AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
931 PM EDT WED AUG 31 2011

.SYNOPSIS...
HIGH PRESSURE WILL BUILD SOUTH ALONG THE EASTERN SEABOARD THROUGH
LATE WEEK...BEFORE SHIFTING OFF THE COAST OVER THE WEEKEND. A COLD
FRONT WILL THEN AFFECT THE REGION EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
SURFACE HIGH PRESSURE WILL EXTEND ACROSS THE AREA TONIGHT...AS
HIGH PRESSURE ALOFT EXTENDS ACROSS THE AREA FROM THE PLAINS. THE
LARGE SCALE SUBSIDENCE AND THE LACK OF MOISTURE WILL RESULT IN
RAINFREE CONDITIONS...AND SKIES THAT ARE MAINLY CLEAR.
HOWEVER...GIVEN SOME BOUNDARY LAYER MOISTURE AS HIGH AS 85 OR 90
PERCENT WE COULD SEE A LITTLE LATE NIGHT STRATUS...WHILE A LITTLE
CIRRUS WILL MOVE IN AROUND THE PERIPHERY OF THE RIDGING ALOFT.

WE CONTEMPLATED ADDING PATCHY FOG SOUTH OF I-16 IN SE GA WHERE
CROSS-OVER TEMPS WILL BE REACHED AND BOUNDARY LAYER MOISTURE IS
GREATEST. HOWEVER...OUR LATEST THINKING IS THAT VISIBILITIES WILL
BE NO LOWER THAN 2 OR 3 MILES...SO HAVE OPTED NOT TO ADD TO THE
FORECAST AT THIS TIME.

RADIATIONAL COOLING WILL BE GOOD GIVEN GEOSTROPHIC WINDS THAT ARE
10 KT OR LESS...AND OVERNIGHT MIN TEMPS WILL BE AT THEIR LOWEST IN
AT LEAST THE PAST 5 OR 6 WEEKS. LOWEST READINGS WILL RANGE FROM
THE MID AND UPPER 60S INLAND...TO THE LOWER AND MIDDLE 70S AT AND
NEAR THE COAST. A FEW OF OUR NORMALLY COOLER SECTIONS FROM
BERKELEY COUNTY TO ALLENDALE COULD EVEN DROP INTO THE LOWER 60S.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
SURFACE HIGH PRESSURE WILL CONTINUE TO EXTEND DOWNWARD ALONG THE
EASTERN SEABOARD...WHILE THE UPPER LEVEL RIDGE BUILDS INTO THE AREA
FROM THE WEST. SURFACE HIGH PRESSURE WILL BEGIN TO TRANSITION
OFFSHORE ON SATURDAY WHILE A TROUGH DEEPENS ACROSS THE WESTERN
ATLANTIC.

HAVE MAINTAINED A RAIN FREE FORECAST AS STRONG SUBSIDENCE
AND PLENTY OF DRY AIR REMAIN ACROSS THE AREA. SOME MODELS EVEN
SUGGEST UPPER TO MID 50 DEWPOINTS ACROSS PORTIONS OF SOUTHEAST SOUTH
CAROLINA ON THURSDAY. HIGH TEMPERATURES WILL REMAIN NEAR SEASONABLY
NORMAL WITH UPPER 80S TO NEAR 90 EXPECTED FOR MOST AREAS AND LOW 90S
EXPECTED ACROSS INLAND PORTIONS OF SOUTHEAST GEORGIA. WITH SUCH LOW
DEWPOINTS...OVERNIGHT LOW TEMPERATES WILL LIKELY DIP BELOW SEASONAL
NORMALS ON THURSDAY WITH MID TO UPPER 60S EXPECTED FOR COASTAL
COUNTIES AND POSSIBLY LOW 60S FARTHER INLAND.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
A DEVELOPING RETURN FLOW COURTESY OF HIGH PRESSURE OVER THE WESTERN
ATLANTIC WILL ALLOW FOR INCREASING MOISTURE ACROSS THE AREA ON
SUNDAY...WHICH COULD BE SUFFICIENT TO SUPPORT DIURNAL ISOLATED
CONVECTION.

MODEL GUIDANCE THEN BEGINS TO DIVERGE SIGNIFICANTLY FOR EARLY NEXT
WEEK AS THE UPPER LEVEL RIDGE BREAKS DOWN AND A LONG WAVE TROUGHS
DEVELOPS OVER THE EASTERN UNITED STATES. MODELS GENERALLY AGREE THAT
THE TROUGH SHOULD STEER TROPICAL CYCLONE KATIA AWAY FROM THE U.S.
EAST COAST...HOWEVER DIFFER LARGELY IN DEVELOPMENT AND TRACKING OF A
POTENTIALLY NEW TROPICAL SYSTEM IN THE GULF OF MEXICO. THE 00Z GFS
APPEARS TO BE AN OUTLIER AS IT TRACKS THIS SYSTEM TOWARD THE REGION
EARLY NEXT WEEK...WHILE THE PREFERRED 00Z ECMWF/00Z CANADIAN SHOW A
FRONT APPROACHING AND EVENTUALLY STALLING OVER THE REGION EARLY NEXT
WEEK. DESPITE ALL OF THE UNCERTAINTY...THERE IS OVERALL AGREEMENT
THAT MUCH DEEPER MOISTURE WILL SPREAD OVER THE REGION EARLY NEXT
WEEK...RESULTING IN INCREASED CHANCES FOR PRECIPITATION AND POSSIBLY
BELOW NORMAL TEMPERATURES.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
THERE COULD BE A LITTLE STRATUS AT KSAV OVERNIGHT INTO THURSDAY
MORNING...OTHERWISE VFR CONDITIONS WILL PREVAIL AT BOTH TERMINALS
THROUGH 00Z FRIDAY.

EXTENDED AVIATION OUTLOOK...VFR CONDITIONS SHOULD PREVAIL THROUGH
SUNDAY.

&&

.MARINE...
HIGH PRESSURE WILL CONTINUE TO BUILD FROM THE NORTH AND NE
TONIGHT...AND WITH SOME PINCHING OF THE GRADIENT AND PRESSURE
RISES ABOUT 1 MB EVERY 3 HOURS WE LOOK FOR NE AND EAST WINDS AS
HIGH AS AROUND 15 KT. LAND BREEZE CIRCULATIONS WILL BACK WINDS
ABOUT 30-50 DEGREES LATE...AS SPEEDS DROP ABOUT 5 OR 10 KT. SEAS
WILL BE A COMBINATION OF SHORT PERIOD WIND WAVES AROUND 5-6
SECONDS...AND LONGER PERIOD SWELLS AROUND 9-10 SECONDS.
SIGNIFICANT WAVE HEIGHT WILL BE UP TO 4 OR 5 FT EARLY...DROPPING
AROUND A FOOT OVERNIGHT.

HIGH PRESSURE WILL BUILD ACROSS THE COASTAL WATERS THROUGH
FRIDAY...THEN SHIFT INTO THE WESTERN ATLANTIC OVER THE WEEKEND AHEAD
OF A COLD FRONT APPROACHING FROM THE NORTHWEST EARLY NEXT WEEK. A
NORTHEAST TO EASTERLY FLOW WILL PREVAIL THROUGH FRIDAY NIGHT BEFORE
VEERING SOUTH ON SATURDAY. A PINCHED GRADIENT WILL RESULT IN 15 TO
20 KNOT WINDS ON THURSDAY...HOWEVER...WINDS AND SEAS ARE EXPECTED TO
REMAIN BELOW SMALL CRAFT ADVISORY CRITERIA THROUGH THE PERIOD.

&&

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

&&

$$