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.
Main Text Page