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Area Forecast Discussion

976
FXUS62 KCHS 221946
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
346 PM EDT MON SEP 22 2014

.SYNOPSIS...
A COLD FRONT WILL MOVE ACROSS THE AREA TONIGHT. TUESDAY THROUGH
EARLY NEXT WEEK...STRONG HIGH PRESSURE WILL EXTEND INTO THE REGION
FROM THE NORTH...AND A TROUGH OF LOW PRESSURE WILL PERSIST NEAR
THE COAST.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
REST OF THE AFTERNOON...THUNDERSTORMS ARE NOW FIRING ALONG THE
APPROACHING COLD FRONT AND HAVE MOVED INTO THE FAR INLAND SOUTHEAST
GEORGIA ZONES. RADAR IMAGERY SHOWS A DEVELOPING OUTFLOW BOUNDARY
LEADING THE MULTI-CELL CLUSTERS WHICH SHOULD CONTINUE TO SPARK NEW
DEVELOPMENT AS IT MOVES THROUGH THE FORECAST AREA INTO THE EVENING.
THE ISOLATED SEVERE THREAT REMAINS IN PLACE...ESPECIALLY ACROSS
SOUTHEAST GEORGIA WHERE THE INSTABILITY IS HIGHEST AND DCAPES ARE
HIGHEST. THIS AREA WILL ALSO LIKELY FEEL MOST OF THE EFFECTS OF THE
UPPER FORCING ARRIVING WITH THE UPPER WAVE MOVING IN. POPS HAVE BEEN
ADJUSTED INTO THE CATEGORICAL RANGE FOR MOST LOCATIONS RIGHT ALONG
THE FRONT FOR AN HOUR OR TWO AS A RESULT. STRONG WIND GUSTS CONTINUE
TO BE THE PRIMARY SEVERE WEATHER THREAT...BUT LARGE HAIL CANNOT BE
TOTALLY RULED OUT WITH THE LOWERING FREEZING LEVEL.

TONIGHT...ANY ONGOING THUNDERSTORM ACTIVITY WILL LIKELY FOCUS ALONG
THE COAST EARLY AND WEAKEN INTO MORE OF AN AREA LINGERING STRATIFORM
RAIN AS WE LOSE SURFACE HEATING AND SURFACE BASED INSTABILITY.
HOWEVER...THE PRECIPITATION WILL LIKELY CONTINUE THANKS TO THE
IMPROVING MID/UPPER LEVEL DYNAMICS AS THE APPROACHING SHORTWAVE
CLOSES OFF INTO A UPPER LOW IN THE EARLY MORNING HOURS. 500 MB TEMPS
FALL TO AROUND -13C OR -14C AND THERE WILL ALSO BE UPPER LEVEL
DIVERGENCE IN THE ENTRANCE REGION OF AN UPPER LEVEL JET THAT
STRETCHES INTO NEW ENGLAND. SO THERE WILL BE ENOUGH LARGER SCALE
SUPPORT FOR PRECIPITATION THROUGH MUCH OF THE NIGHT THAT POPS HAVE
BEEN HIKED UPWARDS INTO THE 50-60 PERCENT RANGE FOR MUCH OF THE
FORECAST AREA. PWATS WILL NOT GET TOO MUCH HIGHER THAN 1.5 INCHES AS
MOST OF THE MOISTURE WILL BE IN THE LOWEST 10 KFT OR SO. AT THE
SURFACE...HIGH PRESSURE WILL BEGIN TO BUILD INTO THE NORTHEAST AND
RIDGE DOWN THE EAST SIDE OF THE APPALACHIANS WITH A TIGHTENING
PRESSURE GRADIENT SUPPORTING STRENGTHENING NORTHEAST FLOW. SKIES
WILL BE OVERCAST THROUGH THE NIGHT. AS COOLER AIR FILTERS IN BEHIND
THE FRONT...EXPECT LOWS TO REACH THE LOW 60S INLAND RANGING TO
MID/UPPER 60S ALONG THE COAST.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
IN THE WAKE OF COLD FROPA...THE OVERALL PATTERN WILL FEATURE STRONG
HIGH PRESSURE CENTERED NORTH OF THE REGION...A PERSISTENT TROUGH OF
LOW PRESSURE OVER COASTAL WATERS/CLOSE TO THE COAST AND A CUT-OFF
UPPER LOW MEANDERING OVER OR CLOSE TO THE REGION. THE SYNOPTIC
PATTERN WILL FAVOR PLENTY OF CLOUDS...BELOW NORMAL HIGH TEMPERATURES
MAINLY IN THE 70S AND SOME DEGREE OF POPS MUCH OF THE TIME...TAPERED
FROM SLIGHT CHANCE/CHANCE INLAND TO CHANCE/LIKELY NEAR THE COAST.
CHANGES SHOULD REMAIN SUBTLE...AND FORECAST DETAILS INCLUDING
DEVIATIONS FROM THIS GENERAL SCENARIO WILL BE ADDRESSED WITHIN
SUBSEQUENT FORECAST PACKAGES...ESPECIALLY WITHIN THE UPCOMING NEAR
TERM PERIODS.

HIGHEST PWATS AND DEEPEST MOISTURE SHOULD REMAIN JUST OFF THE COAST.
HOWEVER...SOME POTENTIAL EXISTS FOR DEEP MOISTURE/AN ASSOCIATED
HEAVY RAIN THREAT TO PERIODICALLY BACK INTO COASTAL
COUNTIES...DEPENDING IN PART ON THE ORIENTATION/EVOLUTION OF THE
UPPER LOW.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
PERHAPS ONE OF THE LONGEST HIGH PRESSURE WEDGE PATTERNS IS IN STORE
FOR THE MEDIUM TO LONG RANGE PERIODS. THE EXPANSIVE SURFACE HIGH
OVER THE NORTHEAST STATES LOOKS TO REMAIN IN PLACE THROUGH SATURDAY
NIGHT WHILE A COASTAL TROUGH MAINTAINS CONSIDERABLE CLOUDINESS...
BETTER RAIN CHANCES AND COOL TEMPS. SUNDAY INTO MONDAY THERE ARE
LARGE MODEL DIFFERENCES REGARDING THE POTENTIAL FOR A DEEP LONGWAVE
UPPER TROUGH MOVING INTO THE EASTERN UNITED STATES. A GULF LOW
PRESSURE SYSTEM MAY LIFT NORTHEAST LATE IN THE WEEKEND OR EARLY NEXT
WEEK.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
THE MAIN FORECAST PROBLEM THIS AFTERNOON AND EVENING IS THE
POTENTIAL FOR DIRECT IMPACTS FROM THUNDERSTORMS ASSOCIATED WITH A
COLD FRONT. THE TIMES FOR TEMPO THUNDER REDUCING VISIBILITIES AND
CAUSING GUSTY WINDS HAVE BEEN ADJUSTED BASED ON CURRENT RADAR AND
MODEL TRENDS. KSAV APPEARS TO HAVE THE BEST CHANCE TO SEE IFR
CONDITIONS FOR A BRIEF PERIOD AS THE THUNDERSTORM COVERAGE WILL
LIKELY BE BETTER THERE. BUT...CONFIDENCE ISN/T QUITE HIGH ENOUGH
SO BOTH SITES ARE KEPT IN THE MVFR RANGE FOR THUNDERSTORM
CONDITIONS. ONCE THUNDERSTORMS MOVE OFF THE COAST AND DISSIPATE
THIS EVENING...STRATIFORM RAIN WILL LIKELY DEVELOP AND LINGER
AROUND THE REGION FOR MUCH OF THE OVERNIGHT. THERE ARE SOME
SIGNALS IN THE MODEL DATA THAT PREVAILING MVFR OR LOWER CEILINGS
COULD OCCUR LATE TONIGHT AND INTO TUESDAY MORNING...BUT CONFIDENCE
ISN/T YET HIGH ENOUGH TO INTRODUCE INTO THE FORECAST.
INSTEAD...WILL GO WITH VFR CEILINGS AND STRENGTHENING NORTHEAST
WINDS FOR THE LATTER PART OF THE FORECAST AS A WEDGE OF HIGH
PRESSURE BEGINS TO BUILD INTO THE REGION.

EXTENDED AVIATION OUTLOOK...THU FRI BRIEF FLIGHT RESTRICTIONS
POSSIBLE IN SCATTERED SHOWERS TUESDAY THROUGH FRIDAY. CHANCES FOR
MVFR CEILINGS INCREASE THURSDAY INTO FRIDAY AS LOW PRESSURE
STRENGTHENS OFF THE COAST.

&&

.MARINE...
TONIGHT...THE APPROACHING COLD FRONT WILL BEGIN TO MOVE INTO AND
THROUGH THE WATERS THIS EVENING. LIGHT SOUTHWEST FLOW WILL TURN
AROUND TO EVENTUALLY BECOME NORTHEASTERLY THANKS TO THE HIGH
PRESSURE BUILDING IN TO THE NORTH. THE STRENGTHENING NORTHEAST
GRADIENT WILL SUPPORT A NOTEWORTHY SURGE LATE TONIGHT WITH ALMOST
ALL OF THE WATERS BECOMING 15-20 KT. FOR NOW IT APPEARS THAT SMALL
CRAFT ADVISORY WINDS WILL BE DELAYED UNTIL TUESDAY WHEN THE GRADIENT
TIGHTENS MORE. ALSO...SINCE THE STRONGEST WINDS WILL NOT ARRIVE
UNTIL AFTER THE TONIGHT PERIOD...SEAS WILL GENERALLY REMAIN 2-3 FT
TONIGHT WITH A FEW 4 FOOTERS PERHAPS BLEEDING INTO THE CHARLESTON
COUNTY WATERS VERY LATE. THUNDERSTORMS ALONG THE FRONT COULD BE
STRONG TO SEVERE THIS EVENING...WITH THE MAIN THREAT BEING WIND
GUSTS OF 40 KT.

HIGH PRESSURE TO THE NORTH...A COASTAL TROUGH AND SOME DEGREE OF
MARINE LAYER INSTABILITY/MIXING WILL CONTRIBUTE TO A PROLONGED
PERIOD OF ELEVATED NORTHEAST WINDS/ASSOCIATED ELEVATED SEAS
TUESDAY THROUGH FRIDAY. TO ADDRESS THE INITIAL SURFACE OF STRONG
NORTHEAST WINDS AND BUILDING SEAS...RAISED SMALL CRAFT ADVISORIES
FOR ALL MARINE ZONES BEGINNING TUESDAY MORNING NORTH...INCLUDING
CHARLESTON HARBOR...AND EARLY TUESDAY AFTERNOON SOUTH. PER GUIDANCE
SIGNALS SUGGESTING A SUBTLE LULL IN WINDS/SEAS THURSDAY...ENDED ALL
ADVISORIES AT 5 AM THURSDAY MORNING. HOWEVER...CONFIDENCE IS NOT
HIGH THAT ANY LULL WILL OCCUR...AND ADJUSTMENTS TO SMALL CRAFT
ADVISORY TIMES COULD BE NEEDED TOWARD MIDWEEK.

RIP CURRENTS...LOCAL GUIDANCE YIELDED A MODERATE RISK FOR RIP
CURRENTS ON AREA BEACHES TUESDAY...MAINLY DUE TO STRONG ONSHORE
WINDS. AN ELEVATED RISK FOR RIP CURRENTS WILL CONTINUE THROUGH AT
LEAST LATE THIS WEEK...AND PERHAPS EVEN INTO THE WEEKEND.

&&

.TIDES/COASTAL FLOODING...
INCREASING NORTHEAST FLOW TUESDAY THROUGH NEXT WEEKEND WILL RESULT
IN MODERATE CHANCES FOR SHALLOW COASTAL/SALT WATER FLOODING AROUND
THE TIMES OF HIGH TIDE ESPECIALLY ALONG THE SC COAST INCLUDING
DOWNTOWN CHARLESTON. SINCE RAINFALL AMOUNTS ARE EXPECTED TO BE
HIGHEST NEAR THE COAST THROUGH THE PERIOD...SOME POTENTIAL EXISTS
FOR A COMBINATION OF FRESH/SALT WATER FLOODING IF HEAVY RAIN FALLS
NEAR THE TIMES OF HIGH TIDE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 11 AM TUESDAY TO 5 AM EDT THURSDAY
     FOR AMZ330-352.
     SMALL CRAFT ADVISORY FROM 8 AM TUESDAY TO 5 AM EDT THURSDAY
     FOR AMZ350.
     SMALL CRAFT ADVISORY FROM 2 PM TUESDAY TO 5 AM EDT THURSDAY
     FOR AMZ354-374.

&&

$$

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




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