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Area Forecast Discussion

879
FXUS62 KGSP 172050
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
450 PM EDT THU APR 17 2014

.SYNOPSIS...
AN UNSEASONABLY COLD AIR MASS WILL REMAIN OVER THE REGION INTO
FRIDAY. A SUBTROPICAL LOW WILL DEVELOP OFF THE CENTRAL GULF COAST
FRI AND TRACK EAST OFF THE SE COAST BY SAT. THIS WILL BRING
WIDESPREAD PRECIPITATION TO THE REGION FRI NIGHT INTO SATURDAY.
BY EARLY NEXT WEEK...TEMPERATURES SHOULD RANGE CLOSE TO NORMAL AHEAD
OF AN APPROACHING COLD FRONT.

&&

.NEAR TERM /THROUGH FRIDAY/...
AS OF 445 PM EDT THURSDAY...SCATTERED HIGH LEVEL CIRRUS FROM GULF
SURFACE CYCLONE IS STARTING TO STREAM IN ACROSS NORTHEAST GA AND THE
WESTERN UPSTATE PER LATEST VISIBLE SATELLITE IMAGERY.  CONVECTION
ASSOCIATED WITH THIS REGION OF CYCLOGENESIS REMAINS STRONG ACROSS
THE WESTERN/CENTRAL GULF...THEREFORE STILL EXPECTING SKY COVER TO
INCREASE OVER THE CWFA THIS EVENING AS UPPER LEVEL WINDS ADVECT
MOISTURE IN.  THIS WILL LIMIT RADIATIONAL COOLING EFFECTS ACROSS THE
SOUTHERN APPALACHIANS...WHICH COMBINED WITH NON ZERO SURFACE WINDS
WILL INHIBIT FROST POTENTIAL. ASIDE FOR MINOR TWEAKS TO TEMPERATURES
AND DEWPOINTS IN THE EXTREME NEAR TERM...NO OTHER CHANGES WERE MADE
TO THE FCST WITH THIS UPDATE.

PREVIOUS DISCUSSION...

AS OF 230 PM EDT THURSDAY...DRY COLD AIR DAMMING HAS KEPT IT COOL
AND A LITTLE BREEZY OVER THE FORECAST AREA TODAY. GOING INTO
TONIGHT...A BLOSSOMING CLOUD SHIELD ASSOCIATED WITH CYCLOGENESIS
OVER THE NRN GOM WILL OVERSPREAD THE REGION. THIS SHOULD LIMIT FROST
POTENTIAL TO JUST A FEW LOCATIONS ACROSS THE NRN MTNS AND THE NRN
FOOTHILLS. IN ADDITION...AS PRESSURES FALL ALONG THE GULF
COAST...THIS WILL STRENGTHEN THE AGEOSTROPHIC COMPONENT OF THE SFC
FLOW WHICH SHOULD RESULT IN AN INCREASE IN MIXING LATE TONIGHT. AND
IF THAT/S NOT ENOUGH TO LIMIT FROST POTENTIAL...A STRATO-CU DECK MAY
SPREAD IN FROM THE SOUTHWEST LATE TONIGHT.

THERE CONTINUE TO BE MODEL DIFFERENCES WITH THE STRENGTH AND TIMING
OF THE LOW AS IT MOVES OUT OF THE GOM AND ACROSS WRN FL ON FRIDAY.
THE GFS HAS A PRONOUNCED LLVL POT VORT MAV THAT RESULTS IN A
CONSIDERABLY STRONGER SFC LOW THAN THE NAM. CONSIDERING THE
WIDESPREAD THUNDERSTORM ACTIVITY THAT IS DEVELOPING OVER THE GOM
ATTM JUST DOWN STREAM OF A SRN STREAM SHORT WAVE MOVING ACROSS SOUTH
TX...I THINK THE GFS MAY BE ON TO SOMETHING. MOST OF THE GFS
ENSEMBLE MEMBERS BRING PCPN ACROSS THE FORECAST AREA BY LATE
TOMORROW AS WELL. THE NEW HPC QPF ALSO FOLLOWS RATHER CLOSELY TO THE
GFS PCPN OUTPUT. ALL OF THIS LENDS ME THE CONFIDENCE TO INCREASE
POPS INTO THE LIKELY RANGE ACROSS THE SOUTHERN PART OF THE FORECAST
AREA BY LATE AFTERNOON.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
AS OF MIDDAY THU...CUTOFF UPPER LOW WILL CROSS THE DEEP SOUTH LATE
FRI AND TRACK SLOWLY ACRS GA/SC THRU SATURDAY...FINALLY MOVING ONTO
THE ATLANTIC EARLY SUNDAY. THE MAJOR GLOBAL MODELS...I.E. THE
GFS...EC AND GEM...ARE BASICALLY ON THE SAME PAGE WRT THE SFC LOW
TRACK AND EVOLUTION. NAM FAMILY...INCLUDING THE SREF MEAN AND NCEP
HIRES WINDOWS...FEATURE A WEAKER UPPER LOW AND A MORE SOUTHERN TRACK
WHICH EFFECTIVELY KEEP US NOTABLY DRIER THAN THE SOLUTIONS FROM THE
OTHER CAMP.

IN TERMS OF PWAT...MOISTURE FROM THE SYSTEM NEVER GETS ALARMINGLY
HIGH AS FAR NORTH AS OUR CWFA EVEN ON THE OPERATIONAL GFS OR NAEFS
ANOMALY PLOTS. CONVECTION LOOKS TO BE LIMITED WITH THE LOW STAYING TO
OUR SOUTH ON ALL THE OPNL GUIDANCE AND ALMOST ALL ENSEMBLE MEMBERS.
SREF HAS JUST ONE MEMBER TRACKING IT FAR ENOUGH NORTH TO BRING EVEN
MODEST CAPE INTO THE PIEDMONT DURING THE DAY SATURDAY. HOWEVER LAPSE
RATES REMAIN FAVORABLE SATURDAY AND SATURDAY NIGHT UNDER THE UPPER
TROUGH...SO CONVECTIVE RAIN RATES ARE NOT OUT OF THE QUESTION.

BRISK EASTERLY MIDLEVEL FLOW WOULD LIKELY PROVIDE ENHANCEMENT ALONG
THE BLUE RIDGE. HOW STRONG THE SFC WINDS GET IN THE HIGHER ELEVATIONS
WILL DEPEND ON HOW CLOSE THE LOW COMES. THIS BEARS WATCHING AS
SOLUTIONS CONVERGE BUT AT THIS TIME IT APPEARS GUSTS WOULD REMAIN
BELOW ADVY CRITERIA.

PWATS AND DEEP LAYER RH DIMINISH THRU THE DAY SATURDAY...SO RATES
SHOULD TAPER OFF. AS THE LOW PULLS AWAY SATURDAY NIGHT PRECIP AND SKY
COVER WILL DIMINISH LEAVING A DRY AND MOSTLY SUNNY SUNDAY.

MAX TEMP FORECAST IS SOMEWHAT CHALLENGING SATURDAY. THE EAST COAST
HIGH WILL HAVE RECENTLY DEPARTED...BUT WITH ONGOING PRECIP AND
ANOTHER FAIRLY STRONG HIGH MOVING INTO QUEBEC AND NEW ENGLAND UNDER
CONFLUENT UPPER HEIGHT PATTERN...THE WEDGE COULD HOLD ON THRU
SATURDAY. KEPT TEMPS NEAR THE LOWER END OF GUID FRI NIGHT INTO
SAT...THEN ALLOWING A RETURN TO NEAR NORMAL TEMPS SUNDAY.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
AS OF 215 PM EDT THURSDAY...THE GLOBAL MODELS CONTINUE TO AGREE THAT
AN UPPER LOW PRESSURE TROUGH WILL SHIFT OFFSHORE SUN NIGHT WITH AN
UPPER RIDGE OF HIGH BUILDING ACROSS THE SE THROUGH MONDAY. A
POSITIVELY TILTED UPPER TROUGH WILL THEN APPROACH FROM THE CENTRAL
CONUS MON NIGHT AND CROSSES THE REGION ON TUE. ANOTHER UPPER RIDGING
WILL BUILD IN BY WED.

AT THE SURFACE...CONDITIONS SHOULD DRY OUT BY SUN NIGHT AS A SURFACE
LOW MOVES FURTHER OFFSHORE AND A DRY HIGH BUILDS IN FROM THE NORTH.
THE HIGH WILL WEAKEN AND SHIFT OFFSHORE BY MON NIGHT AS A COLD FRONT
APPROACHES FROM THE WEST. MODELS SUGGEST THAT THE FROPA WILL OCCUR
TUE AFTERNOON. HOWEVER...THE FRONT WILL WEAKEN CONSIDERABLY AS IT
CROSSES THE MTNS. HENCE...I/VE INHERITED THE LOW END CHANCE POP OVER
THE PIEDMONT WITH SOLID CHANCE POP IN THE MTNS. ALSO HAVE MENTIONED
THUNDER AS MODELS CONTINUE TO HIGHLIGHT SOME DEGREE OF INSTABILITY
ALONG AND AHEAD OF THE FRONT. WED/THU LOOKS DRY AS A DRY HIGH BUILDS
ACROSS THE REGION. TEMPERATURES WILL BE AROUND NORMAL ON MON WITH A
FEW DEGREES ABOVE NORMAL TUE INTO THU.

&&

.AVIATION /21Z THURSDAY THROUGH TUESDAY/...
AT KCLT AND ELSEWHERE...LATEST MODEL RUNS AND GUIDANCE ARE NOT AS
AGGRESSIVE IN BRINGING IN STRATO-CU OVERNIGHT. HOWEVER...THERE IS A
GOOD CU FIELD NOT TOO FAR SOUTHWEST OF THE FORECAST AREA. I HAVE
DECIDED TO KEEP THE LOW END VFR CIGS...THOUGH I DID PUSH THEM BACK A
FEW HOURS AT ALL SITES. KCLT HAS ABOUT THE BEST CHANCE OF SEEING A
DECK UP AROUND 4KFT DEVELOPING LATE TONIGHT...AT LEAST PER THE
GUIDANCE. WINDS WILL BE ENE THIS AFTN...TURNING BACK TO NE OR EVEN
NNE LATE TONIGHT AS ANOTHER SURGE OF COOLER AIR MOVES IN FROM THE
NORTH. AS THIS HAPPENS...WIND SPEEDS SHOULD PICK BACK UP TO 8 TO 10
KTS BY AROUND SUNRISE.

OUTLOOK...CEILING AND VISIBILITY RESTRICTIONS WILL RETURN FRIDAY
EVENING THROUGH SATURDAY AS A COLD FRONT APPROACHES FROM THE WEST
AND A LOW PRESSURE SYSTEM DEVELOPS ALONG THE COAST. FAIR AND DRIER
WEATHER WITH NO RESTRICTIONS SHOULD RETURN FOR SUNDAY AND MONDAY.

CONFIDENCE TABLE...

            20-02Z        02-08Z        08-14Z        14-20Z
KCLT       HIGH 100%     HIGH 100%     HIGH  89%     HIGH  88%
KGSP       HIGH 100%     HIGH 100%     HIGH  99%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     HIGH  87%     HIGH  85%
KHKY       HIGH 100%     HIGH 100%     HIGH  99%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH  85%     HIGH  85%

THE PERCENTAGE REFLECTS THE NUMBER OF GUIDANCE MEMBERS AGREEING
WITH THE SCHEDULED TAF ISSUANCE FLIGHT RULE CATEGORY. COMPLETE
HOURLY EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLES AND ENSEMBLE
FORECASTS ARE AVAILABLE AT THE FOLLOWING LINK: (MUST BE LOWER CASE)

WWW.WEATHER.GOV/GSP/AVIATION_TABLES

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...NONE.
SC...NONE.

&&

$$
SYNOPSIS...JOH
NEAR TERM...CDG/MCAVOY
SHORT TERM...WIMBERLEY
LONG TERM...JOH
AVIATION...MCAVOY






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