932  
FXUS62 KCHS 181453  
AFDCHS  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHARLESTON SC  
1053 AM EDT FRI AUG 18 2017  
   
SYNOPSIS
 
 
ATLANTIC HIGH PRESSURE WILL REMAIN IN CONTROL AS TROUGHING  
PERSISTS INLAND THROUGH LATE THIS WEEK. A COLD FRONT WILL  
DISSIPATE AS IT APPROACHES THE AREA THIS WEEKEND. HIGH PRESSURE  
WILL THEN RETURN EARLY NEXT WEEK BEFORE ANOTHER COLD FRONT  
LIKELY AFFECTS THE AREA FOR MID WEEK.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
TWO MINOR CHANGES TO THE FORECAST.  
1. CONVERGENCE ALONG THE SOUTHERN HALF OF THE COASTAL WATERS HAS  
ALLOWED SHOWERS TO DEVELOP. SOME RUMBLES OF THUNDER ARE  
CERTAINLY POSSIBLE, MAINLY ALONG THE MCINTOSH COAST. WE ADDED  
SLIGHT CHANCE POPS TO PORTIONS OF THE COASTAL WATERS, EXTENDING  
UP TO THE IMMEDIATE MCINTOSH COASTLINE. THESE SHOWERS SHOULD  
GRADUALLY DISSIPATE OVER THE NEXT FEW HOURS AS CONVECTIVE  
DEVELOPMENT SHIFTS TO THE LAND AREAS.  
2. ADJUSTED THE SKY COVER  
PER LATEST SATELLITE IMAGERY TO REFLECT LESS CLOUDS OVER LAND  
AND MORE OVER WATERS, THEN HAVE CLOUDS GRADUALLY INCREASING INTO  
THIS AFTERNOON.  
OTHERWISE, THE TEMPERATURES AND DEW POINTS ARE FAIRLY CLOSE TO  
THE CURRENT FORECAST.  
 
REST OF TODAY: THE RECENT STRONG RIDGING ATOP THE AREA IS STILL  
EVIDENT, BUT IS FLATTENED AS A SHORT WAVE PASSES BY TO THE N  
THIS AFTERNOON WITHIN THE WESTERLY FLOW THAT COVERS A GOOD CHUNK  
OF THE CONUS. FURTHER S WE FIND A PROMINENT TUTT LOW HEADING  
W/WNW FROM THE BAHAMAS TOWARD THE FL KEYS. MEANWHILE AT THE  
SURFACE THE LOCAL REGION REMAINS SITUATED BETWEEN THE PIEDMONT  
TROUGH INLAND AND THE SUB-TROPICAL RIDGE ALIGNED E-W ACROSS FL  
INTO THE GULF OF MEXICO.  
 
IT'LL BE ANOTHER ABNORMALLY HOT DAY DUE TO THE COMBINATION OF  
BOTH 850 MB AND 925 MB TEMPS THAT ARE AT LEAST 2-3 STANDARD  
DEVIATIONS ABOVE NORMAL. THROW IN SOME COMPRESSIONAL HEATING  
WITH SOME DOWNSLOPE OFF THE APPALACHIANS AND MAX TEMPS WILL PEAK  
AT 94-97F MOST PLACES. THE EXCEPTIONS WILL BE ALONG THE  
SHORELINE WHERE THE RESULTANT SEA BREEZE "LIMITS" HIGHS TO THE  
UPPER 80S AND LOWER 90S.  
 
THESE TEMPERATURES COMBINED WITH DEW POINTS IN THE MID AND UPPER  
70S WILL YIELD HEAT INDICES APPROACHING 110 DEGREES NEAR AND  
EAST OF I-95. THE CURRENT FORECAST HAS A FEW POINTS TOUCHING  
THIS TEMPERATURES FOR AN HOUR, BUT THE AREAL COVERAGE APPEARS  
LIMITED AND IT'S NOT THE MINIMUM TWO HOURS TO NECESSITATE A  
HEAT ADVISORY. EVEN WITH THE SURGE IN DEW POINTS WITH THE  
AFTERNOON SEA BREEZE, THE INCREASE IN HEAT INDEX SHOULD BE BRIEF  
ENOUGH TO KEEP THE COASTAL COUNTIES OUT OF ADVISORY CRITERIA.  
OF COURSE, IF TEMPERATURES AND/OR DEW POINTS ARE 1-2 DEGREES  
WARMER THAN EXPECTED, THEN THAT WILL BE ENOUGH TO TIP THE BUCKET  
IN FAVOR OF AN ADVISORY. FURTHER INLAND, DEW POINTS WILL MIX  
OUT ENOUGH WITH THE WESTERLY TRAJECTORIES AND HEAT INDICES WILL  
AVERAGE 105-107 DEGREES.  
 
WARM MID LEVELS, MOISTURE CONFINED GENERALLY TO THE FIRST 2-3  
MILES OF THE TROPOSPHERE AND THE ONLY FORCING FROM THE SEA  
BREEZE AND OUTFLOW BOUNDARIES, OUR RISK FOR CONVECTION IS HELD  
DOWN THIS AFTERNOON. MOST OF THE HI-RES MODELS NOW SHOW LIMITED  
CONVECTION ALONG THE COAST STARTING AT ABOUT 2 PM AND THEN  
TRANSITIONING INLAND INTO THE EVENING. DRIER AIR SHOWN ON WATER  
VAPOR SATELLITE IMAGERY COULD LIMIT THIS POTENTIAL, BUT WE'RE  
HESITANT TO GO ANY LOWER THAN SLIGHT CHANCE POP'S GIVEN MLCAPE  
OVER 2,000 J/KG. WHATEVER STORMS DO FORM HAVE THE POTENTIAL FOR  
LOCALLY HEAVY RAINS GIVEN PWAT 120-130% OF NORMAL AND WEAK  
STEERING FLOW. DCAPES AS HIGH AS 1,200 J/KG COULD PRODUCE A  
STRONG WIND THREAT.  
 
TONIGHT: LINGERING BOUNDARIES AND THE SEA BREEZE FAR INLAND  
DURING THE EVENING WILL MAINTAIN SMALL POP'S ALONG THE FAR NW  
TIER THROUGH MID TO LATE EVENING, BEFORE CHANCES DROP BELOW 15%  
DURING THE POST-MIDNIGHT HOURS. MOISTURE CONVERGENCE WILL ALLOW  
FOR SOME NOCTURNAL CONVECTION OUT NEAR THE GULF STREAM, BUT  
NOTHING ONSHORE. SOME OF THE STATISTICAL GUIDANCE SHOWS A LITTLE  
FOG CLOSER TO DAYBREAK SATURDAY WITH CONDENSATION PRESSURE  
DEFICITS DOWN UNDER 20 MB. BUT CROSS-OVER TEMPS WILL BE HARD TO  
REACH, AND THE GROUND IS SO WARM. THUS NO MENTION OF FOG AT THIS  
TIME. IT'LL BE ANOTHER VERY WARM AND MUGGY NIGHT, AND RECORD  
HIGH MINIMUM TEMPS WILL BE CHALLENGED.  
 

 
   
SHORT TERM /SATURDAY THROUGH MONDAY/
 
 
WEAK TROUGHING SATURDAY WILL GIVE WAY TO A BUILDING RIDGE EARLY NEXT  
WEEK. THIS WILL CAUSE AN APPROACHING COLD FRONT TO STALL AND WEAKEN  
NORTH AND WEST OF THE AREA. HOWEVER, DEEP MOISTURE AND SOME  
CONVERGENCE FROM THE INLAND TROUGH AND SEA BREEZE SHOULD YIELD  
SCATTERED CONVECTION EACH DAY. A FEW STRONG TO SEVERE THUNDERSTORMS  
ARE POSSIBLE, BUT THE SEVERE STORM THREAT OVERALL IS LOW AS THE  
STORMS WILL PRIMARILY BE OF THE PULSE VARIETY. LOCALLY HEAVY  
RAINFALL WILL BE POSSIBLE TOO GIVEN THE ABUNDANT MOISTURE AND  
RELATIVELY WEAK STORM MOTIONS, AND THIS COULD POSE A RISK OF  
FLOODING, ESPECIALLY NEAR THE COAST AS TIDES WILL BE RUNNING HIGH.  
TEMPS IN THE LOWER/MID 90S COMBINED WITH HIGH HUMIDITY WILL LEAD TO  
HEAT INDEX VALUES PEAKING OVER 100 DEGREES MOST LOCALES EACH  
AFTERNOON, PROBABLY HIGHEST ACROSS SOUTHEAST GA ON SATURDAY WHERE  
VALUES COULD NEAR ADVISORY LEVELS (110 DEGREES).  
 

 
   
LONG TERM /MONDAY NIGHT THROUGH THURSDAY/
 
 
VERY STRONG UPPER RIDGING WILL PREVAIL INTO TUESDAY BEFORE A TROUGH  
AND COLD FRONT MOVE IN FOR THE MIDDLE TO LATTER PORTION OF THE WEEK.  
THIS WILL LEAD TO INCREASINGLY UNSETTLED WEATHER TOWARD MID WEEK,  
ESPECIALLY THURSDAY AS THE FRONT MOVES INTO THE AREA. TEMPERATURES  
SHOULD REMAIN ABOVE NORMAL THROUGH THE PERIOD, EXCEPT POSSIBLY  
THURSDAY DEPENDING ON THE LOCATION OF THE FRONT.  
 

 
   
AVIATION /15Z FRIDAY THROUGH TUESDAY/
 
 
VFR THROUGH 12Z SATURDAY.  
 
EXTENDED AVIATION OUTLOOK: MAIN CONCERN THROUGH TUESDAY IS SHOWERS  
AND THUNDERSTORMS, MAINLY EACH AFTERNOON/EARLY EVENING.  
 

 
   
MARINE
 
 
TODAY: THE WATERS WILL BE LOCATED IN BETWEEN AN INLAND TROUGH  
AND STRONG RIDGING TO THE S-SE. SW AND W WINDS THIS MORNING WILL  
BACK TO S-SW THIS AFTERNOON WITH SEA BREEZE CIRCULATIONS  
DEVELOPING. THERE IS ACTUALLY A FAIRLY TIGHT GRADIENT THAT  
DEVELOPS AND COMBINED WITH A BOOST FROM THE SEA BREEZE WILL BUMP  
WINDS UP AS HIGH AS 13-18 KT THE SECOND PART OF THE DAY. SEAS  
WILL AVERAGE 2 OR 3 FT.  
 
TONIGHT: A DECENT PACKING OF THE GRADIENT BETWEEN THE INLAND  
TROUGH AND SUB-TROPICAL RIDGE, PLUS NOCTURNAL LOW LEVEL JETTING  
WILL SUPPORT S-SW WINDS AS HIGH AS 15 OR 20 KT AND SEAS 3-4 FT.  
 
SATURDAY THROUGH WEDNESDAY: THE COASTAL WATERS WILL REMAIN  
BETWEEN AN INLAND TROUGH AND OFFSHORE HIGH PRESSURE THROUGH  
SATURDAY NIGHT WHICH WILL KEEP A TIGHTER THAN NORMAL PRESSURE  
GRADIENT IN PLACE. WINDS COULD PUSH 20 KNOTS AT TIMES BUT THIS  
IS JUST BELOW ADVISORY LEVELS. A WEAKENING COLD FRONT WILL THEN  
APPROACH EARLY NEXT WEEK BUT LIKELY STALL OUT AND NEVER MAKE IT  
THROUGH THE AREA WITH WINDS GENERALLY REMAINING 15 KNOTS OR  
LESS. SEAS WILL MOSTLY BE 4 FEET OR LESS, HIGHEST NEAR THE GULF  
STREAM.  
 

 
   
TIDES/COASTAL FLOODING
 
 
ASTRONOMICAL INFLUENCES WILL LEAD TO A ROUND OF ELEVATED TIDES  
INTO EARLY NEXT WEEK. ONLY SMALL TIDAL DEPARTURES ARE NECESSARY  
AND WE COULD APPROACH SHALLOW COASTAL FLOODING LEVELS WITH THE  
EVENING HIGH TIDES, ESPECIALLY ALONG PARTS OF THE SC COAST,  
INCLUDING DOWNTOWN CHARLESTON.  
 

 
   
CLIMATE
 
 
RECORD HIGH MINIMUMS FOR 18 AUGUST...  
KCHS 78/2010  
KCXM 82/1998  
KSAV 78/2010  
 
RECORD HIGH MINIMUMS FOR 19 AUGUST...  
KCHS 79/2010  
KCXM 81/2009  
KSAV 79/1878  
 

 
   
EQUIPMENT
 
 
THE TEMPERATURE AND DEW POINT SENSORS AT THE DOWNTOWN CHARLESTON  
OBSERVATION SITE (KCXM) COULD PERIODICALLY FAIL. TECHNICIANS  
ARE WORKING TO RESOLVE THE PROBLEM.  
 

 
   
CHS WATCHES/WARNINGS/ADVISORIES
 
 
GA...NONE.  
SC...NONE.  
MARINE...NONE.  
 

 
 

 
NEAR TERM...  
SHORT TERM...  
LONG TERM...  
AVIATION...  
MARINE...  
TIDES/COASTAL FLOODING...  
CLIMATE...  
EQUIPMENT...  
 
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