Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 180103
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
903 PM EDT FRI APR 17 2015

.SYNOPSIS...
A WEAKENING WEDGE OF HIGH PRESSURE WILL PERSIST INLAND THROUGH
SATURDAY NIGHT. A WARM FRONT WILL LIFT NORTH OF THE REGION
SUNDAY...THEN A COLD FRONT WILL SWEEP THROUGH MONDAY EVENING. HIGH
PRESSURE WILL PREVAIL TUESDAY THROUGH THURSDAY. A WEAK COLD FRONT
COULD PUSH THE REGION LATE WEEK.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
AN EXPANDING AREA OF MAINLY LIGHT RAIN CONTINUES TO PUSH EAST-
NORTHEAST ACROSS SOUTH-CENTRAL GEORGIA THIS EVENING. THE RAIN
ALIGNS WELL WITH AN AREA OF ENHANCED 295-305K ISENTROPIC ASSENT
NOTED ON THE LATEST RAP/NAM ISENTROPIC 4-PANELS. THE RAIN IS
POISED TO AFFECT MUCH OF INTERIOR SOUTHEAST GEORGIA OVER THE NEXT
FEW HOURS...POSSIBLY EXTENDING INTO FAR INTERIOR SOUTHEAST SOUTH
CAROLINA EARLY SATURDAY MORNING WHILE WEAKENING. POPS WILL BE
INCREASED TO 80-90 PERCENT WELL INLAND TO ACCOUNT FOR THIS...
TAPERING OFF TO 30-60 PERCENT CLOSER TO THE COAST. THE WEDGE
AIRMASS REMAINS STABLE ENOUGH TO PRECLUDE TSTM DEVELOPMENT
OVERNIGHT. LOWS IN THE LOWER-MID 60S LOOK REASONABLE...ALTHOUGH
SOME SLIGHT ADJUSTMENTS TO HOURLY TEMPERATURES WILL BE NEEDED.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
SATURDAY...A SHORTWAVE TROUGH WILL BE SHIFTING OFF THE COAST DURING
THE MORNING FOLLOWED BY RIDGING ALOFT DURING THE AFTERNOON. POPS
START THE DAY GRADUATED FROM CHANCE NORTH TO SLIGHT CHANCE
SOUTH/WEST. THE RIDGING/DRYING TREND COULD MINIMIZE OR ELIMINATE THE
POTENTIAL FOR DIURNAL SHOWERS DURING THE AFTERNOON. HOWEVER...
LOCALIZED CONVERGENCE WILL DEVELOP ALONG VARIOUS BOUNDARIES
INCLUDING THE REMNANTS OF THE SURFACE WEDGE REGIME WHICH COULD
ACTUALLY SPREAD TOWARD THE S/SW AND THE SEA BREEZE. SINCE MODEL
SOUNDINGS DEPICT A SUBTLE DRYING TREND...A FEW SHOWERS CANNOT BE
RULED OUT ALONG/NEAR THESE BOUNDARIES...THUS SLIGHT CHANCE POPS
ARE CARRIED THROUGH THE AFTERNOON.

UNCERTAINTY REGARDING THE POSITION/STRENGTH OF THE PERSISTENT WEDGE
AND ASSOCIATED CLOUD COVER TRANSLATES TO AN UNCERTAIN TEMPERATURE
FORECAST SATURDAY. HIGHS RANGING FROM THE MID/UPPER 70S
NORTHERN...COASTAL AND INLAND COUNTIES TO THE LOWER 80S NEAR THE
ALTAMAHA RIVER APPEAR REASONABLE.

SATURDAY NIGHT AND SUNDAY...A LONGWAVE TROUGH ACROSS THE CENTRAL
UNITED STATES WILL SHIFT EAST WITH EMBEDDED SHORTWAVE ENERGY LIFTING
INTO THE SOUTHEAST STATES. FOCUSED LOW/MID-LEVEL MOISTURE TRANSPORT
WILL SURGE NORTH...INITIALLY INTO AREAS WEST OF THE REGION...THEN
THIS WARM CONVEYOR BELT MOISTURE WILL SHIFT EAST INTO THE
REGION...SUPPORTED BY DYNAMIC FORCING FOR ASCENT AS DEPICTED BY
UPPER DIVERGENCE/500 MB HEIGHT FALLS. THE WARM CONVEYOR BELT
FEATURING DEEP MOISTURE/PWATS NEARING 2 INCHES...OVERRUNNING ABOVE
THE SURFACE WEDGE REGIME...STRENGTHENING DYNAMIC FORCING FOR ASCENT
AND A DESTABILIZING ENVIRONMENT AS A SURFACE WARM FRONT PRESSES
NORTH INTO/THROUGH THE REGION WILL TRANSLATE EAST ACROSS THE REGION.
WHILE UNCERTAINTY REMAINS REGARDING IMPORTANT FORECAST
DETAILS...ESPECIALLY PRECIPITATION TIMING...THIS SCENARIO SUPPORTS
POPS RAMPING UP TO LIKELY AT LEAST ACROSS INLAND COUNTIES LATE
SATURDAY NIGHT...THEN LIKELY/CATEGORICAL POPS REMAIN IN ORDER
SUNDAY.

THE HIGH PWATS AND RELATIVELY LONG RESIDENCE TIME OF THE WARM
CONVEYOR BELT/PLUME OF HIGH PWATS SUGGEST THE POTENTIAL FOR LOCALLY
HEAVY RAINFALL...PERHAPS STARTING LATE SATURDAY NIGHT THEN
CONTINUING THROUGH SUNDAY. ALSO...DESTABILIZATION...0-6 KM BULK
SHEAR OF 30-40 KNOTS AND ENHANCED HELICITY SUGGESTS THAT POTENTIAL
FOR AT LEAST ISOLATED SEVERE THUNDERSTORMS ALONG AND SOUTH OF THE
WARM FRONT...PARTICULARLY SUNDAY AFTERNOON/EVENING.

THE WARM CONVEYOR BELT/ASSOCIATED STRONGEST FORCING AND DEEPEST
MOISTURE SHIFTS OFFSHORE SUNDAY NIGHT...THUS COVERAGE OF SHOWERS
SHOULD GRADUALLY DIMINISH FROM CHANCE/WITH TIME. MAINTAINED CHANCE
POPS EVEN OVERNIGHT...BUT STRONGER DRYING/SUBSIDENCE COULD TRANSLATE
TO LOWER POPS FOR A TIME LATE SUNDAY NIGHT INTO MONDAY.

MONDAY...THE LONGWAVE UPPER TROUGH UPSTREAM WILL DRIVE A SURFACE
COLD FRONT TOWARD THE REGION. WARM ADVECTION AND COMPRESSIONAL
HEATING AHEAD OF THE COLD FRONT WILL YIELD A WARM DAY WITH HIGHS IN
THE MIDDLE 80S COMMON. SURFACE DEWPOINTS ARE EXPECTED TO REMAIN IN
THE MIDDLE 60S...AND THERMODYNAMIC/KINEMATIC PARAMETERS ARE
SUFFICIENT TO SUPPORT THUNDERSTORMS AND PERHAPS EVEN AN ORGANIZED
SEVERE WEATHER EVENT AHEAD OF THE COLD FRONT MONDAY
AFTERNOON/EVENING. HOWEVER...MODEL SOUNDINGS DEPICT A TREND TOWARD
CAPPING/STRONGER DRYING ALOFT...AND THIS COULD IN TURN LIMIT THE
COVERAGE/PERSISTENCE OF DEEP MOIST CONVECTION. THUS...POPS ARE
CAPPED AT CHANCE MONDAY AFTERNOON.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
THE COLD FRONT AND ANY ASSOCIATED CONVECTION WILL SHIFT OFFSHORE
MONDAY NIGHT...FOLLOWED BY DRIER HIGH PRESSURE WHICH WILL THEN
PREVAIL FOR MOST OF THE LONG TERM PERIOD. EXPECT HIGHS IN THE UPPER
70S TO LOWER 80S AND LOWS IN THE MIDDLE 50S TO LOWER 60S. A WEAK
COLD FRONT MAY APPROACH OR MOVE THROUGH NEXT FRIDAY WITH ISOLATED
SHOWERS.

&&

.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
LOW CONFIDENCE FOR THE NEXT 12 HOURS. NAM/RAP SOUNDINGS SHOW ONLY
MID-LEVEL CIGS IMPACTING THE TERMINALS OVERNIGHT WHILE MOST OF THE
TABULAR GUIDANCE SHOWS IFR CIGS REDEVELOP LATER THIS EVENING. WITH
THE WEDGE ALREADY BREAKING DOWN...IT IS UNCLEAR WHETHER ENOUGH
LOW-LEVEL SATURATION WILL OCCUR OVERNIGHT TO SUPPORT IFR
CONDITIONS. CIGS WILL LOWER A BIT AS A MORE CONSOLIDATED AREA OF
SHOWERS OVER SOUTHWEST GEORGIA MOVE EAST INTO THE AREA...BUT
UPSTREAM OBSERVATIONS ARE CURRENTLY NO LOWER THAN MVFR. WILL TAKE
A SLIGHTLY MORE CONSERVATIVE APPROACH THAN THE 18Z TAFS SHOWING
MVFR CONDITIONS PREVAILING AT BOTH TERMINALS BY LATE EVENING AND
PERSIST THROUGH SUNRISE WITH A SCATTERED LAYER AT 006-008 FT. IFR
IS STILL A POSSIBILITY...ESPECIALLY GIVEN CONDITIONS OVER THE PAST
TWO NIGHTS...BUT WITH A WEAK WEDGE THAN PREVIOUS DAYS...PREFER NOT
TO FORECAST IFR ATTM.

EXTENDED AVIATION OUTLOOK...A PERIOD OF VFR IS POSSIBLE SATURDAY
AFTERNOON INTO SATURDAY EVENING...THEN FLIGHT RESTRICTIONS SHOULD
PREVAIL LATER SATURDAY NIGHT THROUGH SUNDAY NIGHT. VFR COULD RETURN
MONDAY...ALTHOUGH A COLD FRONT COULD PRODUCE SHOWERS/THUNDERSTORMS
AND ASSOCIATED FLIGHT RESTRICTIONS MONDAY AFTERNOON AND/OR EVENING.
VFR SHOULD THEN PREVAIL LATER MONDAY NIGHT THROUGH MID WEEK.

&&

.MARINE...
TONIGHT...VARIABLE CONDITIONS OVERNIGHT WITH THE MARINE AREA
CAUGHT BETWEEN WEAK LOW PRESSURE OFFSHORE AND A DECAYING INLAND
WEDGE. SEAS WILL AVERAGE 2-4 FT THROUGH THE NIGHT.

SATURDAY THROUGH WEDNESDAY...EXPECT CHANGEABLE WINDS DURING THE
SATURDAY THROUGH TUESDAY PERIOD. WINDS/SEAS ARE EXPECTED TO REMAIN
BELOW SCA THRESHOLDS SATURDAY/SATURDAY NIGHT. A WEAK PRESSURE
PATTERN WILL ALLOW THE SEA BREEZE CIRCULATION TO DOMINATE SATURDAY
AFTERNOON./EVENING...THEN BUILDING HIGH PRESSURE FROM THE NORTH
WILL TURN SYNOPTIC WINDS TOWARD THE EAST/NORTHEAST SATURDAY NIGHT
INTO SUNDAY. A WARM FRONT WILL ADVANCE NORTH THROUGH THE WATERS
LATER SUNDAY OR SUNDAY NIGHT...AND A TIGHTER GRADIENT WILL SUPPORT
STRONGER WINDS WHICH WILL VEER FROM EAST/NORTHEAST TO THE SOUTH.
ELEVATED SOUTH/SOUTHWEST WILL THEN PREVAIL AHEAD OF AN APPROACHING
COLD FRONT MONDAY. SMALL CRAFT ADVISORY WINDS CANNOT BE RULED OUT
SUNDAY THROUGH MONDAY...BUT FOR NOW THE MOST LIKELY SCENARIO CAPS
WINDS AT 20 KNOTS AND SEAS AT 3-5 FT...HIGHEST BEYOND 20 NM.
IN THE WAKE OF COLD FROPA...OFFSHORE WINDS AND SEAS BELOW SCA
THRESHOLDS SHOULD PREVAIL MONDAY NIGHT THROUGH TUESDAY. AS THE
HIGH SHIFTS EAST...WINDS SHOULD VEER TOWARD THE NORTH/NORTHEAST
MIDWEEK.

&&

.TIDES/COASTAL FLOODING...
DUE TO ENHANCED ASTRONOMICAL HIGH TIDES WITH THE UPCOMING PERIGEE
AND NEW MOON...THE RISK FOR SHALLOW COASTAL FLOODING WILL CONTINUE
THROUGH THE WEEKEND. COASTAL FLOOD ADVISORIES MAY BE
NEEDED...MAINLY WITH THE EVENING HIGH TIDES.

&&

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

&&

$$

ST



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