850  
FXUS62 KCHS 232315  
AFDCHS  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHARLESTON SC  
715 PM EDT SUN SEP 23 2018  
   
SYNOPSIS
 
 
HIGH PRESSURE WILL WEAKEN TONIGHT AS A QUASI-STATIONARY FRONT  
GRADUALLY APPROACHES FROM THE NORTH INTO MONDAY. LOW PRESSURE  
WILL APPROACH FROM THE ATLANTIC MONDAY NIGHT INTO TUESDAY, THEN  
WEAKEN AS IT MOVES NORTHWARD. A COLD FRONT IS FORECAST TO  
APPROACH FROM THE WEST TOWARDS THE SECOND HALF OF THE WEEK AND  
THEN POSSIBLY STALL OVER THE AREA. A SECOND COLD FRONT IS  
EXPECTED NEXT WEEKEND.  
 

 
   
NEAR TERM /UNTIL 6 AM MONDAY MORNING/
 
 
A SURFACE TROUGH HAS BEEN ANALYZED IN THE AREA NEAR BEAUFORT TO  
NORTH CHARLESTON AND MONCKS CORNER, AS EVIDENCED BY SW WINDS IN  
AND AROUND THESE COMMUNITIES, WHILE E AND SE WINDS ARE DEPICTED  
CLOSER TO THE COAST AND OFFSHORE. THIS FEATURE ALONG WITH DEEP  
MOISTURE CONVERGENCE AND A TONGUE OF HIGHER PWAT ACROSS THE AREA  
WILL LEAD TO ISOLATED OR SCATTERED SHOWERS OVER THE CHARLESTON  
TRI-COUNTY DISTRICT TO BEAUFORT COUNTY THIS EVENING, WITH NO  
MORE THAN A FEW STRAY SHOWERS ELSEWHERE.  
 
GIVEN AN UNIMPRESSIVE THERMODYNAMIC ENVIRONMENT, AND LITTLE TO  
NO SURFACE-BASED INSTABILITY NOR NEGATIVE SHOWALTER VALUES, WE  
HAVE REMOVED MENTION OF THUNDER FROM THE ATLANTIC WATERS THIS  
EVENING. WE HAVE ALSO HAD TO INCREASE CLOUD COVER SIGNIFICANTLY  
IN MANY AREAS AS PER RECENT TRENDS, AND MADE SOME ADJUSTMENTS  
TO THE DIURNAL TEMP CURVE WHICH ACCOUNTS FOR THE EXTENSIVE CLOUD  
COVER AND PREVIOUS AND/OR ONGOING SHOWERS.  
 
PREVIOUS DISCUSSION... MOST OF THE SHOWERS FROM THIS AFTERNOON  
SHOULD DIMINISH EARLY THIS EVENING WITH MAINLY DRY CONDITIONS  
OVERNIGHT. SOME STRATUS POSSIBLE AS WELL AS FOG, ESPECIALLY NEAR  
THE CENTRAL SAVANNAH RIVER AREA. CLOSER TO DAYBREAK MORE  
SHOWERS WILL BE POSSIBLE NEAR THE COAST AS MOISTURE CONVERGENCES  
INCREASES AS A COASTAL TROUGH DEVELOPS. LOWS SHOULD RANGE FROM  
THE UPPER 60S/NEAR 70 INLAND TO THE MID/UPPER 70S NEAR THE  
COAST.  
 

 
   
SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/
 
 
MONDAY: A BROAD AREA OF LOW PRESSURE LOCATED ABOUT 500 NM ESE OF THE  
COASTS OF SC AND GA WILL TRAVEL W AND NW AROUND A WEAKNESS IN AN  
OTHERWISE ANOMALOUSLY STRONG H5 RIDGE WITH 594 HEIGHTS AND CENTERED  
NEAR BERMUDA. AT THE SAME TIME A QUASI-STATIONARY FRONT WILL BEGIN  
TO SAG INTO CLOSER PROXIMITY OF THE AREA FROM THE N AS A HUGE 1038  
MB HIGH BUILDS OUT OF SE CANADA. AN EASTERLY FLOW THROUGHOUT THE  
FIRST 10K FT OR SO OF THE TROPOSPHERE WILL MAINTAIN PWAT VALUES  
AROUND 1.6 TO PERHAPS 2 INCHES, WHICH IS NEAR OR ABOVE NORMAL FOR  
THIS TIME OF YEAR. FORCING WILL BE DRIVEN BY COASTAL CONVERGENCE IN  
THE MORNING, AND THEN THE SEA BREEZE, OTHER MESO-SCALE FEATURES AND  
MAYBE A SUBTLE MID LEVEL IMPULSE THAT SOME OF THE GUIDANCES SHOWS  
ARRIVING FROM THE S-SE IN THE AFTERNOON. THIS WILL LEAD TO  
SCATTERED COVERAGE OF CONVECTIVE RAINS, HIGHEST OVER THE COASTAL  
COUNTIES IN THE MORNING, WITH THE BULK OF THE COVERAGE SHIFTING  
INLAND DURING THE AFTERNOON. THERMODYNAMICS MIGHT BE A LITTLE MORE  
FAVORABLE FOR T-STORMS THAN RECENT DAYS, ESPECIALLY WITH MUCAPE  
FORECAST AS HIGH AS 2500 J/KG IF NOT MORE. THUS WE HAVE MENTION OF  
SLIGHT CHANCE T-STORMS THROUGHOUT THE DAY. THE OVERALL STORM MOTION  
IS SLOW ENOUGH TO COMBINE WITH THE ELEVATED MOISTURE TO ALLOW FOR AT  
LEAST LOCALIZED HEAVY RAINFALL. THE HOURLY TEMP CURVE WILL REQUIRE  
ALTERATIONS AT A LATER TIME, PENDING THE EXACT COVERAGE/TIMING OF  
CONVECTION. BUT MAX TEMPS SHOULD HIT THE MID OR UPPER 80S MOST  
PLACES INLAND WITH H8 TEMPS AS WARM AS 16-17C.  
 
MONDAY NIGHT: THE ATLANTIC LOW WILL APPROACH BUT REMAIN ABOUT 200 OR  
SO MILES OFF THE COAST BY DAYBREAK TUESDAY, AS THE FRONT IS ABLE TO  
DROP INTO THE LOCAL REGION WITH SOME WEDGING TO OCCUR INLAND. THE  
MAJORITY OF THE DIURNAL CONVECTION WILL FADE DURING THE EARLY AND  
MIDDLE EVENING HOURS, WITH ANY LATE NIGHT CONVECTION ASSOCIATED WITH  
THE LOW OFFSHORE TO AFFECT NO MORE THAN CHARLESTON AND BERKELEY  
COUNTIES LATE. GIVEN THE WET GROUNDS AND CONDENSATION PRESSURE  
DEFICITS UNDER 10 MB, WE HAVE ADDED MENTION OF PATCHY FOG FAR W-NW  
TIER LATE. MIN TEMPS AGAIN WILL STAY SEVERAL DEGREES ABOVE NORMAL,  
COOLEST FAR INLAND.  
 
TUESDAY: THE BROAD ATLANTIC LOW WILL TURN NORTHWARD AROUND THE  
WESTERN SIDE OF THE SUB-TROPICAL RIDGE, AND WILL STAY OFFSHORE OF  
THE SC AND GA COASTS AS IT HEADS TOWARD THE NC OUTER BANKS AT NIGHT.  
THE INLAND WEDGE GRADUALLY BREAKS DOWN AND A LIGHT PRESSURE PATTERN  
SETTLES IN FOR THE AFTERNOON AS THE FRONT BECOMES DIFFUSE. ALTHOUGH  
THERE IS STILL DECENT MOISTURE, IT IS LESS THAN RECENT DAYS, AND  
THAT ALONG WITH LESS FORCING AND SOME SUBSIDENCE TO THE W OF THE  
ATLANTIC LOW, CONVECTIVE RAINS ARE NO MORE THAN 20-30%. WITH LESS O  
A CHANCE OF RAIN, PLUS H8 TEMPS UP TO 17 OR 18C, MAX TEMPS WILL HIT  
87-91F MOST COMMUNITIES INLAND FRONT THE IMMEDIATE SHORELINE.  
 
WEDNESDAY: DEEP SUB-TROPICAL ATLANTIC RIDGING WILL BUILD INTO THE  
AREA, WITH A DISTANT UPSTREAM COLD FRONT STILL TO THE W OF THE  
APPALACHIANS AS A BROAD MID AND UPPER TROUGH IS LOCATED OVER THE  
CENTRAL STATES. A DEEP OFFSHORE FLOW THROUGHOUT THE VERTICAL AND THE  
LACK OF FORCING OTHER THAN MAINLY THE SEA BREEZE AND MAYBE SOME  
OUTFLOWS THAT ARRIVE FROM THE W-NW, WILL LIMIT CHANCES OF SHOWERS  
AND T-STORMS TO NO MORE THAN 20-30%, HIGHEST INLAND. IT'LL BE  
ANOTHER ABNORMALLY WARM DAY FOR LATE SEPTEMBER, WITH H8 TEMPS UP TO  
17 OR 18C AND THE OFFSHORE TRAJECTORIES. AS A RESULT, MOST PLACES  
INLAND FROM THE INTRA-COASTAL WILL HIT 90F, OR FAR ABOVE NORMAL.  
 

 
   
LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/
 
 
WEDNESDAY NIGHT A COLD FRONT WILL BE SLOWLY APPROACHING FROM THE  
NORTHWEST. THIS TREND WILL CONTINUE INTO THURSDAY. THE FRONT MAY  
THEN STALL OVER OR NEARBY OUR AREA FRIDAY. ANOTHER COLD FRONT IS  
THEN FORECASTED TO APPROACH FROM THE NORTHWEST ON SATURDAY. THE  
RAINFALL POTENTIAL WILL DEPEND ON THE APPROXIMATE LOCATION OF THE  
FRONTS. FOR NOW IT APPEARS TO EQUATE TO ABOVE NORMAL POPS. HOWEVER,  
MORE ADJUSTMENTS TO THE FORECAST ARE EXPECTED.  
 

 
   
AVIATION /00Z MONDAY THROUGH FRIDAY/
 
 
MUCH OF THE VALID 00Z TAF CYCLE WILL BE VFR. HOWEVER, IF  
STRATOCUMULUS/ALTOCUMULUS CLOUDS ARE ABLE TO DIMINISH ENOUGH  
TONIGHT, DUE TO THE WET GROUNDS, ESPECIALLY AT KCHS, THERE COULD  
BE SOME FOG AND/OR STRATUS CONCERNS AND POSSIBLE FLIGHT  
RESTRICTIONS AROUND 09-13Z MONDAY. ANOTHER CHANCE OF AT LEAST  
TEMPORARY FLIGHT RESTRICTIONS WILL OCCUR FROM ABOUT 15-22Z  
MONDAY DUE TO SCATTERED SHRA AND MAYBE A TSRA IN ASSOCIATION  
WITH THE SEA BREEZE AND OTHER MESO-SCALE BOUNDARIES.  
 
EXTENDED AVIATION OUTLOOK: THE GREATEST RISK FOR FLIGHT RESTRICTIONS  
WILL TRANSPIRE THURSDAY AND FRIDAY DUE TO SCATTERED CONVECTION WITH  
A COLD FRONT IN THE VICINITY.  
 

 
   
MARINE
 
 
TONIGHT: A TROUGH HAS BEEN DEPICTED JUST INLAND FROM THE SC  
COAST, AND THIS HAS ALTERED WINDS SOMEWHAT IN CHARLESTON HARBOR  
AND NEAR THE COAST. BUT THE SYNOPTIC PATTERN WILL CHANGE TO ONE  
THAT FEATURES EASTERLY WINDS ACROSS THE ENTIRE MARINE AREA  
DURING TONIGHT AS THE TROUGH FALLS APART. ON AVERAGE, WINDS  
WILL HOLD AT OR BELOW 15 KT, WITH SEAS AS LARGE AS 2-4 FT.  
ISOLATED TO SCATTERED SHOWERS WILL OCCUR THROUGH THE NIGHT, WITH  
A FEW T-STORMS TO OCCUR AFTER MIDNIGHT.  
 
MONDAY THROUGH TUESDAY: LOW PRESSURE IN THE ATLANTIC, ABOUT MIDWAY  
BETWEEN BERMUDA AND THE BAHAMAS AT THE START OF THE PERIOD, WILL  
TRAVEL CLOCKWISE AROUND STRONG HIGH PRESSURE THAT COVERS MUCH OF THE  
W AND NW ATLANTIC. IT REACHES IT'S CLOSEST POINT TO THE MARINE AREA  
TUESDAY, WHEN IT IS POSITIONED ABOUT 75-100 NM E OF AMZ350. THERE IS  
ENOUGH OF AN INLAND WEDGE MONDAY AND A BOOST FROM THE SEA BREEZE TO  
ALLOW FOR EASTERLY WINDS AS HIGH AS NEAR 15 KT, WITH THE ASSOCIATED  
SEAS TO 3 OR 4 FT. PROVIDED THE LOW IN THE ATLANTIC STAYS RELATIVELY  
WEAK, WINDS ON TUESDAY WILL ACTUALLY DECREASE, BUT SEAS DO STAY  
SOMEWHAT ELEVATED, AND AS HIGH AS 3-5 FT.  
 
WEDNESDAY THROUGH SATURDAY: THE SUB-TROPICAL RIDGE WILL BE THE  
DOMINANT FEATURE, AS ANY UPSTREAM COLD FRONT WILL NOT IMPACT THE  
WATERS UNTIL LATE IN THE FORECAST PERIOD. WINDS AND SEAS SHOULD STAY  
BELOW ANY ADVISORY THRESHOLDS, ALTHOUGH THE WAVE WATCH GUIDANCE  
INDICATES SOME LONG PERIOD SWELLS ORIGINATING FROM TC KIRK THAT WILL  
BEGIN ARRIVING DURING THE LATTER PART OF THE UPCOMING WEEK. THIS  
COULD ALTER THE FORECAST OF SEAS BEGINNING THURSDAY OR FRIDAY.  
 
RIP CURRENTS: MODEST ONSHORE WINDS, ASTRONOMICAL INFLUENCES AND  
SMALL SWELL ENERGY WILL CAUSE A MODERATE RISK OF RIP CURRENTS AT  
AREA BEACHES THROUGH AT LEAST MONDAY.  
 
THE RISK OF RIP CURRENTS COULD REMAIN ELEVATED THROUGHOUT THE WEEK,  
DEPENDING UPON HOW MUCH SWELL ENERGY THERE IS FROM AL98 (THE CLOSEST  
ATLANTIC LOW) AND TC KIRK SPINNING IN THE CENTRAL ATLANTIC.  
 

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

 
 

 
NEAR TERM...  
SHORT TERM...  
LONG TERM...  
AVIATION...RJB  
MARINE...  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab SC Page
The Nexlab GA Page Main Text Page