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Area Forecast Discussion

374
FXUS62 KCHS 271757
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
157 PM EDT SUN JUL 27 2014

.SYNOPSIS...
AN ATLANTIC RIDGE OF HIGH PRESSURE WILL REMAIN SOUTH OF THE AREA
TODAY...THEN GIVE WAY TO A STRONG COLD FRONT THAT WILL CROSS THE
REGION LATE MONDAY. THE FRONT WILL BECOME STATIONARY SOUTH AND
SOUTHEAST OF THE AREA THROUGH THE MIDDLE OF THE WEEK...THEN SLOWLY
MOVE BACK TOWARDS THE SOUTHEAST COAST AS A COASTAL TROUGH LATE IN
THE WEEK AND INTO THE FIRST PART OF THE WEEKEND.

&&

.NEAR TERM /THROUGH TONIGHT/...
FLAT UPPER RIDGING CENTERED OVER THE GULF OF MEXICO WILL BE THE
DOMINANT FEATURE ALOFT FOR SOUTHEAST GEORGIA AND SOUTHEAST SOUTH
CAROLINA TODAY...CONFINING ANY ISOLATED SHOWERS ASSOCIATED WITH
PROGRESSIVE SHORTWAVE ENERGY NORTH OF THE FORECAST AREA. SOLID
WEST FLOW IN THE LOW TO MID LEVELS AND A STRONG CAPPING INVERSION
WILL ADDITIONALLY SUPPRESS SHOWER/THUNDERSTORM DEVELOPMENT THROUGH
THE AFTERNOON. PRECIPITABLE WATER VALUES WILL FALL BELOW 1.5
INCHES BY LATE AFTERNOON ACCORDING TO MODEL FORECAST SOUNDINGS AT
BOTH CHARLESTON AND SAVANNAH...LENDING FURTHER CONFIDENCE IN THE
ONGOING RAIN-FREE FORECAST.

THE MAIN CONCERN FOR TODAY WILL BE HOT AND HUMID CONDITIONS UNDER
THE DEEP OFFSHORE FLOW AND RISING THICKNESS VALUES. WILL INDICATE
HIGH TEMPERATURES RANGING FROM THE MID/UPPER 90S INLAND TO UPPER
80S/LOWER 90S AT THE BEACHES. NEARLY ALL GUIDANCE SOURCES SUGGEST
THAT THERE WILL BE SUFFICIENT MIXING WITHIN OFFSHORE FLOW TO ALLOW
DEWPOINT TEMPERATURES TO FALL INTO THE UPPER 60S TO LOWER 70S
DURING THE TIME OF PEAK HEATING THIS AFTERNOON. THUS...HEAT
INDICES OF 105 TO 108 DEGREES WILL FALL JUST SHORT OF THE LOCAL
LATE-SEASON HEAT ADVISORY CRITERIA OF 110 DEGREES. TRENDS WILL
STILL BE CLOSELY MONITORED DURING THE NEXT SEVERAL HOURS...AS ANY
SLIGHT VARIATION IN TEMPERATURES OR DEWPOINTS...ESPECIALLY IF LOW
LEVEL MOISTURE POOLS IN CONJUNCTION WITH THE RESULTANT SEA
BREEZE...COULD BRING CONDITIONS INTO HEAT ADVISORY THRESHOLDS.
EITHER WAY...CONDITIONS WILL BE SUFFICIENTLY HOT AND HUMID TO
WARRANT EXTRA CAUTION WHEN OUTSIDE THIS AFTERNOON.

TONIGHT...A BROAD AND AMPLIFYING TROUGH WILL START TO CARVE OUT
OVER THE EASTERN STATES...CAUSING A DEEPENING SURFACE LOW TO MOVE
INTO THE NORTHEAST STATES. THIS WILL SEND A STRONG COLD FRONT
THROUGH THE OHIO VALLEY TO NEAR THE APPALACHIAN CHAIN LATE.
CONVECTIVE INHIBITION WILL REMAIN IN PLACE...AND ALONG WITH A
SPARSE AMOUNT OF MOISTURE...SUGGESTS A CONTINUATION OF RAIN-FREE
CONDITIONS. CONSIDERING A BUILD UP OF THE EXTREME DAYTIME
HEAT...ALONG WITH AND THE PLANETARY BOUNDARY LAYER NEVER FULLY
DECOUPLING...EXPECT TEMPERATURES TO ONLY FALL INTO THE MID OR
UPPER 70S INLAND AND THE LOWER 80S AT THE COAST. IN
ADDITION...THERE IS THE CHANCE THAT UPSTREAM CONVECTIVE DEBRIS
CLOUDS COULD BE MORE ABUNDANT...FURTHER EVIDENCE OF A VERY WARM
NIGHT...WITH TEMPERATURES MUCH OF THE NIGHT REMAINING ABOVE 80
DEGREES. THESE TEMPERATURES WILL BE CLOSE TO RECORD HIGH MINS FOR
JULY 28TH. SEE CLIMATE SECTION BELOW FOR DETAILS.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
MONDAY AND MONDAY NIGHT...THE EXPANSIVE MID/UPPER LEVEL RIDGE WILL
BECOME FURTHER SUPPRESSED TO THE SOUTH AS AN ANOMALOUSLY DEEP TROUGH
CARVES OUT ACROSS THE EASTERN CONUS. HEIGHTS WILL STEADILY FALL AS
THE PRIMARY EMBEDDED SHORTWAVE MOVES ACROSS THE NORTHEAST. THIS
TROUGH WILL DRIVE A STRENGTHENING SURFACE LOW THROUGH NEW ENGLAND
WITH A STRONG TRAILING COLD FRONT THAT WILL CROSS THE FORECAST AREA
IN THE EVENING/OVERNIGHT HOURS. MODEL SOUNDINGS SHOW A DISTINCT CAP
IN PLACE THROUGH THE MORNING AND MUCH OF THE AFTERNOON WHICH WILL
KEEP MOST OF THE DAY DRY. CONVECTIVE INITIATION IS ANTICIPATED IN
THE MID/LATE AFTERNOON ALONG AND JUST AHEAD OF THE FRONT AS IT MOVES
THROUGH THE MIDLANDS. THUNDERSTORMS WILL THEN SPREAD INTO AND
THROUGH THE FORECAST AREA THROUGH THE EVENING. THE LACK OF
CONVECTION FOR MUCH OF THE DAY WILL ALLOW FOR PLENTY OF INSOLATION
AND WITH VERY WARM LOW LEVEL THICKNESSES AND COMPRESSIONAL HEATING
AHEAD OF THE FRONT...A VERY WARM DAY IS EXPECTED. IN FACT...LOW
LEVEL FLOW WILL LIKELY BE WESTERLY FOR MOST OF THE DAY AND HIGHS ARE
EXPECTED TO SOAR INTO THE UPPER 90S TO AROUND 100 IN MANY AREAS.
WITH THESE KIND OF TEMPERATURES AND DEWPOINTS IN THE LOW TO MID
70S...HEAT INDEX VALUES WILL RISE TO AROUND 110 DEGREES FOR A FEW
HOURS. HEAT ADVISORIES MAY BE NEEDED AND WILL CONTINUE TO BE
MENTIONED IN THE HWO.

REGARDING THE SEVERE THREAT...THE TREMENDOUS SURFACE HEATING WILL
RESULT IN SURFACE BASED CAPE VALUES IN THE 3000-4000 J/KG RANGE BY
LATE AFTERNOON. IT APPEARS AS THOUGH IT WILL BE AN IMPRESSIVE NEAR
STORM ENVIRONMENT...ESPECIALLY GIVEN THE DYNAMICS AND
THERMODYNAMICS. THE MID/UPPER LEVEL TROUGH ALOFT WILL BRING WITH IT
30-40 KNOTS OF FLOW AT 500 MB...RESULTING IN DEEP LAYER SHEAR VALUES
OF AROUND 25-30 KNOTS. MODEL SOUNDINGS SHOW INDICATIONS OF THE
PRESENCE OF AN ELEVATED MIXED LAYER WHICH WILL SERVE TO STEEPEN
LAPSE RATES AND PROVIDE ENOUGH MID LEVEL DRY AIR TO PRODUCE DCAPE
VALUES IN EXCESS OF 1000 J/KG. HAIL CAPES ARE ALSO QUITE IMPRESSIVE
AND THE POTENTIAL FOR LARGE HAIL IS CERTAINLY THERE...AS WELL AS
STRONG DAMAGING WIND GUSTS. OVERALL COVERAGE IS EXPECTED TO REMAIN
SCATTERED...BUT WHATEVER DOES DEVELOP HAS THE POTENTIAL TO BE QUITE
STRONG. GIVEN THE AMOUNT OF INSTABILITY AND PRESENCE OF SHEAR...THE
MODE OF CONVECTION WILL LIKELY BE DISCRETE AND SOME COMBINATION OF
MULTICELLS WITH SOME SUPERCELL CHARACTERISTICS. THE TIME PERIOD OF
GREATEST SEVERE THREAT CONTINUES TO BE THE SAME...AS EARLY AS 4-5 PM
CONTINUING THROUGH ABOUT MIDNIGHT. ONE POTENTIAL FLY IN THE OINTMENT
FOR BOTH THE HEAT ADVISORY AND SEVERE THUNDERSTORM RISK IS THE
POTENTIAL FOR DEWPOINTS TO MIX OUT MORE THAN ADVERTISED IN THE
WESTERLY FLOW. THIS WOULD REDUCE HEAT INDICES AND FURTHER REDUCE
CONVECTIVE COVERAGE. IT IS SOMETHING WORTH WATCHING WITH SUBSEQUENT
MODEL RUNS. WE CONTINUE TO BE IN THE SPC SLIGHT RISK FOR MONDAY AND
WE WILL MAINTAIN THE CURRENT HWO MENTION.

TUESDAY THROUGH WEDNESDAY...A MUCH COOLER PERIOD IS EXPECTED THROUGH
MID WEEK BEHIND THE DEPARTING COLD FRONT. WITH THE ANOMALOUSLY DEEP
MID/UPPER LEVEL TROUGH ACROSS THE EASTERN CONUS...HEIGHTS WILL
CONTINUE TO FALL AND HIGH TEMPERATURES ARE EXPECTED TO TOP OUT IN THE
UPPER 80S IN MANY AREAS...WHICH WOULD BE A FEW DEGREES BELOW NORMAL.
TUESDAY LOOKS MAINLY DRY BEHIND THE FRONT. THERE MAY BE A BIT MORE
ACTIVITY ON WEDNESDAY AS THE MODELS SHOW MORE CONVECTIVE RESPONSE IN
THE DEVELOPING ONSHORE FLOW.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
THE DEEP EASTERN CONUS TROUGH WILL REMAIN IN PLACE THROUGH THE END
OF THE WEEK AND INTO THE START OF NEXT WEEKEND. IN FACT SOME OF THE
MODELS SHOW THE TROUGH STRENGTHENING FURTHER AND DEVELOPING A CLOSED
LOW ACROSS THE UPPER MIDWEST. AT THE SURFACE...THE REMNANT COLD
FRONT OFFSHORE MAY ACTUALLY DRIFT BACK WESTWARD AS A COASTAL TROUGH
BY THE END OF THE PERIOD. IF THIS WERE TO OCCUR DEEP MOISTURE WOULD
INCREASE ACROSS THE FORECAST AREA AND AN INCREASE IN CONVECTIVE
COVERAGE APPEARS LIKELY BY THE LATTER PART OF THE FORECAST.
TEMPERATURES WILL REMAIN BELOW CLIMO THROUGH THE PERIOD.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
VFR CONDITIONS WILL PREVAIL THROUGH 18Z MONDAY. DECENT MIXING THIS
AFTERNOON WILL SUPPORT BREEZY SOUTHWEST WINDS UP TO 20 KT THROUGH
SUNSET. THE LOW LEVEL JET WILL FURTHER INCREASE OVERNIGHT AND
SUPPORT 25 TO 30 KT WINDS AT 2 KFT BY 06Z EARLY MONDAY MORNING.
CONSIDERING THAT WINDS WILL NOT DECOUPLE TONIGHT...WIND SHEAR
VALUES WILL FALL SHORT OF CRITERIA FOR INCLUSION IN THE TAFS.

EXTENDED AVIATION OUTLOOK...PRIMARILY VFR. A COLD FRONT IS EXPECTED TO
MOVE THROUGH THE AREA LATE MONDAY...WHICH COULD RESULT IN CEILING
AND VISIBILITY REDUCTIONS AS THUNDERSTORMS PASS OVER THE AREA
MONDAY EVENING AND MONDAY NIGHT.

&&

.MARINE...
TODAY...THE COASTAL WATERS WILL BECOME LOCATED WITHIN A FAIRLY
TIGHT GRADIENT BY JULY STANDARDS...SITUATED BETWEEN THE SUB-TROPICAL
ATLANTIC RIDGE TO THE SOUTHEAST AND DEVELOPING LOW PRESSURE IN THE
GREAT LAKES. THIS WILL SUPPORT SOUTHWEST WINDS AVERAGING 12 TO 17
KT...WITH EVEN SOME HIGHER GUSTS. SEAS WILL RANGE FROM FROM 2 TO
4 FT...HIGHEST ACROSS THE OUTER GEORGIA WATERS AND PARTS OF
CHARLESTON COUNTY WATERS.

TONIGHT...THERE IS A FURTHER PACKING OF THE ISOBARS IN RESPONSE TO
THE NORTHERN STATES LOW INTENSIFYING AND A COLD FRONT HEADING
TOWARD THE APPALACHIANS. IN FACT THERE IS AS MUCH AS A 3 MB SPREAD
FROM NORTH TO SOUTH THAT DEVELOPS ACROSS OUR MARINE ZONES.
GEOSTROPHIC WINDS TOP OUT AT 25-30 KT...BUT GIVEN THE WARM
ADVECTION REGIME ONLY A PORTION OF THIS WILL MIX DOWN TO THE
SURFACE OF THE WATER. WHAT IS ANTICIPATED IS FOR SW WINDS TO CLIMB
THROUGH THE NIGHT...UP TO SOME 17-22 KT. GUSTS WILL OCCASIONALLY
REACH AROUND 25 KT...BUT FOR THE MOMENT THEIR FREQUENCY AND
SPATIAL COVERAGE IS NOT ENOUGH TO JUSTIFY HOISTING THE SMALL CRAFT
ADVISORY FLAGS. LATER FORECAST SHIFTS WILL RE- ADDRESS THIS
SITUATION TO SEE IF ANY ADVISORIES WOULD BE NECESSARY. SEAS WILL
CLIMB ANOTHER FOOT OR SO FROM TODAY...CAPPED AT 4 OR 5 FT.

MONDAY THROUGH THURSDAY...A STRONG COLD FRONT IS EXPECTED TO
APPROACH AND MOVE ACROSS THE LOCAL WATERS MONDAY EVENING THROUGH
MONDAY NIGHT. AHEAD OF THE FRONT...THE TIGHT GRADIENT WILL SUPPORT
WINDS IN THE 15-20 KT RANGE...WITH SMALL CRAFT ADVISORY CONDITIONS
NOT EXPECTED. STRONG THUNDERSTORMS WILL LIKELY ACCOMPANY THE FRONT
THROUGH THE EARLY TUESDAY MORNING HOURS. BEHIND THE FRONT...THE
GRADIENT WILL RELAX CONSIDERABLY. WINDS WILL BE 15 KT OR LESS AND
WILL TAKE ON A MORE EASTERLY COMPONENT THROUGH MID WEEK.

&&

.CLIMATE...
RECORD HIGH MINIMUM TEMPS FOR JULY 27TH...
KCHS...79 SET IN 2005.
KCXM...82 SET IN 2005.
KSAV...80 SET IN 1885.

RECORD HIGH MINIMUM TEMPS FOR JULY 28TH...
KCHS...79 SET IN 1981.
KCXM...83 SET IN 1999.
KSAV...81 SET IN 1878.

&&

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

&&

$$

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






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