Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 251957
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
357 PM EDT SAT APR 25 2015

.SYNOPSIS...
A COLD FRONT WILL MOVE SOUTH THROUGH THE AREA SUNDAY EVENING INTO
SUNDAY NIGHT. DRY HIGH PRESSURE WILL PREVAIL MONDAY INTO TUESDAY.
A GULF OF MEXICO STORM SYSTEM WILL IMPACT THE REGION MID WEEK WITH
DRY HIGH PRESSURE THEN LIKELY RETURNING THROUGH THE END OF THE
WEEK.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
A RATHER SIGNIFICANT NULL IN CONVECTION IS IN PROGRESS ACROSS THE
REGION LATE THIS AFTERNOON AS WEAK SUBSIDENCE HAS OVERSPREAD THE
AREA IN THE WAKE OF A WEAK MID-LEVEL DISTURBANCE. SATELLITE SHOWS
SOME SIGNIFICANT BREAKS IN THE CLOUD CANOPY OVER GEORGIA WHICH IS
HELPING TO PUSH TEMPERATURES BACK INTO THE UPPER 70S TO LOWER 80S
ACROSS SOUTHEAST GEORGIA WITH MID 80S ALONG THE ALTAMAHA RIVER.
THE ATMOSPHERE ACROSS SOUTHEAST SOUTH CAROLINA AND SOUTHEAST
GEORGIA WILL CONTINUE TO DESTABILIZE OVER THE NEXT FEW HOURS AS
POCKETS OF FOCUSED INSOLATION HELP TO DRIVE TEMPERATURES UP AS
DEWPOINTS RISE INTO THE LOWER 70S. MODIFIED RAP/H3R POINT
SOUNDINGS SUPPORT SBCAPES NEAR 2000 J/KG WITH LIFTED INDICES AS
LOW AS -4 TO -5C AND SHOWALTER INDICES -2 TO -3C--MODERATE VALUES
FOR LATE APRIL. LOW-LEVEL CONVERGENCE IS PROGGED TO INCREASE
ACROSS CENTRAL GEORGIA OVER THE NEXT FEW HOURS AS UPPER FORCING
ASSOCIATED WITH THE RIGHT ENTRANCE REGION OF THE UPPER JET
OVERSPREAD THE DEEP SOUTH. KMXX/KJGX RADAR TRENDS ALREADY SHOW A
RECENT INCREASE IN SHOWER/TSTM ACTIVITY ALONG THE ALABAMA/GEORGIA
LINE...WHICH SUGGESTS THE FORCING WITH THE UPPER JET IS FINALLY
BEGINNING TO CATCH UP TO THE RIBBON OF HIGHER INSTABILITY THAT IS
BEGINNING TO EXPAND OVER SOUTHERN GEORGIA INTO FAR SOUTHERN SOUTH
CAROLINA.

THE MAIN QUESTION FOR THE EVENING HOURS IS HOW FOCUSED WITH ANY
RISK FOR SEVERE WEATHER BE. HIGH RESOLUTION MODELS...INCLUDING THE
H3R...HAVE BEEN INDICATING SOME SORT OF QLCS MOVING ACROSS
SOUTHEAST GEORGIA AND POSSIBLY AS FAR NORTH AS BEAUFORT OVER THE
NEXT SEVERAL HOURS AS THE CORRIDOR OF JET ENHANCED UPPER FORCING
TRAVERSES THE AREA. 0-6KM BULK SHEAR OF 55-65 KT IS CERTAINLY
SUPPORTIVE OF CONVECTIVE ORGANIZATION WITH POSSIBLE SUPERCELLS...
BUT THE EVENTUAL DEGREE OF INSTABILITY AND DEEP LAYERED FORCING
WILL BE ULTIMATE DETERMINING FACTORS ON HOW CONCENTRATED THE
SEVERE THREAT WILL BECOME. GIVEN THE CONSISTENT TRENDS NOTED IN
THE HIGH RESOLUTION MODELS ALL DAY...ITS HARD TO IGNORE THE
QLCS/BOW ECHO SIGNAL NOTED IN THE MODEL REFLECTIVITY PROGS.
CONVECTIVE TRENDS CONTINUE TO BE MONITORED AND A SEVERE TSTM OR
TORNADO WATCH MAY EVENTUALLY BE NEEDED FOR PARTS OF THE FORECAST
AREA OVER THE NEXT FEW HOURS.

THE PRIMARY HAZARDS LOOK TO BE DAMAGING WINDS...BUT ISOLATED
TORNADOES CAN NOT BE RULED OUT GIVEN THE DEGREE OF 0-1KM SR-
HELICITY IN PLACE. THE RISK FOR SEVERE WEATHER LOOKS TO BE THE
HIGHEST ROUGHLY ALONG/SOUTH OF A METTER-SPRINGFIELD-BEAUFORT
LINE...INCLUDING THE SAVANNAH METRO AREA. THE SEVERE PROBABILITIES
DROP OFF QUITE A BIT NORTH OF THIS LINE...INCLUDING THE
CHARLESTON TRI-COUNTY AREA.

WILL CARRY CHANCE TO LIKELY POPS ACROSS THE AREA THIS EVENING...
HIGHEST SOUTH OF THE I-16 CORRIDOR. WILL REFRAIN FROM INCLUDING
SEVERE WEATHER ATTRIBUTES IN THE GRIDDED AND TEXT FORECASTS FOR
NOW UNTIL CONVECTIVE TRENDS BECOME A BIT MORE APPARENT. ONCE THE
QLCS EXITS THE COAST...MAINLY DRY CONDITIONS WILL PREVAIL WITH
CLEARING SKIES. THE BOUNDARY LAYER LOOKS TO REMAIN COUPLED THROUGH
THE NIGHT WITH MODEST LOW-LEVEL JETTING REMAINING IN PLACE. THE
RISK FOR FOG LOOKS LOW AS A RESULT...BUT SOME AREAS OF LOW STRATUS
COULD DEVELOP JUST BEFORE SUNRISE...ESPECIALLY IN SOUTHEAST
GEORGIA. LOWS WILL ONLY DROP INTO THE UPPER 60S WITH A FEW MID 60S
POSSIBLE FOR AREAS BORDERING THE SOUTHERN/EASTERN MIDLANDS AND THE
SANTEE RIVER.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
SUNDAY...LOW PRESSURE WILL SHIFT OFF THE MID ATLANTIC COAST
EARLY...ALLOWING A BACKDOOR COLD FRONT TO APPROACH THE SOUTHEAST
DURING THE DAY. DEEP WESTERLY FLOW AHEAD OF THE FRONT WILL ALLOW
FAIRLY SIGNIFICANT HEATING OVER THE AREA...WITH AFTERNOON TEMPS
EXPECTED TO PEAK IN THE UPPER 80S TO AROUND 90 DEGREES. GIVEN THIS
AMOUNT OF HEATING...LOW LVL MIXING WILL BE ENHANCED AND WILL LIKELY
RESULT IN BREEZY CONDITIONS THROUGHOUT THE AFTERNOON AS A 30-40 KT
LOW LVL JET PASSES OVER THE SOUTHEASTERN UNITED STATES. AS THE FRONT
BEGINS TO ENTER NORTHERN AREAS DURING THE EVENING...IT WILL BE
SOMEWHAT MOISTURE STARVED. HOWEVER...WE COULD SEE A FEW SHOWERS
AND/OR THUNDERSTORM DURING LATE AFTERNOON INTO EARLY OVERNIGHT HOURS
AS THE FRONT CONTINUES A SOUTHWARD SHIFT OVER THE AREA. SFC WINDS
WILL QUICKLY BECOME NORTH BEHIND THE FRONT...WITH RAPIDLY COOLING
AND DRYING CONDITIONS FROM NORTH TO SOUTH SUNDAY NIGHT. IN
GENERAL...LOW TEMPS WILL RANGE IN THE MID 50S NORTH TO LOW 60S
SOUTH.

MONDAY AND TUESDAY...DRY HIGH PRESSURE WILL BUILD FROM THE
NORTH/NORTHWEST BEHIND A COLD FRONT WELL TO OUR SOUTH. AS A
RESULT...TEMPS WILL BE MUCH COOLER WITHIN NORTHERLY FLOW. IN
GENERAL...MONDAY AFTERNOON HIGHS WILL RANGE FROM THE LOW TO MID
70S...THEN COOL A FEW MORE DEGREES ON TUESDAY AS HIGH PRESSURE
BECOMES CENTERED OVER THE SOUTHEAST. BY TUESDAY EVENING...MOISTURE
RETURNS AS ONSHORE FLOW DEVELOPS WELL AHEAD OF LOW PRESSURE SHIFTING
OVER THE NORTHERN GULF OF MEXICO. A FEW SHOWERS WILL BE
POSSIBLE...MAINLY SOUTH OF INTERSTATE 16 BEFORE SUNSET.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
RAIN CHANCES WILL BE INCREASING FROM THE SOUTHWEST TUESDAY NIGHT AS
DEVELOPING LOW PRESSURE ALONG THE NORTHERN GULF COAST MOVES TOWARD
THE AREA ALONG WITH DEEPER MOISTURE. STILL SOME TIMING/STRENGTH
DIFFERENCES THOUGH WITH THIS SYSTEM WITH THE ECMWF CONTINUING TO BE
FASTER AND STRONGER WITH THE LOW AS IT MOVES THROUGH SOUTHEAST
GEORGIA WEDNESDAY/WEDNESDAY NIGHT. THE LOW SHOULD BE PULLING AWAY
FROM THE AREA THURSDAY WITH IMPROVING CONDITIONS...ALTHOUGH CANNOT
RULE OUT SOME LINGERING SHOWERS AS DUE TO THE SLOW PROGRESSION OF
THE UPPER TROUGH. FRIDAY AND SATURDAY LOOK DRIER WITH TEMPS GETTING
BACK TO AT OR JUST ABOVE NORMAL.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
CONDITIONS WILL RETURN TO VFR SHORTLY AT BOTH KCHS/KSAV IN THE
WAKE OF SHOWERS/TSTMS. EXPECT TSTMS TO REDEVELOP ALONG/SOUTH I-16
OVER THE NEXT FEW HOURS WHICH COULD IMPACT KSAV. INTRODUCED A
TEMPO GROUP FROM 22-01Z TO ACCOUNT FOR THIS. NO IMPACTS ARE
EXPECTED ATTM AT KCHS. ACTIVITY WILL DIMINISH BY MID-EVENING WITH
VFR PREVAILING. SOME CONCERN FOR CIGS/PATCHY FOG TO REDEVELOP LATE
AS SKIES CLEAR...BUT ENOUGH WIND SHOULD PREVENT TOO MUCH FOG.
BETTER CHANCES FOR CIGS WILL BE KSAV WHERE MORE RAIN WILL FALL.
WILL SHOW MVFR CIGS THERE FROM 09-12Z...BUT KEEP KCHS VFR FOR NOW.

EXTENDED AVIATION OUTLOOK...BREEZY SUNDAY AFTERNOON WITH GUSTS OF 25
TO POSSIBLY 30 KTS. VFR CONDITIONS SHOULD THEN PREVAIL INTO TUESDAY
BEFORE SHOWERS AND TSTMS RETURN MAINLY TUESDAY NIGHT INTO WEDNESDAY
NIGHT.

&&

.MARINE...
TONIGHT...GUSTY WINDS NEAR 40 KT BRIEFLY OCCURRED EARLIER ACROSS
THE SOUTHERN SOUTH CAROLINA WATERS IN RESPONSE TO MESOSCALE
INFLUENCES FROM A WAKE LOW. THIS FEATURE IS NOW WELL OFFSHORE AND
CONDITIONS WILL SUBSIDE A BIT. MODEST SOUTH/SOUTHWEST WINDS WILL
PERSIST OVERNIGHT WITH SPEEDS 15-20 KT NEARSHORE WATERS AND 20-25
KT OFFSHORE WATERS. A SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR
THE GEORGIA OFFSHORE LEG THROUGH THE NIGHT. SEAS WILL AVERAGE 2-4
FT NEARSHORE WATERS AND 4-6 FT OFFSHORE WATERS.

SUNDAY THROUGH THURSDAY...WINDS WILL REMAIN ELEVATED FROM THE WEST
ON SUNDAY AHEAD OF A BACKDOOR COLD FRONT THAT PUSHES SOUTH THROUGH
THE WATERS EARLY SUNDAY NIGHT. WE COULD SEE A BRIEF PERIOD OF SMALL
CRAFT ADVISORY LEVEL CONDITIONS IN THE CHS HARBOR SUNDAY AFTERNOON
AS STRONG HEATING OVER LAND PROMOTES MIXING OF A 30-40 KT LOW LVL
JET TO SFC. BEHIND THE COLD FRONT...A NORTHERLY SURGE WILL ENSUE
WITH CONDITIONS NEAR ADVISORY LEVELS MAINLY BEYOND 20 NM SUNDAY
NIGHT. HIGH PRESSURE WILL THEN CONTINUE BUILDING FROM THE NORTH ON
MONDAY...THEN BECOME CENTERED OVER THE AREA ON TUESDAY...LEADING TO
A WEAKENING PRESSURE GRADIENT THROUGH THE PERIOD AND IMPROVED
WINDS/SEAS. CONDITIONS SHOULD THEN GO BACK DOWNHILL AS DEVELOPING
LOW PRESSURE APPROACHES FROM THE SOUTHWEST WEDNESDAY AND PASSES
THROUGH THE AREA WEDNESDAY NIGHT...WITH SMALL CRAFT ADVISORIES
POSSIBLY BEING NEEDED FOR AT LEAST THE WATERS BEYOND 20 NM FROM
SHORE. THERE IS STILL SOME UNCERTAINTY THOUGH REGARDING THE
STRENGTH/TRACK/TIMING OF THE LOW AND THUS FORECAST CONFIDENCE IS LOW
AT THIS TIME.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 11 PM THIS EVENING TO 11 AM EDT
     SUNDAY FOR AMZ374.

&&

$$

ST/DPB


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