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Area Forecast Discussion

357
FXUS62 KCHS 011953
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
353 PM EDT WED APR 1 2015

.SYNOPSIS...
HIGH PRESSURE WILL BUILD IN FROM THE NORTH TONIGHT BEHIND A WEAK COLD
FRONT. HIGH PRESSURE WILL SHIFT OFF THE EASTERN SEABOARD THURSDAY.
A ROBUST FAST MOVING COLD FRONT WILL CROSS THE REGION SATURDAY
MORNING...FOLLOWED BY COOLER HIGH PRESSURE THROUGH SUNDAY. THE HIGH
WILL SHIFT INTO THE ATLANTIC AND WILL PUSH WARMER AND MORE HUMID AIR
INTO THE REGION THROUGH THE FIRST HALF OF NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
A POTENT SHORTWAVE HAS PUSHED OFF THE COAST AS OF EARLY THIS
AFTERNOON WITH STRONG NEGATIVE VORTICITY ADVECTION PRODUCING
SUBSIDENCE IN ITS WAKE. THE RAIN SHOWERS ALONG WITH MUCH OF THE
LOW AND MID LEVEL CLOUDINESS PUSHED OFF THE COAST MIDDAY WITH ONLY
SCATTERED CUMULUS REMAINING OVER SOME INLAND LOCALES. THE WEAK
BACK DOOR COLD FRONT HAS CROSSED THE SAVANNAH RIVER WITH DRIER AIR
FILTERING IN FROM THE NORTHEAST. DRY HIGH PRESSURE WILL CONTINUE
TO BUILD OVERNIGHT BUT WILL SHIFT STEADILY EAST INTO THE WESTERN
ATLANTIC AS THE UPPER RIDGE PROGRESSES. THIS EVENING WE EXPECT
A BRIEF PERIOD OF MOSTLY CLEAR SKIES BEFORE WEAK ONSHORE FLOW
BRINGS ABOUT A LOW-LEVEL MOISTURE INCREASE ACROSS SOUTHEAST GA.
PARTLY TO MOSTLY CLOUDY SKIES WILL DEVELOP OVERNIGHT ACROSS FAR SW
GA ZONES. MEANWHILE...SKIES WILL BE MOSTLY CLEAR FARTHER NORTH
ALLOWING FOR BETTER RADIATIONAL COOLING. OVERNIGHT LOWS WILL RANGE
FROM UPPER 50S FAR SOUTH TO AROUND 50 TO THE NORTH.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
THURSDAY...AN UPPER LEVEL RIDGE AND AN AREA OF SURFACE HIGH
PRESSURE WILL SHIFT WELL OFF THE COAST. LOW LEVEL FLOW WILL BECOME
ONSHORE WHICH WILL HELP SURFACE DEWPOINTS INCREASE INTO THE LOWER
60S ACROSS SOUTHEAST GA AND UPPER 50S TO NEAR 60 ELSEWHERE BY LATE
IN THE DAY. MEANWHILE...A SHORTWAVE TROUGH ALOFT WILL ADVANCE INTO
THE REGION FROM THE WEST/SOUTHWEST DURING THE AFTERNOON...WHICH
COULD AID IN THE DEVELOPMENT OF ISOLATED SHOWERS AND THUNDERSTORMS
DURING THE AFTERNOON ACROSS MAINLY WESTERN ZONES. EVEN THOUGH WEAK
INSTABILITY DEVELOPS ACROSS THE SOUTHWEST BY
AFTERNOON...MAINTAINED ONLY SLIGHT CHANCE/LOWER END CHANCE POPS
DUE TO LACK OF A LOW LEVEL FOCUS. HIGHS WILL MAINLY BE FROM
75-80F...EXCEPT NEAR THE COAST WHERE A VIGOROUS ONSHORE FLOW WILL
KEEP TEMPS IN THE UPPER 60S.

CONVECTION COULD PERSIST INTO THURSDAY EVENING ESPECIALLY ACROSS
WESTERN/SOUTHWESTERN COUNTIES...SO MAINTAINED AT LEAST SLIGHT CHANCE
POPS IN THE SOUTHWEST THROUGH MUCH OF THE EVENING. AS WINDS VEER
TOWARD THE SOUTH OVERNIGHT...DEWPOINTS IN THE 60S WILL MAINTAIN A
LONGER RESIDENCE OVER THE COOLER WATERS...WHICH COULD RESULT IN SOME
SEA FOG ACROSS THE NEARSHORE WATERS. SOME OF THE FOG MAY TRY TO MOVE
A SHORT DISTANCE INLAND. WATER TEMPERATURES NEAR 60 DEGREES IN MANY
AREAS AND DECENT WINDS OF NEAR 10 KNOTS ARE LIMITING FACTORS FOR
DENSE FOG DEVELOPMENT...UNLIKE BACK IN EARLY TO MID MARCH WHEN SHELF
WATER WERE MUCH COOLER. IT WILL BE A MILD NIGHT WITH LOWS MAINLY IN
THE LOWER 60S.

FRIDAY...A DECENT AMOUNT OF SUNSHINE...850 MB TEMPERATURES OF 13 TO
14 DEGREES CELSIUS AND COMPRESSIONAL HEATING IN ADVANCE OF A COLD
FRONT SHOULD PUSH HIGH TEMPERATURES INTO THE LOWER TO MID 80S. AN
ISOLATED SHOWER/THUNDERSTORM CANNOT BE RULED OUT IF/WHERE LOW LEVEL
CONVERGENCE DEVELOPS EAST OF I-95...BUT CAPPED/FAIRLY DRY MODEL
SOUNDINGS SUGGEST THAT THE POTENTIAL FOR CONVECTION REMAINS TOO
LIMITED TO JUSTIFY A MENTION WITHIN PUBLIC FORECASTS. ALSO OF
NOTE... SEA FOG/STRATUS COULD CONTINUE TO BRUSH THE BEACHES/SEA
ISLANDS.

FRIDAY NIGHT...AHEAD OF AN APPROACHING COLD FRONT...TEMPERATURES
WILL ONLY DROP INTO THE LOWER 60S. CLOUD COVER WILL BE ON THE
INCREASE ESPECIALLY LATE AS A FAST MOVING STORM SYSTEM AND
ASSOCIATED COLD FRONT APPROACH THE WESTERN COUNTIES LATE. MAINTAINED
A SLIGHT CHANCE FOR SHOWERS AND PERHAPS A THUNDERSTORMS IN THE NORTH
AND WEST LATE.

SATURDAY...THE COLD FRONT WILL ZIP ACROSS THE REGION WITH AN
ASSOCIATED CHANCE FOR SHOWERS AND PERHAPS A FEW THUNDERSTORMS ALONG
AND AHEAD OF THE FRONT. ALL 12Z MODEL GUIDANCE AND 00Z ECMWF
INDICATE A VERY QUICK COLD FRONTAL PASSAGE IN THE MORNING FOLLOWED
BY A FAIRLY RAPID CLEARING...WITH MAINLY SUNNY SKIES EXPECTED IN THE
WEST AND NORTH BY MID AFTERNOON. MODELS DIFFER SIGNIFICANTLY ON HOW
COOL THE AIR IS BEHIND THE FRONT. THE 12Z GFS AND NAM INDICATE MUCH
COOLER AIR THAN DOES THE 00Z AND 12Z ECMWF AND ESPECIALLY THE 12Z
CANADIAN MODEL. BASED ON A TREND TOWARD MORE COOLER AIR IN THE WAKE
OF THE FRONT...LOWERED HIGHS A BIT INTO THE LOWER TO MID 70S.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
POST FROPA HIGH PRESSURE WILL PROVIDE COOLER AND RAIN-FREE
CONDITIONS SATURDAY NIGHT THROUGH SUNDAY NIGHT. THEN...HIGH PRESSURE
WILL PUSH OFFSHORE AND A RETURN FLOW FEATURING WARMER TEMPERATURES
AND INCREASING MOISTURE WILL PREVAIL MONDAY THROUGH WEDNESDAY. ODDS
FAVOR RAIN-FREE CONDITIONS THROUGH MONDAY. THEN...EVEN THOUGH
SIGNIFICANT MOISTURE/PRECIPITATION SHOULD BE HELD WEST OF THE REGION
BY THE ATLANTIC RIDGE THROUGH MIDWEEK...LOW POPS ARE JUSTIFIED
TUESDAY-WEDNESDAY.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
MOST OF THE CLOUD COVER WILL DIMINISH THIS AFTERNOON AS THE UPPER
SHORTWAVE MOVES OFFSHORE. SCATTERED VFR CUMULUS WILL PERSIST UNTIL
SUNSET. LATER TONIGHT ONCE THE BACK DOOR FRONT FULLY DROPS
THROUGH THE AREA AND SOME LOW-LEVEL MOISTURE BEGINS TO MOVE INTO
SOUTHEAST GA WE SHOULD SEE STRATOCUMULUS OVERSPREAD KSAV. THE
LOWEST CEILINGS WILL REMAIN SW OF THE TERMINAL SO WE MAINTAINED
VFR CONDITIONS AT THIS TIME. HOWEVER...THE EDGE OF MVFR CEILINGS
WILL LIKELY BE VERY CLOSE TO KSAV AND LATER ISSUANCES MAY NEED TO
BRING A FEW HOURS OF MVFR CEILINGS INTO KSAV FOR LATER TONIGHT
INTO THU MORNING.

EXTENDED AVIATION OUTLOOK...SCATTERED SHOWERS AND PERHAPS A
THUNDERSTORM ARE EXPECTED SATURDAY MORNING WHICH COULD RESULT IN A
PERIOD OR TWO OF SUB VFR CIGS/VSBYS. OTHERWISE...MAINLY VFR
CONDITIONS ARE EXPECTED.

&&

.MARINE...
A NORTHEAST SURGE IS DEVELOPING ACROSS THE SC WATERS THIS
AFTERNOON BEHIND THE BACK DOOR COLD FRONT AS IT DROPS SOUTH. WE
EXPECT 15-20 KT WINDS THROUGH MUCH OF THE NIGHT WITH SEAS BUILDING
AS HIGH AS 5 FT OVER OUTER PORTIONS. WINDS IN CHARLESTON HARBOR
MAY GUST A LITTLE ABOVE 20 KT LATE THIS AFTERNOON WITH THE INITIAL
SURGE BEFORE DIMINISHING A BIT THIS EVENING.

HIGHER DEWPOINT AIR WILL SPREAD ACROSS THE STILL-CHILLY SHELF WATERS
ON THURSDAY AND THEN LIKELY PERSIST THROUGH FRIDAY NIGHT OR SATURDAY
MORNING. CONDITIONS WILL BECOME INCREASINGLY FAVORABLE FOR SEA FOG
LATER THURSDAY OR THURSDAY NIGHT...POSSIBLY CONTINUING INTERMITTENTLY
OR CONTINUOUSLY THROUGH FRIDAY NIGHT/SATURDAY MORNING. MAINTAINED
MENTION FOR PATCHY OR AREAS OF FOG THROUGH FRIDAY...THEN DID NOT
INCLUDE IT FRIDAY NIGHT OR SATURDAY MORNING DUE TO THE POSSIBILITY
OF SOMEWHAT LOWER DEWPOINT AIR MOVING IN. WATERS TEMPERATURES NEAR
60...AS OPPOSED TO LOWER TO MID 50S DURING OUR BIGGER FOG EVENTS A
FEW WEEKS AGO IS A LIMITING FACTOR FOR DENSE SEA FOG.

AS SURFACE HIGH PRESSURE SHIFTS OFFSHORE...MODERATE ONSHORE FLOW IS
EXPECTED THURSDAY. AHEAD OF AN APPROACHING COLD FRONT S/SW WINDS
WILL INCREASE FRIDAY/FRIDAY NIGHT...AND SCA CONDITIONS ARE POSSIBLE
ESPECIALLY ACROSS AMZ350 AND AMZ374.

THE COLD FRONT WILL CROSS THE WATERS SATURDAY...AND STRONG HIGH
PRESSURE WILL THEN BUILD INTO THE REGION ENHANCING COLD ADVECTION
AND SUPPORTING A SURGE OF NORTHEAST WINDS AND ELEVATED SEAS DURING
THE LATTER HALF OF THE WEEKEND INTO EARLY NEXT WEEK. SMALL CRAFT
ADVISORIES APPEAR LIKELY WITH THIS EVENT.

THE HIGH PRESSURE WILL SHIFT OFFSHORE NEXT WEEK...AND WINDS WILL
VEER THROUGH THE E/SE TO S/SW. WINDS/SEAS SHOULD REMAIN BELOW SCA
LEVELS THROUGH MID WEEK.

&&

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

&&

$$
SYNOPSIS...
NEAR TERM...JRL
SHORT TERM...MTE
LONG TERM...SPR
AVIATION...JRL/MTE
MARINE...JRL/MTE



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