Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 300512
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
112 AM EDT SAT APR 30 2016

.SYNOPSIS...
A STATIONARY FRONT WILL LINGER JUST NORTH OF THE AREA OVERNIGHT
THROUGH SATURDAY...THEN WILL PULL FARTHER NORTH AS A WARM FRONT
SATURDAY NIGHT. LOW PRESSURE WILL PASS TO THE DISTANT NORTH SUNDAY
AND MONDAY...AS ATLANTIC HIGH PRESSURE STRETCHES ACROSS THE LOCAL
AREA. A DEVELOPING LOW WILL PASS OVER OR NEAR THE AREA DURING THE
MIDDLE OF NEXT WEEK...FOLLOWED BY HIGH PRESSURE BUILDING INTO THE
AREA TOWARDS THE END OF THE WEEK.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
THE PRIMARY CONCERN THROUGH SUNRISE IS THE POTENTIAL FOR FOG AND
STRATUS. ALREADY SEEING STRATUS DEVELOPING ACROSS PORTIONS OF THE
CHARLESTON TRI-COUNTY AREA WITH CEILINGS AS LOW AS 300 FT. RAP/NAM
SOUNDINGS SUGGEST A POTENTIAL FOR STRATUS BUILD DOWN...ESPECIALLY
IN THE BEAUFORT-CHARLESTON CORRIDOR...BUT THIS WILL BE DEPENDENT
ON WHETHER WINDS DIMINISH ENOUGH TO SUPPORT FOG. WILL ADD AREAS
OF FOG IN THIS AREA WITH PATCHY FOG ELSEWHERE TO TREND. A DENSE
FOG ADVISORY CAN NOT BE COMPLETELY RULED OUT...MAINLY FOR COASTAL
SOUTH CAROLINA...POSSIBLY AS FAR INLAND AS WALTERBORO AND MONCKS
CORNER. OTHERWISE...ADJUSTED OVERNIGHT LOWS UP SLIGHTLY PER GOING
TRENDS.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH MONDAY/...
SATURDAY...A WEDGE-TYPE FRONT WILL LIE IN NORTHERN SC AND DIPS A
LITTLE FURTHER SOUTH AS HIGH PRESSURE MOVES EAST ACROSS LONG ISLAND
AND BRIDGES ACROSS THE FRONT TO NEAR THE BAHAMAS. RETURN FLOW FROM
THE SE AND SOUTH AROUND THE HIGH WILL SUPPORT ANOTHER DAY WITH FAR
ABOVE NORMAL TEMPS. SINCE WE ARE CONCERNED ABOUT HOW MUCH CIRRUS
OUTFLOW THERE WILL BE FROM THE HUGE COMPLEX OF CONVECTION THAT WAS
ONGOING TODAY IN TN/MS VALLEY WE HAVEN/T GONE AS WARM AS THE LOW
LEVEL THICKNESS FORECAST. WE/LL BE A LITTLE MORE CONSERVATIVE AND
SHOW MAX TEMPS UPPER 80S/LOWER 90S INLAND FROM US-17...AND WITH AN
ONSHORE SYNOPTIC FLOW AND AN EARLIER START TO THE SEA BREEZE THE
COASTAL REGIONS WILL BE LIMITED TO UPPER 70S/LOWER 80S AT BEST.

CONVECTIVE CHANCES WILL BE HELD IN CHECK BY THE LACK OF ANY FOCUSING
MECHANISM OTHER THAN THE SEA BREEZE...AND LIMITED MOISTURE. PLUS
THERE IS STILL SOME WEAK RIDGING ALOFT AND AT LEAST SOME CAP TO
REMAIN IN PLACE. OUR FORECAST THUS SHOWS NOTHING MORE THAN SLIGHT
CHANCE POPS IN THE AFTERNOON...MAINLY FAR WEST/NW ZONES.

ISOLATED CONVECTION COULD LINGER INTO THE NIGHT FAR INLAND AS THE
MEAN FLOW ALOFT FROM THE WEST COULD ALLOW FOR UPSTREAM CONVECTION TO
SLIDE IN DURING THE NIGHT. DEBRIS CLOUD COVER AND LOW LEVEL WARM
ADVECTION WILL PRODUCE A VERY WARM NIGHT...WITH LOWS AT BEST 65-70
OR ABOUT 10F ABOVE NORMAL.

SUNDAY THROUGH MONDAY...LOW PRESSURE ALOFT WILL TRACK THROUGH THE
CENTRAL PLAINS INTO THE UPPER MIDWEST THE SECOND DAY OF THE
WEEKEND...THEN BECOMES ABSORBED BY NORTHERN STREAM ENERGY BY
MONDAY.THE MEAN TROUGH POSITION WILL REMAIN FAR TO THE NORTH/NW
EARLY NEXT WEEK...WITH GENERALLY A ZONAL FLOW TO PREVAIL LOCALLY. AT
THE SURFACE WEAK LOW PRESSURE WILL SHIFT THROUGH THE OHIO VALLEY
SUNDAY...THEN PASSES OFF THE DELMARVA AND TO THE SOUTH OF NEW
ENGLAND MONDAY. HIGH PRESSURE CENTERED BETWEEN BERMUDA AND THE MID
ATLANTIC COAST SUNDAY WILL BE FORCED EAST AS THIS OCCURS...BUT IT/S
ANTICYCLONIC FLOW ATTEMPTS TO HOLD ON LOCALLY. A COLD FRONT TRAILING
FROM THE LOW WILL STRUGGLE TO MOVE EAST SUNDAY...MAKING IT TO NEAR
THE APPALACHIANS MONDAY AS THE ATLANTIC RIDGE HOLDS ON. BOTH DAYS
WILL EXPERIENCE A CONTINUATION OF UNUSUALLY WARM WEATHER WITH 850 MB
TEMPS SOME 2 STANDARD DEVIATIONS ABOVE NORMAL. HOWEVER HOW WARM WE
DO GET IS GEARED TOWARD THE INITIATION OF DIURNAL CONVECTION...SO
FOR NOW WE HAVE MID AND UPPER 80S FOR HIGHS AND MID AND UPPER 60S
FOR LOWS.

WHILE THERE ARE SUBTLE MID LEVEL IMPULSES THAT TRAVERSE THE
REGION...OVERALL FORCING FOR ASCENT IS MISSING OUTSIDE OF THE SEA
BREEZE AND MESO-SCALE BOUNDARIES THAT ARE ABLE TO FORM. AN INCREASE
IN INSTABILITY AND CAPE...MODEST UPWARD VERTICAL VELOCITIES AND
ADEQUATE LAPSE RATES WILL SUPPORT AN INCREASED RISK FOR MAINLY
DIURNALLY INDUCED SHOWERS AND T-STORMS. WE/LL CAP POPS IN THE CHANCE
RANGE BUT WITH PWATS AROUND 2 STANDARD DEVIATIONS GREATER THAN
NORMAL THERE COULD BE A LOCALLY HEAVY RAIN RISK.

THE SEVERE CHANCES ARE VERY LOW BUT NON-ZERO WITH DCAPES OF 1000-
1200 J/KG SUGGESTING A STRONG WIND THREAT. PLUS 500 MB TEMPS ARE AS
COLD AS -11/-12C AND CAPE IN THE HAIL GROWTH ZONE IS ALSO FAVORABLE
FOR AT LEAST MARGINALLY SEVERE HAIL. PROBABILITIES THOUGH ARE LOW
ENOUGH WHERE THERE IS NO NEED TO INCLUDE IN THE HAZARDOUS WEATHER
OUTLOOK AT THIS STAGE.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
THE MODELS START TO DIVERGE MONDAY NIGHT. THE 00Z GFS HAS WEAK HIGH
PRESSURE IN PLACE MONDAY NIGHT INTO TUESDAY...FOLLOWED BY A LOW
DEVELOPING TO OUR SOUTH TUESDAY NIGHT. THE LOW WILL PASS OVER OR
NEAR US WEDNESDAY FOLLOWED BY BUILDING HIGH PRESSURE THURSDAY. THE
00Z ECMWF DOES NOT HAVE HIGH PRESSURE IN PLACE MONDAY NIGHT INTO
TUESDAY...INSTEAD IT HAS UNSETTLED WEATHER OVER OUR
AREA...ORIGINATING IN THE GULF OF MEXICO. A LOW WILL DEVELOP TO OUR
SOUTH TUESDAY. THE LOW WILL PASS OVER OR NEAR US TUESDAY NIGHT
FOLLOWED BY POSSIBLY ANOTHER LOW ON WEDNESDAY. HIGH PRESSURE BUILDS
IN ON THURSDAY. THE 00Z CANADIAN HAS WEAK HIGH PRESSURE IN PLACE
MONDAY NIGHT INTO TUESDAY. A LOW WILL DEVELOP TO OUR SOUTHWEST
TUESDAY...PASSING TO OUR WEST TUESDAY NIGHT. HIGH PRESSURE BUILDS IN
BEHIND THAT. GIVEN ALL THE DIFFERENT SOLUTIONS...OPTED TO GO WITH
SLIGHT CHANCE POPS MONDAY NIGHT FOLLOWED BY CHANCE POPS TUESDAY INTO
WEDNESDAY. WE START DRYING OUT WEDNESDAY NIGHT INTO THURSDAY. WITH A
TROUGH OVER THE EAST COAST DURING THIS TIME FRAME TEMPERATURES WILL
BE BELOW NORMAL WEDNESDAY AND THURSDAY.

&&

.AVIATION /05Z SATURDAY THROUGH WEDNESDAY/...
KCHS...BECOMING INCREASINGLY CONCERNED THAT LOW CIGS WILL DEVELOP
AT THE TERMINAL OVER THE NEXT FEW HOURS. SURFACE WINDS ARE STAYING
UP...BUT THERE ARE SIGNS THEY MAY WEAKEN A BIT PRIOR TO SUNRISE.
ALREADY SEEING CIGS AS LOW AS 300-400 FT AT KMKS-KDYB-KRBW AS OF
03/05Z OBSERVATIONS...SO WILL GO AHEAD AN INTRODUCE A TEMPO GROUP
FOR LIFR CIGS AROUND 400 FT 08-11Z PER GOING TRENDS. WILL KEEP
VSBYS NO LOWER THAN 4SM UNTIL SIGNS SHOW WINDS BEGINNING TO
SUBSIDE. ESTIMATE AT 30 PERCENT CHANCE THAT CIGS WILL DROP BELOW
AIRFIELD MINIMUMS. AMENDMENTS WILL MOST LIKELY BE NEEDED OVER THE
COMING HOURS AS TRENDS BECOME MORE APPARENT. CONDITIONS SHOULD
IMPROVE TO VFR BY 13Z.

KSAV...BEST FOG PARAMETERS LOOK TO REMAIN JUST EAST AND NORTHEAST
OF THE TERMINAL THROUGH SUNRISE...ALTHOUGH LOW CIGS COULD COME
UNCOMFORTABLY CLOSE OVER THE COMING HOURS. WILL SHOW A TEMPO
GROUP FOR SCT004 FROM 09-11Z...BUT AN AMENDMENT MAY BE NEEDED TO
ESTABLISH A CEILING. WILL ALSO KEEP VSBYS AT MVFR THRESHOLDS SINCE
THE BETTER FOG PARAMETERS ARE DISPLACED FROM THE TERMINAL. VFR BY
13Z.

EXTENDED AVIATION OUTLOOK...UNSETTLED CONDITIONS FOR THE EARLY AND
MIDDLE PART OF NEXT WEEK WILL RESULT IN PERIODIC FLIGHT RESTRICTIONS
IN SHRA/TSRA AT THE LOCAL TERMINALS.

&&

.MARINE...
OVERNIGHT...WEAK OFFSHORE HIGH PRESSURE WILL MAINTAIN SE WINDS
10 KT OR LESS ACCOMPANIED BY SEAS 1-2 FT.

SATURDAY THROUGH WEDNESDAY...HIGH PRESSURE WILL BE THE MAIN LARGE
SCALE FEATURE THAT PREVAILS INTO EARLY NEXT WEEK...BEFORE AN AREA OF
LOW PRESSURE COULD IMPACT THE WATERS BY THE MIDDLE OF THE WEEK.
OUTSIDE OF CONVECTION THERE ARE NO REAL CONCERNS IN REGARDS TO WINDS
AND SEAS...WITH NO ADVISORY CONDITIONS TO OCCUR. EVEN WITH SOME
BOOST OF SEA BREEZE CIRCULATIONS WINDS WILL REMAIN AT OR BELOW 12-18
KT AND SEAS WON/T GET ANY HIGHER THAN 3 OR 4 FT.

HOWEVER...MARINERS ARE ADVISED THAT CHANCES OF SHOWERS AND T-STORMS
WILL BE ON THE RISE EARLY NEXT WEEK...PEAKING IN THE CHANCE RANGE
LATER MONDAY OR TUESDAY THROUGH WEDNESDAY. REMEMBER TO HAVE A SAFE
BOATING PLAN IN PLACE BEFORE YOU SET OUT JUST IN CASE YOU DO
ENCOUNTER STORMS.

&&

.CLIMATE...
RECORD HIGHS FOR 30 APRIL...
KCHS 91/1990...
KCXM 88/1987...
KSAV 93/1990...

&&

.EQUIPMENT...
THE CHARLESTON HARBOR NOS TIDE GAUGE HAS STOPPED REPORTING ALL
METEOROLOGICAL AND TIDE DATA. THE NATIONAL OCEAN SERVICE IS
INVESTIGATING THE CAUSE OF THE OUTAGE. AN ESTIMATED TIME FOR DATA
RESTORATION IS UNKNOWN AT THIS TIME.

&&

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

&&

$$

ST



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