Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 210821
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
421 AM EDT SUN SEP 21 2014

.SYNOPSIS...
HIGH PRESSURE WILL BRIEFLY BUILD OVER THE AREA TODAY AND TONIGHT. A
COLD FRONT WILL THEN ADVANCE THROUGH THE REGION MONDAY AND STALL
SOUTH OF THE AREA THROUGH NEXT WEEKEND. A BROAD AREA OF HIGH
PRESSURE WILL BUILD IN FROM THE NORTH TUESDAY AND WILL PREVAIL
INTO NEXT WEEKEND...WHILE A TROUGH OF LOW PRESSURE DEVELOPS AND
STRENGTHENS OFF THE COAST.

&&

.NEAR TERM /THROUGH TONIGHT/...
SKIES HAVE BEEN CLEARING OUT OVERNIGHT WITH THE CHARLESTON TRI-
COUNTY REGION THE LAST TO SEE CLOUDS SCOUR OUT AS THE UPPER LEVEL
DEFORMATION ZONE SLOWLY PULLS AWAY TO THE NE. MEASURABLE RAINS
HAVE ENDED ALONG THE SANTEE RIVER AREA WITH A FEW SPRINKLES PRIOR
TO 6 AM BEFORE SKIES CLEAR THERE. BY DAYBREAK...CLEAR SKIES SHOULD
BE THE RULE JUST ABOUT ALL LOCALES WITH TEMPS IN THE MID TO UPPER
60S.

IN THE WAKE OF THE DEPARTING SURFACE LOW OFF THE CAROLINA COAST
TODAY...SIGNIFICANT MID LEVEL HEIGHT RISES AND A DOWNSLOPING LOW
LEVEL WIND COMPONENT SHOULD ENSURE A MUCH WARMER DAY...ESPECIALLY
ALONG COASTAL AREAS WHERE HIGH TEMPS SATURDAY FAILED TO TOP THE MID
70S UNDER CLOUD COVER AND STEADY NNE WINDS. FULL-SUNSHINE LOW LEVEL
THICKNESS VALUES SUGGEST SOME AREAS MAY TOUCH 90 DEGREES THIS
AFTERNOON AND WITH THE SUBSIDENCE AND SINKING PROFILES...THIS SEEMS
PLAUSIBLE.

EXPECT A TRANQUIL OVERNIGHT PERIOD WITH LOW TEMPS RANGING FROM THE
MID 60S INLAND TO 70-75 ON THE BEACHES WHERE W/SW WINDS WILL PERSIST.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
A POTENT SHORTWAVE DROPPING INTO THE CAROLINAS ON MONDAY WILL DROP A
COLD FRONT INTO THE AREA DURING THE AFTERNOON. A WARM ADVECTION
REGIME AND PRE-FRONTAL COMPRESSION WILL RESULT IN A WARM DAY WITH
HIGH TEMPS IN THE UPPER 80S TO AROUND 90. HOWEVER THERE SHOULD BE
DECENT MOISTURE POOLING ALONG AND AHEAD OF THE FRONT. DEWPOINTS WILL
RISE INTO THE MIDDLE 60S DURING THE AFTERNOON...WHICH COMBINED WITH
THE UPPER 80S HIGH TEMPS COULD YIELD SURFACE-BASED CAPES BETWEEN
1000 AND 1500 J/KG. SCATTERED TSTMS ARE EXPECTED TO DEVELOP IN THE
EARLY AFTERNOON AND CONTINUE THROUGH THE EVENING AS THE FRONT DROPS
SOUTH. ALTHOUGH A FEW STRONG/SEVERE STORMS CANNOT BE RULED OUT...
LAPSE RATES ARE NEARLY MOIST ADIABATIC WHICH WILL LIMIT UPWARD
VERTICAL MOTION. HOWEVER THERE WILL BE A CONSIDERABLE AMOUNT OF LOW
AND MID LEVEL DRY AIR WHICH MAY SUPPORT ISOLATED DOWNBURSTS.

MONDAY NIGHT INTO WEDNESDAY THE ECMWF DIVERGES FROM THE GFS/NAM/CAN
BY CUTTING OFF THE UPPER SHORTWAVE ACROSS THE DEEP SOUTH AND MOVING
THE COASTAL TROUGH INLAND BY WEDNESDAY. INTERESTINGLY THIS IS
SIMILAR TO THE LAST TWO RUNS OF THE ECMWF...THOUGH THE REMAINDER OF
THE MODEL SUITE CONTINUES TO INSIST ON THE MORE PROGRESSIVE UPPER
PATTERN. WE HAVE TAKEN A MIDDLE GROUND BY KEEPING MOST OF THE
EXTENSIVE PRECIP OVER THE COASTAL WATERS BUT BROUGHT 30-40 POPS INTO
COASTAL AREAS ESPECIALLY WEDNESDAY. IF THE ECMWF WERE TO VERIFY
PERFECTLY WE WOULD SEE MORE EXPANSIVE PRECIP ACROSS ALL LAND AREAS
ON WEDNESDAY. COLD ADVECTION WITH HIGH PRESSURE BUILDING IN FROM THE
NORTH AND INCREASING ISENTROPIC CLOUD COVER WILL DROP HIGH TEMPS
INTO THE LOWER 80S TUESDAY AND UPPER 70S WEDNESDAY.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
THE LONG TERM PERIODS LOOK RATHER WET AS BROAD UPPER LEVEL LOW
PRESSURE LINGERS OVER THE SOUTHEAST UNITED STATES AND AN EXPANSIVE
COASTAL TROUGH JUST OFF THE COAST MAINTAINS MOISTURE AND LIFT ACROSS
THE AREA. A COOL WEDGE OF HIGH PRESSURE WILL REMAIN TO THE NORTH
THROUGH SATURDAY BEFORE SHIFTING EAST INTO THE ATLANTIC SUNDAY. WE
EXPECT FAIRLY GOOD COVERAGE OF SHOWERS AND ISOLATED TSTMS WEDNESDAY
NIGHT THROUGH FRIDAY WITH DECREASING RAIN CHANCES OVER THE WEEKEND
AS AN UPPER RIDGE BUILDS OVER THE WESTERN ATLANTIC. TEMPS WILL
REMAIN AT OR BELOW NORMAL.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
VFR CONDITIONS WILL PREVAIL AT KSAV/KCHS THROUGH 12Z MONDAY.

EXTENDED AVIATION OUTLOOK...BRIEF FLIGHT RESTRICTIONS POSSIBLE IN
THUNDERSTORMS MONDAY AFTERNOON INTO MONDAY NIGHT.

&&

.MARINE...
WITH A GRADUALLY WEAKENING PRES GRADIENT AND BACKING DIRECTIONAL
TRENDS TO A MORE OFFSHORE COMPONENT WE INITIALIZED BY LOWERING
ADVISORY FLAGS SOUTH OF EDISTO BEACH INCLUDING THE NEAR SHORE
GEORGIA WATERS. WE WILL ALLOW UNTIL LATE MORNING BEFORE DESCENDING
ADVISORIES OFF THE CHARLESTON COUNTY COAST AND BEYOND 20 NM OFF
THE GEORGIA COAST. A PROGRESSIVE PATTERN THROUGH TONIGHT FOR THE
MARINE AREA WITH OFFSHORE WINDS BECOMING SW TONIGHT AHEAD OF THE
NEXT FRONT APPROACHING FROM THE NW. WITH A LOW LEVEL WARM AIR
ADVECTION PATTERN...THERE SHOULD BE A SURGE WITH SW FLOW TONIGHT
BUT GENERALLY BELOW 20 KT. SEAS SHOULD COME DOWN NICELY TODAY AND
REMAIN 2-4 FT...HIGHEST WELL OFFSHORE.

A FAIRLY WEAK GRADIENT WILL EXIST ON MONDAY BEFORE A COLD FRONT
DROPS THROUGH THE WATERS. HIGH PRESSURE BUILDING FROM THE NORTH
MONDAY NIGHT THROUGH LATE WEEK WILL ALLOW A STRONG COASTAL TROUGH TO
DEVELOP WITH PERSISTENT NORTHEAST FETCH OVER THE WATERS. THE
STRONGEST GRADIENT IS NOW EXPECTED TUESDAY INTO WEDNESDAY...THEN THE
TROUGH MAY BROADEN AND SHIFT INLAND LATE IN THE WEEK. SMALL CRAFT
ADVISORY CONDITIONS ARE LIKELY ACROSS ALL OR PORTIONS OF THE WATERS
DURING MID-WEEK...POSSIBLY PERSISTING INTO LATE WEEK.

&&

.TIDES/COASTAL FLOODING...
LOW CHANCES FOR SHALLOW SALT WATER FLOODING TODAY AND MONDAY...THEN
INCREASING NORTHEAST FLOW TUESDAY THROUGH THE WEEKEND WILL RESULT IN
MODERATE CHANCES FOR COASTAL FLOODING. IT WILL TAKE MORE THAN A 1
FOOT POSITIVE ANOMALY TO PRODUCE FLOODING SO IT IS NOT A SLAM DUNK
BUT CHANCES ARE FAIRLY GOOD FOR CHARLESTON HARBOR TO REACH 7.0 FT
MLLW WITH SOME EVENING HIGH TIDES.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 AM EDT THIS MORNING FOR AMZ350-
     374.

&&

$$

NEAR TERM...
SHORT TERM...JRL
LONG TERM...JRL
AVIATION...
MARINE...JRL
TIDES/COASTAL FLOODING...





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