RAIN  
DURING THE EVENING HIGH TIDE WHICH COULD CAUSE SIGNIFICANT  
FLOODING IN AREAS LIKE DOWNTOWN CHARLESTON DUE TO THE  
COMBINATION OF FRESHWATER AND TIDAL FLOODING.  
 

 
   
TIDES/COASTAL FLOODING
 
 
WITH A FORECAST OF STORM SURGE INUNDATION IN THE 2-4 FT RANGE,  
A STORM SURGE WARNING REMAINS UP FROM EDISTO BEACH NORTH THROUGH  
THE SOUTH SANTEE RIVER. THE BEST CHANCE OF SEEING THE PEAK  
INUNDATION OF 4 FEET WILL BE IF ISAIAS MAKES LANDFALL IN OUR  
FORECAST AREA, MAINLY IN AREAS JUST TO THE RIGHT/NORTH OF  
LANDFALL. WHERE THE SS WARNING IS IN PLACE, NO COASTAL FLOOD  
ADVISORIES/WARNINGS WILL BE ISSUED, ALTHOUGH SOUTH OF THE STORM  
SURGE WARNING AREA WE WILL ISSUE THOSE PRODUCTS AS NEEDED.  
HOWEVER, AT THIS TIME WE DO NOT ANTICIPATE REACHING MAJOR  
COASTAL FLOOD THRESHOLDS SOUTH OF EDISTO BEACH SO A COASTAL  
FLOOD WARNING IS UNLIKELY. HOWEVER, THIS COULD CHANGE IF ISAIAS  
ENDS UP BEING SLOWER TO ARRIVE AND/OR STRONGER ALLOWING THE PEAK  
SURGE TO BETTER ALIGN WITH THE HIGH TIDE.  
 

 
   
CLIMATE
 
 
DAILY RAINFALL RECORDS FOR AUGUST 3...  
 
KCHS: 4.60 INCHES  
KCXM: 2.60 INCHES  
KSAV: 4.17 INCHES  
 

 
   
CHS WATCHES/WARNINGS/ADVISORIES
 
 
GA...TROPICAL STORM WARNING FOR GAZ116>119-138>141.  
HIGH RISK FOR RIP CURRENTS UNTIL 8 PM EDT THIS EVENING FOR GAZ117-  
119-139-141.  
HIGH SURF ADVISORY UNTIL 2 AM EDT TUESDAY FOR GAZ117-119-139-  
141.  
WIND ADVISORY UNTIL 2 AM EDT TUESDAY FOR GAZ101.  
SC...TROPICAL STORM WARNING FOR SCZ044-045-047>052.  
HIGH RISK FOR RIP CURRENTS UNTIL 8 PM EDT THIS EVENING FOR  
SCZ048>051.  
HIGH SURF ADVISORY UNTIL 2 AM EDT TUESDAY FOR SCZ048-051.  
FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR SCZ044-045-048>050-  
052.  
WIND ADVISORY UNTIL 2 AM EDT TUESDAY FOR SCZ042-043.  
STORM SURGE WARNING FOR SCZ049-050.  
HIGH SURF ADVISORY UNTIL 8 AM EDT TUESDAY FOR SCZ049-050.  
MARINE...TROPICAL STORM WARNING FOR AMZ330-350-352-354-374.  

 
 

 
 
NEAR TERM...BSH/RJB  
SHORT TERM...MS  
LONG TERM...MS  
AVIATION...RJB/MS  
MARINE...RJB/MS  
HYDROLOGY...  
TIDES/COASTAL FLOODING...  
CLIMATE...  
 
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