Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 261751
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
151 PM EDT SAT JUL 26 2014

.SYNOPSIS...
AN ATLANTIC RIDGE OF HIGH PRESSURE WILL REMAIN SOUTH OF THE AREA
INTO EARLY NEXT WEEK...WITH A WEAK TROUGH OF LOW PRESSURE PREVAILING
INLAND. A COLD FRONT WILL APPROACH AND CROSS THE REGION LATE MONDAY
AND BECOME STATIONARY SOUTH AND SOUTHEAST OF THE AREA THROUGH THE
MIDDLE OF NEXT WEEK. THE FRONT WILL THEN SLOWLY DISSIPATE OFF THE
SOUTHEAST COAST THROUGH LATE NEXT WEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...
THE FORECAST AREA REMAINS ON THE PERIPHERY OF A DEEP MOISTURE AXIS
CURRENTLY STRETCHING ALONG THE SOUTHEAST COASTLINE THIS MORNING.
PRECIPITABLE WATER VALUES FROM THE 12Z CHARLESTON SOUNDING WERE
STILL NEARLY 2 INCHES...AND WITH WEAK UPPER LEVEL ENERGY AND
LINGERING MESOSCALE BOUNDARIES IN THE AREA...ISOLATED SHOWERS ARE
ONGOING ALONG AND SOUTH OF THE ALTAMAHA RIVER IN SOUTHEAST
GEORGIA.

THE UPPER TROUGH ALONG THE EAST COAST WILL SHIFT INTO THE ATLANTIC
AS THE DAY PROGRESSES...ALLOWING UPPER RIDGING TO PRESS TOWARD THE
AREA FROM THE WEST/SOUTHWEST. AT THE SAME TIME...THE WEAKENING
SURFACE TROUGH WILL TRANSITION INTO A LEE-SIDE PIEDMONT FEATURE
LATER TODAY. DRIER MID LEVEL AIR WILL FILTER TOWARD THE SOUTHEAST
COAST...AND FORECAST MODEL SOUNDINGS SHOW PRECIPITABLE WATER
VALUES DECREASING LATE IN THE DAY AS A RESULT. DIURNAL CUMULUS
HAS INITIATED ACROSS THE FORECAST AREA AS TEMPERATURES RAPIDLY
WARM AND THE ATMOSPHERE CONTINUES TO DESTABILIZE. HIGH RESOLUTION
MODELS DEPICT A MORE TYPICAL SUMMERTIME AFTERNOON CONVECTIVE
PATTERN...WITH ISOLATED THUNDERSTORMS DEVELOPING GENERALLY ALONG
AND EAST OF THE SEABREEZE BY EARLY AFTERNOON. POTENTIAL COVERAGE
OF PRECIPITATION APPEARS NOTABLY LOWER THAN YESTERDAY...A RESULT
OF MORE PERSISTENT MID LEVEL CAPPING AND THE INTRUSION OF DRIER
AIR IN THE MID LEVELS. THERE STILL WILL BE SUFFICIENT MESOSCALE
BOUNDARY INTERACTIONS...SEABREEZE FORCING...AND LOW LEVEL MOISTURE
TO SUPPORT A SLIGHT CHANCE FOR SHOWERS/THUNDERSTORMS DURING THE
FAVORED AFTERNOON/EARLY EVENING HOURS.

DEEP WEST FLOW AND A SLIGHT INCREASE IN HEIGHTS ALOFT LATER TODAY
WILL SUPPORT A VERY WARM AND HUMID AFTERNOON. TEMPERATURES WILL
PEAK IN THE LOW TO MID 90S WITH HEAT INDICES IN THE 100 TO 105
DEGREE RANGE. ALTHOUGH THESE VALUES ARE BELOW HEAT ADVISORY
THRESHOLDS...CAUTION IS ADVISED IF OUTSIDE DURING THE TIME OF
MAXIMUM AFTERNOON HEATING.

TONIGHT...IN THE ABSENCE OF ANY SUPPORT ALOFT...LINGERING CONVECTION
THIS EVENING WILL TAPER OFF AROUND SUNSET. EVEN DRIER AIR ABOVE
THE BOUNDARY LAYER MOVES INTO THE REGION...AND SKIES WILL BECOME
MOSTLY CLEAR. SINCE THERE IS A BUILD UP OF HEAT DURING THE
DAYTIME...NIGHTTIME LOWS WILL NOT FALL BELOW THE LOWER OR MIDDLE
70S INLAND...WITH UPPER 70S ALONG THE BARRIER ISLAND COMMUNITIES.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
SUNDAY AND SUNDAY NIGHT...THE FORECAST AREA WILL BE POSITIONED UNDER
THE EASTERN PERIPHERY OF AN EXPANSIVE UPPER LEVEL RIDGE THAT IS
CENTERED ACROSS THE CENTRAL PLAINS AND THE FOUR CORNERS REGION. AT
THE SURFACE....THE REGION WILL BE CAUGHT BETWEEN HIGH PRESSURE TO
THE EAST AND AN INLAND TROUGH OF LOW PRESSURE. MODEL SOUNDINGS SHOW
A NOTABLE CAP AND SUBSIDENCE INVERSION AROUND 800 MB WHICH IS
EXPECTED TO KEEP THINGS QUIET. IN FACT...GIVEN THE LACK OF
CONVECTIVE ACTIVITY IN THE MODELS...A DRY FORECAST IS IN PLACE. THE
BIGGEST FORECAST CONCERN WILL BE RISING TEMPERATURES. WITHIN THE
INLAND SURFACE TROUGH...WINDS WILL TAKE ON MORE OF A WESTERLY
COMPONENT. THIS WILL ALLOW HIGHS TO REACH THE MID TO UPPER 90S. WHEN
COMBINED WITH DEWPOINTS IN THE LOW TO MID 70S...HEAT INDEX VALUES
WILL REACH INTO THE 105-110 RANGE IN THE AFTERNOON WHICH IS JUST
BELOW HEAT ADVISORY THRESHOLDS. THE DRY FORECAST THEN CONTINUES
OVERNIGHT AHEAD OF AN APPROACHING COLD FRONT. LOWS WILL ONLY FALL
INTO THE UPPER 70S IN MOST AREAS.

MONDAY THROUGH TUESDAY...EARLY IN THE WEEK THE MID/UPPER LEVEL
PATTERN ACROSS THE EASTERN CONUS WILL AMPLIFY SIGNIFICANTLY AS A
SERIES OF SHORTWAVES ROTATE ACROSS THE GREAT LAKES REGION. THIS WILL
SUPPRESS THE WEEKEND UPPER RIDGE FURTHER SOUTH AND ALLOW FOR A DEEP
TROUGH WITH FALLING HEIGHTS. THE MOST NOTABLE FEATURE EARLY IN THE
WEEK IS THE COLD FRONT THAT IS PROGGED TO APPROACH THE AREA AND
CROSS MONDAY NIGHT. THIS IS A STRONG COLD FRONT FOR LATE JULY DRIVEN
BY AN INCREASINGLY ANOMALOUSLY DEEP TROUGH. IN FACT...BY MONDAY
EVENING HEIGHTS IN THE CORE OF THE TROUGH TO THE NORTH WILL BE ON
THE ORDER OF 3 TO 4 STANDARD DEVIATIONS BELOW NORMAL. A TROUGH THIS
DEEP DRIVING A STRONG COLD FRONT IS ALMOST SURE TO BRING ABOUT AN
ACTIVE ROUND OF THUNDERSTORMS LATE MONDAY. MID LEVEL FLOW INCREASES
INTO THE 40-50 KNOT RANGE WHICH RESULTS IN DEEP LAYER SHEAR VALUES
OF AROUND 30-35 KNOTS RIGHT ALONG THE FRONT. MODEL SOUNDINGS SHOW AN
ATMOSPHERE THAT WOULD BE SUPPORTIVE OF AN ENVIRONMENT CAPABLE OF
PRODUCING DISCRETE CELLS WITH SUPER-CELL STRUCTURES PRODUCING LARGE
HAIL AND DAMAGING WINDS. THERE WILL BE NO SHORTAGE OF SURFACE BASED
INSTABILITY WITH TREMENDOUS SURFACE HEATING AND COMPRESSIONAL
HEATING LEADING TO HIGHS IN THE UPPER 90S IN MANY AREAS. IN
FACT...WE COULD SEE HEAT INDEX VALUES OF 110 FOR A FEW HOURS AND A
HEAT ADVISORY MAY BE NEEDED FOR PARTS OF THE AREA. THE SHEAR IS
UNIDIRECTIONAL BUT THE VARIOUS THERMODYNAMIC INDICES POINT TO AN
INCREASING THREAT OF A SEVERE WEATHER EVENT MONDAY EVENING. NOT
SURPRISINGLY...MUCH OF THE FORECAST AREA IS IN THE DAY 3 SLIGHT RISK
FROM SPC. WE WILL ADD A MENTION TO THE HWO WITH THIS PACKAGE. THE
FRONT AND ASSOCIATED THUNDERSTORM ACTIVITY WILL PUSH THROUGH AND
OFFSHORE THROUGH TUESDAY. THICKNESSES DROP OFF CONSIDERABLY UNDER
THE TROUGH ALOFT AND A MUCH COOLER DAY IS EXPECTED. TUESDAY HIGHS
WILL LIKELY REMAIN IN THE UPPER 80S FOR MOST AREAS.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
THE PATTERN IN THE LONG TERM WILL BE DOMINATED BY THE ANOMALOUSLY
DEEP MID/UPPER LEVEL TROUGH ACROSS THE EASTERN CONUS. THE LOWERING
HEIGHTS AND THICKNESS VALUES WILL RESULT IN A MUCH COOLER PERIOD
WITH BELOW NORMAL TEMPERATURES THROUGH THE END OF THE WEEK. WEAK
HIGH PRESSURE WILL BE POSITIONED TO THE NORTHEAST WHICH WILL RESULT
IN AN EASTERLY ONSHORE FLOW THROUGH MUCH OF THE PERIOD. CHANCES FOR
CONVECTION WILL START OFF LOWER WEDNESDAY...BUT WILL INCREASE SLOWLY
THROUGH THE END OF THE FORECAST WITH A DEVELOPING COASTAL TROUGH IN
THE VICINITY.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
ISOLATED CONVECTION HAS INITIATED BY EARLY AFTERNOON...HOWEVER
INCREASING DRY AIR AT MID LEVELS WILL LIKELY LIMIT OVERALL
THUNDERSTORM COVERAGE TODAY. HAVE ADDED VCTS AT KCHS TO ACCOUNT
FOR LATEST RADAR TRENDS...YET CONFIDENCE IN DIRECT IMPACTS AT
EITHER TAF SITE IS TOO LOW TO EXPLICITLY MENTION AT THIS TIME.
WILL MONITOR CONVECTIVE DEVELOPMENT AND MAKE AMENDMENTS IF
NECESSARY. VFR CONDITIONS WILL OTHERWISE PREVAIL THROUGH 18Z
SUNDAY.

EXTENDED AVIATION OUTLOOK...PRIMARILY VFR. A COLD FRONT IS EXPECTED TO
MOVE THROUGH THE AREA LATE MONDAY. BRIEF CEILING AND VISIBILITY
REDUCTIONS WILL BE POSSIBLE WITH THUNDERSTORMS THAT DEVELOP IN THE
EVENING HOURS.

&&

.MARINE...
TODAY...THE MARINE ZONES WILL BE LOCATED NEAR THE NORTHWEST
PERIPHERY OF THE SUB-TROPICAL ATLANTIC RIDGE THAT EXTENDS ACROSS
SOUTHERN FLORIDA. AT THE SAME TIME...A WEAK TROUGH WILL AGAIN BE
FOUND INLAND OVER THE MID-ATLANTIC AND SOUTHEAST STATES. THIS
RESULTS IN LIGHT TO MODERATE WEST/SW WINDS BACKING AROUND MORE TO
THE SOUTH/SW WITH THE SEA BREEZE CIRCULATIONS THIS AFTERNOON. THE
WAVE SPECTRUM IS MAINLY FROM WIND DRIVEN WAVES OF 2 TO 3 FT...BUT
ALSO A LITTLE SMALL 8 SECOND SWELL FROM THE SOUTHEAST.

TONIGHT...THERE IS LITTLE CHANGE IN THE OVERALL SYNOPTIC
PATTERN...ALTHOUGH THE INLAND TROUGH DOES BECOME A LITTLE BETTER
DEFINED. WINDS WILL CLOCK BACK AROUND TO THE SW/WEST THROUGH THE
NIGHT WITH AT MOST ONLY A SMALL NOCTURNAL SURGE. SEAS WILL PRETTY
MUCH HOLD IN A STEADY STATE...AVERAGING 2-3 FT.

SUNDAY THROUGH WEDNESDAY...EARLY IN THE PERIOD THE SETUP WILL
FEATURE A TIGHTENING SOUTHWEST GRADIENT ACROSS THE WATERS AS A COLD
FRONT APPROACHES FROM THE NORTHWEST. THE STRONG COLD FRONT WILL
CROSS THE WATERS MONDAY NIGHT...LIKELY ACCOMPANIED STRONG
THUNDERSTORMS. DURING THIS SUNDAY/MONDAY TIME PERIOD...THE GRADIENT
WILL SUPPORT SOUTHWEST WINDS IN THE 15-20 KT RANGE AT TIMES.
MARGINAL SMALL CRAFT ADVISORY CONDITIONS WILL BE POSSIBLE
PERIODICALLY...ESPECIALLY FOR THE OUTER GEORGIA WATERS. THEN FOR THE
LATTER PART OF THE FORECAST PERIOD...THE GRADIENT WILL SLACKEN AND
WINDS WILL BECOME MORE NORTHEASTERLY WITH TIME. WINDS AND SEAS ARE
EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS TUESDAY AND
WEDNESDAY.

&&

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

&&

$$

NEAR TERM...WMS
SHORT TERM...BSH
LONG TERM...BSH
AVIATION...WMS
MARINE...





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