Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 232007 AAC
AFDGSP

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
307 PM EST TUE DEC 23 2014

.SYNOPSIS...
A MOIST AIRMASS WILL REMAIN OVER THE REGION TONIGHT BEFORE A COLD
FRONT CROSSES ON WEDNESDAY. HIGH PRESSURE WILL ARRIVE ON THURSDAY
AND LINGER INTO FRIDAY. ANOTHER FRONTAL SYSTEM WILL CROSS THE AREA
ON SATURDAY AND SUNDAY...FOLLOWED BY HIGH PRESSURE ON MONDAY.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
AS OF 230 PM...KGSP RADAR CONTINUES TO SHOW AN AREA OF LIGHT TO
MODERATE RAIN...WITH EMBEDDED HEAVY RAIN AND THUNDER...MOVING NWD
OVER THE PIEDMONT OF THE CAROLINAS. THIS AREA OF PRECIP WILL
GRADUALLY WEAKEN OVER NC AS IT MOVES FARTHER N AND DEEPER OVER THE
WEDGE...BUT WILL STILL REQUIRE ALTERING THE PRECIP PROBABILITY TO
ACCT FOR VERY HIGH PROBABILITY RIGHT AWAY OVER THE NC FOOTHILLS AND
WRN PIEDMONT. WILL ALSO NOT RULE OUT AN OCCASIONAL LIGHTNING STRIKE.
MEANWHILE...OVER PORTIONS OF THE TN VALLEY AND SRN APPALACHIANS
OUTSIDE THE WEDGE...SOME TSTM ACTIVITY MAY SKIRT THE SW MTNS OF NC
THROUGH LATE AFTERNOON. BRIEFLY LOWERED THE POP IN BETWEEN THESE TWO
AREAS...BUT EXPECT THAT INCREASING FORCING SHOULD HELP MORE STORMS
TO DEVELOP...AND THE ATMOSPHERE WILL CATCH UP WITH THE FCST. TEMPS
ARE STILL IN GOOD SHAPE.

LOOKS LIKE AN ACTIVE 18-24 HRS ACROSS THE WESTERN CAROLINAS AND NE
GEORGIA. A MULTITUDE OF FORCING WILL CONVERGE UPON THE FCST AREA
THROUGH TONIGHT...WITH PLENTY OF DEEP MOISTURE. IT IS GOOD BET THAT
THE ENTIRE FCST AREA WILL SEE RAIN AT SOME POINT...THOUGH PERHAPS
NOT CONTINUOUSLY AS A 100 PCT POP MIGHT SUGGEST TO SOME. THE MAIN
ISSUES OVERNIGHT CONCERN RAINFALL AMTS AND WIND ON THE RIDGETOPS. A
45-55KT LOW LEVEL JET WILL TRANSLATE E ACROSS THE MTNS IN THE 06Z TO
12Z WED TIME FRAME. THIS COULD RESULT IN SOME STRONG WIND GUSTS ON
THE RIDGETOPS...BUT USUALLY THIS DOES NOT RESULT IN HIGH WINDS DOWN
IN THE VALLEYS DURING NIGHTTIME WARM ADVECTION FLOWS. WILL NOT ISSUE
ANY ADVISORIES FOR WIND. AS FOR HEAVY RAIN...EXPECT THE PASSAGE OF
SEVERAL SHOWERS THAT COULD CONTAIN FAIRLY HIGH RAINFALL RATES.
HOWEVER...THE TREND IN QPF IN THE MODELS AND OTHER GUIDANCE HAS BEEN
DOWNWARD...AND ANTECEDENT CONDITIONS ARE RELATIVELY DRY IN SPITE OF
RECENT LIGHT RAIN. THIS DOES NOT EXACTLY LOOK LIKE THE CASE WHERE
CONVECTION WILL RACE AHEAD OF THE FRONT AND MOVE ALONG THE GULF
COAST TO DISRUPT THE MOISTURE TRANSPORT TO THE CAROLINAS. IF
ANYTHING...IT LOOKS A BIT MORE LIKE THE CASE WHERE MOISTURE
TRANSPORT COULD BE ENHANCED A BIT BY A SLOW MOVING SYSTEM. AT ANY
RATE...CONFIDENCE IS STILL NOT HIGH ENOUGH THAT WE WILL HAVE ANY
SORT OF FLOODING WHERE THE RAIN IS LIKELY TO BE THE HEAVIEST...NEAR
THE BLUE RIDGE IN GEORGIA AND ALONG THE SC/NC BORDER. SO...A WATCH
WILL NOT BE ISSUED AT THIS TIME. EXPECT THE WEDGE TO HOLD ACROSS THE
REGION OVERNIGHT...MEANING TEMPS WILL BE NEARLY STEADY...PERHAPS
RISING A DEGREE OR TWO IN THE EARLY MORNING HOURS.

ON WEDNESDAY...AS OUR FLOOD THREAT GOES DOWN...OUR SEVERE WEATHER
THREAT CREEPS UP A BIT. SHEAR WILL BE PLENTIFUL AND THE LATEST
GUIDANCE SHOWS SUFFICIENT INSTABILITY TO GIVE PAUSE ABOUT A HIGH
SHEAR/LOW CAPE QLCS SCENARIO. HOWEVER...THE INGREDIENTS APPEAR TO
FAIL TO COME TOGETHER AT THE SAME TIME AND PLACE.  THE BEST SHEAR
WILL LIKELY RACE OFF TO THE EAST WITH THE LOW LEVEL JET EARLY IN THE
MORNING. MEANWHILE...IT WILL TAKE SOME TIME TO DESTROY THE REMNANT
OF THE WEDGE...PERHAPS NOT UNTIL MIDDAY OR EARLY AFTERNOON FROM THE
SOUTH. THAT DOES PROVIDE FOR SOME SUFFICIENT OVERLAP DURING THAT
TIME...BUT THE STRONG FORCING REMAINS TO THE WEST. EVEN AT THAT...
THE UPPER TROF SHOULD BE DEAMPLIFYING THROUGH THE DAY WITH THE SHORT
WAVE STEADILY SHEARING OUT. MOST OF OUR GOOD SEVERE QLCS CASES
FEATURE A STRONGLY ADVECTIVE SRN STREAM WAVE THAT ORGANIZES THE
CONVECTION INTO A LINE...BUT THIS ONE APPEARS TO NOT HAVE THAT KIND
OF FORCING...AND IT ONLY WEAKENS WITH TIME AS THE WAVE FINALLY MOVES
IN FROM THE WEST IN THE EVENING. WILL HAVE TO KEEP AN EYE ON
IT...BUT FOR NOW AGREE WITH DAY2 OUTLOOK IN KEEPING THE MARGINAL
RISK TO THE S AND E. TEMPS WILL BE TRICKY DEPENDING ON WEDGE
DESTRUCTION.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
AT 200 PM EST TUESDAY...THE COLD FRONT SHOULD BE MAKING ITS WAY
ACROSS THE EASTERN PORTION OF THE FORECAST AREA CIRCA 00Z THU. THE
12Z NAM AND SREF ARE STILL SLOWER WITH THE FRONT THAN THE GLOBAL
MODELS...BUT THE GAP HAS CLOSED TO MORE LIKE 3 OR SO HOURS SLOWER.
HENCE...EXPECT NUMEROUS SHOWERS AND PERHAPS SOME EMBEDDED
THUNDERSTORMS TO BE ONGOING WED EVENING ESSENTIALLY ALONG AND EAST
OF THE I-77 CORRIDOR. THE FRONT AND ASSOCIATED SHOWERS SHOULD THEN
MOVE WELL EAST OF THE AREA BY MIDNIGHT. THE EXCEPTION WILL BE IN THE
MOUNTAIN NW FLOW UPSLOPE AREAS WHERE SCATTERED RAIN/SNOW SHOWERS
WILL PERSIST INTO THE EARLY CHRISTMAS. IN FACT...THE UPPER TROUGH
AXIS WILL TRAIL THE FRONT BY ABOUT 6 HOURS OR SO AND THE ADDED UPPER
SUPPORT MAY LEND TO AN UPTICK IN UPSLOPE PRECIP AFTER MIDNIGHT FOR A
WHILE EVEN LOW LEVEL WIND FIELDS ARE NOT IDEAL FOR UPSLOPE PRECIP.
ALSO...SOME MID LEVEL CLOUDS ASSOCIATED WITH TROUGH WILL SLOW
CLEARING EAST OF THE MOUNTAINS.

HIGH PRESSURE CENTERED ALONG THE GULF COAST WILL RIDGE NORTHWARD
OVER THE REGION DURING CHRISTMAS. HENCE...EXPECT PLENTY OF SUNSHINE
AND TEMPS A LITTLE ABOVE CLIMO. THE MODELS FORECAST THE CENTER OF
THE HIGH TO MIGRATE DIRECTLY OVER THE WESTERN CAROLINAS AND NE GA
THU NIGHT AND THEN MOVE EAST OF THE AREA ON FRIDAY. GENERALLY CLEAR
CONDITIONS WILL PREVAIL UNTIL LATE IN THE DAY FRIDAY WHERE TIME
HEIGHTS INDICATE THAT MID AND HIGH CLOUDINESS WILL BE ON THE
INCREASE. TEMPS WILL BE CONTINUE A LITTLE ABOVE CLIMO THU NIGHT AND
FRI AS AIRMASS IS NON POLAR IN ORIGIN.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
AS OF 130 PM TUESDAY...HIGH PRESSURE WILL DRIFT OFF THE SOUTHEAST
COAST EARLY SATURDAY AS HEIGHTS FALL GENTLY OVER THE ERN STATES IN
RESPONSE TO TWO SEPARATE SHORTWAVES. THE LEADING NRN STREAM WAVE WILL
CROSS ONTARIO SATURDAY INDUCING SFC CYCLOGENESIS. LATEST GFS/EC/GEM
HAVE TRENDED SLOWER WITH THE ASSOCIATED FRONT...CONTINUING TO SHOW IT
EXTENDING TO THE GULF COAST...BUT NOW DELAYING ITS WEAK PUSH ACRS THE
SRN APPALACHIANS UNTIL SAT NIGHT.

THE SRN STREAM WAVE COMES INTO PLAY SAT NIGHT/SUNDAY AS IT DIGS INTO
TEXAS AND A SFC LOW FORMS ALONG THE FRONT INVOF THE GULF COAST. THIS
GENERAL EVOLUTION IS DEPICTED ON ALL THREE MODELS THOUGH THE DETAILS
AND TIMING ARE NOT CONSISTENT. CONSENSUS DOES SUPPORT INCREASING POPS
BEGINNING LATE SAT...THEN TAPERING OFF MONDAY. THE GEM PROGRESSION OF
THE SHORTWAVE IS FASTEST...MAKING THE LOW SPIN UP OVER LA/MS...BEFORE
THE FRONT REACHES THE MTNS. SAID LOW REMAINS AN OPEN FRONTAL WAVE.
THE EC IS CLOSE TO THE GEM WITH THE GENESIS OF THE LOW /BUT CLOSES IT
OFF/ AND IS MUCH SLOWER IN BRINGING IT THRU OUR CWFA. THE GFS IS
SLOWER WITH THE WAVE SUCH THAT IT FORMS TO OUR SOUTH AND KEEPS US ON
THE COOL SIDE OF THE SYSTEM. THUS THE GFS IS THE ONLY MODEL
SUGGESTING A SIGNIFICANT WINTRY PRECIP THREAT WITH THE EVENT...THAT
BEING SUN NIGHT INTO MON OVER THE NC MTNS. NONE OF THE CONSULTED
GUIDANCE INDICATES QPF WILL BE ENOUGH TO CAUSE NOTABLE HYDRO
CONCERNS.

TEMPS WILL REMAIN SEVERAL DEGREES ABOVE CLIMO THRU SATURDAY...WITH
THE DIURNAL RANGE BEING LIMITED SUNDAY BY CLOUDS/PRECIP. ONCE THE
FRONT/WAVE MOVE THROUGH TEMPS WILL RETURN TO NEAR CLIMO.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
AT KCLT...FLYING CONDITIONS APPEAR RATHER GRIM OVER THE NEXT 24
HOURS...WITH WIDESPREAD IFR/LOW IFR CONDITIONS EXPECTED TO CONTINUE
AT LEAST THROUGH THE OVERNIGHT HOURS. AM MONITORING TSRA OVER
UPSTATE SC AND WILL AMEND TAF IF LTG APPROACHES FROM THE S/SW. WILL
NOT RULE OUT SOME BRIEF MVFR PERIODS...ESPECIALLY AFTER ANY SHOWER
ACTIVITY...BUT CONFIDENCE IS FAIRLY HIGH THAT IFR WILL BE AS GOOD AS
IT GETS FOR LONG STRETCHES OF TIME. THE GUIDANCE ACTUALLY GOES WITH
VERY LOW IFR TONIGHT AS ISENTROPIC LIFT CONTINUES OVER THE TOP OF
THE PERSISTENT OLD WEDGE ACROSS THE PIEDMONT. HAVE OPTED TO TRY TO
STAY MORE OPTIMISTIC WITH MERELY LOW IFR FOR NOW...BUT EXPECT AT
LEAST A TEMPO GROUP WITH LATER ISSUANCES TO ACCOUNT FOR PERIODS OF
VLIFR. THROUGH THE EVENING AND MUCH OF THE OVERNIGHT...THERE SHOULD
BE AN EASTERLY CROSSWIND. AS THE LOW LEVEL JET ARRIVES AFTER
MIDNIGHT...EXPECT A PERIOD WITH SOME LOW LEVEL WIND SHEAR FROM THE
SOUTH...WHICH RAMPS UP AROUND 06Z. HARD TO SAY WHEN A WIND SHIFT
WILL START TO OCCUR TO SE AFTER SUNRISE AS WEDGES ARE SOMETIMES
STUBBORN. COMPROMISED ON 13Z IN ORDER TO BEGIN A PROB30 FOR
TSRA...WHEN IN FACT THE WIND SHIFT MIGHT NOT BE UNTIL CLOSER TO THE
END OF THE PERIOD. THE TIMING WILL BE FINE TUNED LATER.

ELSEWHERE...VERY SIMILAR TO KCLT OUTSIDE THE MTNS WITH WIDESPREAD
IFR/LIFR CONDITIONS AND A NE WIND UNDER THE WEDGE. THE ONLY
DIFFERENCE WILL BE TIMING THE DESTRUCTION OF THE WEDGE WITH A
SOUTHERLY WIND SHIFT...WHICH SHOULD BEGIN BY 14Z OR SO AT KAND BUT
IS NOT LIKELY TO REACH THE REST OF THE NON-MTN TAFS BEFORE THE END
OF THE PERIOD. AT KAVL...WIND WILL BE MORE SE THROUGH THE PERIOD.

OUTLOOK...DEEP MOISTURE WILL CONTINUE AT ALL TAF SITES UNTIL A COLD
FRONT CROSSES THE AREA FROM THE WEST LATE WED. WIDESPREAD RAIN
SHOWERS AND ASSOCIATED LOW RESTRICTIONS WILL CONTINUE THROUGH FROPA.
DRY HIGH PRESSURE WILL RETURN THROUGH LATE WEEK.

CONFIDENCE TABLE...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       MED   71%     HIGH  83%     HIGH  95%     MED   75%
KGSP       MED   75%     HIGH  80%     HIGH  95%     HIGH  85%
KAVL       HIGH  80%     MED   78%     HIGH  89%     HIGH 100%
KHKY       MED   71%     MED   73%     HIGH  85%     HIGH  90%
KGMU       MED   75%     HIGH  80%     HIGH  95%     HIGH  85%
KAND       LOW   59%     HIGH  83%     HIGH  93%     MED   72%

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...RWH
NEAR TERM...PM
SHORT TERM...LG
LONG TERM...WIMBERLEY
AVIATION...PM



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