National Weather Service Weather Forecast Office

Area Forecast Discussion

790
FXUS62 KCHS 181941
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
341 PM EDT TUE JUN 18 2013

.SYNOPSIS...
A COLD FRONT WILL APPROACH FROM THE NORTHWEST TONIGHT...THEN
SLOWLY PROGRESS THROUGH THE AREA WEDNESDAY AND THURSDAY. HIGH
PRESSURE WILL THEN BUILD FROM THE NORTH INTO THE WEEKEND AND
SETTLE OVER THE ATLANTIC EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
TONIGHT...THE LARGE SCALE PATTERN FEATURES AN EXPANSIVE TROUGH
OVER THE EASTERN STATES...WITH A DECENT MID LEVEL IMPULSE TO MOVE
THROUGH THIS EVENING. THIS SHORT WAVE IN SYNC WITH A COUPLED JET
STRUCTURE OVER EASTERN TN AND WESTERN NC IS PROVIDING AMPLE
FORCING FOR ASCENT...AND WILL DO SHOW INTO THIS EVENING. AT THE
SURFACE THERE IS GOOD FORCING AS WELL...THE RESULT OF AN
INCREASING 20-30 KT LOW LEVEL JET IN ADVANCE OF A SLUGGISH MOVING
COLD FRONT EXTENDING SW FROM A WAVE OF LOW PRESSURE IN WEST-
CENTRAL NC. VARIOUS SMALL SCALE BOUNDARIES ARE EVIDENT...THE
RESULTANT SEA BREEZE WHICH IS CRANKING UP NEAR 20
MPH...DIFFERENTIAL HEATING BOUNDARIES...LAKE MARION/MOULTRIE
BREEZES AND OUTFLOWS FROM BOTH ONGOING AND UPSTREAM CONVECTION.
THIS ALL LEADS TO SCATTERED TO LIKELY POPS OVER MUCH OF THE
FORECAST AREA INTO THIS EVENING WHEN THE SHORT WAVE PULLS EAST. A
FEW OF THE STORMS WILL BE STRONG OR SEVERE...WITH DECENT
THERMODYNAMICS...SUCH AS SBCAPES OF 2000-2500 J/KG...LIFTED
INDICES -6 TO -8C AND BETTER MID LEVEL LAPSE RATES THAN LATE AT
6-6.5 C/KM. THE UPDRAFTS WILL BECOME ENHANCED BY THE DYNAMICS
ALOFT AND THE UPWARD MOTION CAUSED BY THE LIFTING DUE TO THE
BOUNDARIES IN THE LOWER TROPOSPHERE. THIS TAPS INTO SOME DRY AIR
SHOWN ON SOUNDINGS BETWEEN ABOUT 8-18K FEET...WITH THE RESULTING
DCAPES AS HIGH AS 800-1200 J/KG. THUS MICROBURSTS ARE THE MAIN
WEATHER HAZARD THROUGH ABOUT 8 OR 9 PM...WITH ISOLATED SEVERE
WEATHER TO PERSIST DURING THIS TIME.

TEMPORARY NVA DEVELOPS IN WAKE OF THE SHORT WAVE LATER THIS
EVENING AND OVERNIGHT...BUT THE COLD FRONT DRAWS CLOSER TO THE
CWFA LATE. WITH THE FORECAST AREA IN CLOSE PROXIMITY TO THE RIGHT
REAR QUAD OF THE UPPER 60-75 KT NEAR THE APPALACHIANS...WE/LL HAVE
TO HOLD ONTO AT LEAST SMALL POPS DURING THE LATE EVENING AND
OVERNIGHT HOURS. THE FRONT THOUGH BECOMES ALIGNED WITH THE FLOW
ALOFT AS IT POSITIONS ITSELF MORE EAST-WEST ORIENTED...TRAILING
FROM THE SURFACE WAVE THAT CLOSES INTO A WEAK LOW OFF THE MID
ATLANTIC REGION OVERNIGHT. GIVEN THAT THERE LOOKS TO BE DECENT
CINH IN PLACE AND SOME STABILIZATION OCCURS THIS LOOKS TO LIMIT
THE STRENGTH OF ANY NOCTURNAL ACTIVITY.

MEANWHILE...PWATS ARE NEAR THE 75TH PERCENTILE THROUGH
TONIGHT...AS HIGH AS 1.9 TO 2.0 INCHES. WHILE THERE IS ENOUGH
PROGRESSION TO SHOWERS AND T-STORMS...THE SATURATED CONDITIONS
COULD NONETHELESS STILL PRODUCE SOME LOCALLY HEAVY RAINFALL. AND
SOME OF THE GROUNDS ARE STILL RATHER SOAKED...MOST NOTABLY A
SWATH OF 1-3 INCH AND LOCALLY HIGHER AMOUNTS ACROSS MUCH OF THE
NORTHERN AND CENTRAL ZONES FROM RAINS WITHIN THE PAST 24-36 HOURS.
SO IT MIGHT NOT TAKE MUCH TO PRODUCE AT LEAST MINOR FLOODING
CONCERNS.

EVAPORATIVE COOLING DUE TO THE RAIN HAS BEEN IMPLIED IN THE HOURLY
TEMP GRIDS THROUGH THIS EVENING...WITH MID AND UPPER 70S WHERE
RAIN OCCURS THROUGH SUNSET. WHERE RAIN DOESN/T FALL TEMPS WILL
BE IN THE 80S FOR SEVERAL MORE HOURS. THEREAFTER...THE TEMP CURVE
WILL SHOW FAIRLY STEADY TEMPS...BEFORE LATE NIGHT HEAT LOSS IS
MAXIMIZED AND LOWS DROP TO THE LOWER AND MIDDLE 70S MOST
COMMUNITIES.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
WEDNESDAY AND THURSDAY...A COLD FRONT WILL SLOWLY SHIFT OVER
SOUTHEAST SOUTH CAROLINA AND SOUTHEAST GEORGIA BY WEDNESDAY MORNING
BEFORE GRADUALLY STALLING SOUTH OF THE REGION BY THURSDAY. AS THIS
OCCURS...PWATS NEAR 2.0 INCHES ALONG WITH MID LVL FORCING FROM AN H5
SHORTWAVE SHOULD FAVOR PERIODS OF SHOWERS/THUNDERSTORMS THROUGH MUCH
OF WEDNESDAY...ESPECIALLY ALONG/NEAR THE PASSING COLD FRONT. HAVE
THEREFORE INCREASED POPS TO 50-60 WEDNESDAY AFTERNOON MAINLY ALONG
COASTAL LOCATIONS IN SOUTHEAST GEORGIA CLOSER TO THE FRONT. AT THIS
TIME...CLOUD COVER WILL REMAIN A LIMITING FACTOR FOR STRONGER
THUNDERSTORMS TO DEVELOP OVER THE REGION. HOWEVER...WEAK WIND FIELDS
WILL LIKELY RESULT IN SLOW STORM MOTIONS THAT MAY CONTRIBUTE TO
HEAVY RAINFALL DURING A FEW THUNDERSTORMS.

A GRADUAL DECREASE IN OVERALL POPS SHOULD THEN OCCUR FROM NORTH TO
SOUTH WEDNESDAY NIGHT INTO THURSDAY AS THE FRONT SLOWLY SLIPS TO THE
SOUTH OF THE FORECAST AREA...BUT HAVE MAINTAINED A MENTION OF PRECIP
OVER ALL LOCATIONS THROUGH THURSDAY NIGHT...WITH 20 POPS OVER INLAND
SOUTH CAROLINA TO 30-40 POPS OVER LOCATIONS SOUTH OF INTERSTATE 16
IN GEORGIA. OVERALL HIGH TEMPS ARE EXPECTED TO PEAK IN THE MID/UPPER
80S OVER MOST LOCATIONS WEDNESDAY AND THURSDAY...WITH SLIGHTLY COOLER
TEMPS IN THE MID 80S OVER SOUTH CAROLINA BEHIND THE FRONT. OVERNIGHT
LOWS ARE EXPECTED TO BE IN THE UPPER 60S TO LOWER 70S.

FRIDAY...SURFACE HIGH PRESSURE WILL BUILD INTO THE REGION FROM THE
NORTH...WITH THE PATTERN EVENTUALLY TRANSITIONING INTO AN INLAND
WEDGE/COASTAL TROUGH TYPE SCENARIO. A FEW MODELS INDICATE A WEAK
WAVE/SFC LOW DEVELOPING ALONG THE STALLED FRONT JUST OFFSHORE LATE
MORNING/EARLY AFTERNOON BEFORE DRIFTING ONSHORE DURING THE EVENING.
A SLIGHT CHANCE TO CHANCE OF SHOWERS AND/OR THUNDERSTORMS WILL
THEREFORE BE MAINTAINED IN THE CURRENT FORECAST...WITH HIGHEST
CHANCES OVER SOUTHEAST GEORGIA COUNTIES. OVERALL HIGH TEMPS SHOULD
REMAIN IN THE MID/UPPER 80S WITH SLIGHTLY COOLER TEMPS ALONG COASTAL
COUNTIES AND THE TRI COUNTY AREA DUE TO ENHANCED CLOUD COVER AND
HIGH PRESSURE NUDGING IN FROM THE NORTH.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
A WEAK INLAND WEDGE/COASTAL TROUGH PATTERN WILL PERSIST INTO
SATURDAY...BEFORE THE TROUGH DAMPENS BY SUNDAY AS HIGH PRESSURE
SETTLES OVER THE ATLANTIC. THE FORECAST AREA WILL THEN REMAIN
ALONG THE WESTERN PERIPHERY OF THE ATLANTIC HIGH PRESSURE INTO
EARLY NEXT WEEK. SHOWER/THUNDERSTORM CHANCES WILL RANGE FROM
AROUND 20 TO 30 PERCENT EACH AFTERNOON AND EARLY EVENING...BEFORE
DIMINISHING OVERNIGHT AS MORE TYPICAL FOR EARLY SUMMER. THE
INITIALLY WEAK AND ILL DEFINED UPPER LEVEL PATTERN WILL TRANSITION
INTO BROAD RIDGING ALOFT...SUPPORTING A GRADUAL WARMING TREND
THROUGH THE PERIOD. HIGH TEMPERATURES AROUND 90 DEGREES AWAY FROM
THE COASTLINE SATURDAY AND SUNDAY WILL WARM INTO THE LOWER 90S BY
MONDAY AND TUESDAY.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
CONVECTION WILL CONTINUE TO FIRE IN ADVANCE OF A COLD FRONT AND
ALONG NUMEROUS MESO-SCALE BOUNDARIES...WITH ADDITIONAL FORCING FOR
ASCENT TO RESULT FROM A SHORT WAVE THAT MOVES THROUGH LATE THIS
AFTERNOON/EVENING. WE HAVE SHOWN TEMPORARY FLIGHT RESTRICTIONS
FROM TSRA INTO THE MVFR RANGE FROM ABOUT 20-21Z THROUGH 24-01Z AT
BOTH SITES AS THE SHORT WAVE MOVES THROUGH. GUSTY WINDS...HEAVY
RAINS AND POSSIBLE IFR CONDITIONS WILL ALSO OCCUR. IN WAKE OF THE
SHORT WAVE THE AIRFIELDS WILL REMAIN VFR TONIGHT...BUT THE COLD
FRONT WILL APPROACH LATE TONIGHT/WEDNESDAY MORNING AND STALL. WITH
THE FRONT CLOSE BY ADDITIONAL SHRA/TSRA WILL DEVELOP AGAIN EARLY
WEDNESDAY. FLIGHT RESTRICTIONS ARE AGAIN POSSIBLE.

EXTENDED AVIATION...MVFR CIGS/VSBYS POSSIBLE WEDNESDAY THROUGH
THURSDAY DUE TO SHOWERS AND/OR THUNDERSTORMS....WITH THE GREATEST
CHANCE BEING WEDNESDAY AFTERNOON. LOW CHANCE OF MVFR CIGS/VSBYS
FRIDAY AFTERNOON/EVENING WITH A COASTAL TROUGH/SFC LOW SHIFTING
ONSHORE.

&&

.MARINE...
TONIGHT...THE PRESSURE GRADIENT IS CONTINUING TO TIGHTEN BETWEEN
THE STRONG SUB-TROPICAL ATLANTIC RIDGE TO THE SE AND AN UPSTREAM
COLD FRONT TO THE NW. THAT ALONG WITH SEA BREEZE CIRCULATION IS
GENERATING SOME GUSTS NEAR AND OVER 20 KT AT VARIOUS COASTAL SITES
AND IN CHARLESTON HARBOR. THE PACKING OF THE ISOBARS WILL BECOME
GREATEST EARLY TONIGHT BETWEEN THE COLD FRONT AND ATLANTIC
HIGH...AS GEOSTROPHIC WINDS CLIMB TO 25-30 KT. THIS IS SUPPORTIVE
OF A MARGINAL SCA ACROSS OUR OUTER GA WATERS FOR SOME 6 FOOTERS
AND FREQUENT GUSTS OF 25 KT. THE OTHER WATERS LOOK TO REMAIN JUST
BELOW ANY HEADLINE CONDITIONS...WITH WINDS PEAKING AT 15 OR 20 KT
AND SEAS AT 3-5 FT.

WEDNESDAY THROUGH SATURDAY...WINDS/SEAS ARE EXPECTED TO REMAIN BELOW
SMALL CRAFT ADVISORY CRITERIA AS A COLD FRONT SHIFTS OVER THE
COASTAL WATERS WEDNESDAY BEFORE STALLING NEAR/OVER SOUTHERN GA
WATERS ON THURSDAY. AS THE FRONT SLOWLY SHIFTS SOUTH...SOUTHERLY
WINDS WILL VEER BEHIND THE FRONT AND EVENTUALLY BECOME
NORTHEAST/EAST THURSDAY. HIGH PRESSURE WILL THEN BUILD FROM THE
NORTH OVER THE WATERS INTO EARLY FRIDAY...WITH WIND SPEEDS
APPROACHING 15-20 KTS OVER NORTHERN SC WATERS AND SEAS BUILDING TO
3-4 FT. ENHANCED EAST/NORTHEAST FLOW MAY CONTINUE INTO SATURDAY
AS A COASTAL TROUGH DEVELOPS OFFSHORE AND GRADUALLY SHIFTS OVER LAND.

RIP CURRENTS...WE HAVE MAINTAINED A MODERATE RISK AT THE BEACHES
THROUGH THIS EVENING...DUE TO SOUTH AND SW WINDS AT 15 TO 20 MPH
AND A 2 FOOT SWELL EVERY 10 SECONDS. THE RISK FOR TOMORROW HOWEVER
IS LOW WITH MUCH LESS WIND.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EDT WEDNESDAY FOR AMZ374.

&&

$$

SYNOPSIS...
NEAR TERM...
SHORT TERM...DPB
LONG TERM...WMS
AVIATION...
MARINE...