Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 251727
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
127 PM EDT SAT APR 25 2015

.SYNOPSIS...
AN AREA OF LOW PRESSURE WILL MOVE EAST FROM KENTUCKY AND
TENNESSEE TO NORTH CAROLINA TONIGHT...THEN OFF THE COAST ON
SUNDAY.  COOL AND DRY HIGH PRESSURE DROPS DOWN FROM CENTRAL
CANADA SUNDAY AND WILL BE OUR WEATHER MAKER FOR THE FIRST HALF OF
THE WEEK.  EXPECT LOW PRESSURE TO MOVE EAST FROM TEXAS IN MID WEEK
AND BRING MORE RAIN WEDNESDAY AND THURSDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
1715 UTC UPDATE...WINDS WERE UPDATED TO INCORPORATE THE LATEST
NAM GUIDANCE. POPS AND QPF WERE REDUCED FURTHER BASED ON RADAR
TRENDS.

1400 UTC UPDATE...POPS WERE ADJUSTED BASED ON RADAR
TRENDS...FAVORING THE EAST OVER THE WEST. WINDS WERE UPDATED FROM AN
EQUALLY WEIGHTED BLEND OF THE LATEST ADJMAV AND NAM. AREAS OF RAIN
INDUCED FOG WERE UPDATED BASED ON OBSERVATIONS. TEMPERATURE EAST OF
KCLT WERE ADJUSTED SLIGHTLY FOR A BETTER FIT WITH NEIGHBORING
OFFICES.

AS OF 650 AM...I WILL ISSUE A QUICK UPDATE TO ADJUST POPS AND
TEMPERATURES TO OBSERVATIONS. OVERALL...THE CURRENT FORECAST APPEARS
ON TRACK.

AS OF 525 AM...WIDESPREAD CONVECTION ACROSS THE NORTHERN HALF OF AL
WILL CONTINUE TO TRACK EAST THIS MORNING. A LARGE AREA OF -RA WILL
GRADUALLY INCREASE ACROSS THE WESTERN CAROLINAS AHEAD OF THE DEEP
CONVECTION. I WILL UPDATE THE FORECAST TO MAKE MINOR TWEAKS TO THE
POP TRENDS. IN ADDITION...I WILL UPDATE TEMPERATURES TO INDICATE
VALUES NEARLY STEADY THROUGH SUNRISE.

AS OF 340 AM...HIGH DEBRIS CLOUDS WILL LIKELY INCREASE ACROSS THE
CWA THROUGH SUNRISE. FURTHER WEST...WATER VAPOR LOOP INDICATED A
CLOSED MID LEVEL LOW OVER THE CENTRAL PLAINS...DRIFTING TO THE EAST-
NORTHEAST. SOUTH EAST OF THE MID LEVEL LOW...A LARGE BAND OF
CONVECTION WAS TRACKING EAST ACROSS NRN MS/AL. THIS ACTIVITY LARGELY
ORIGINATED NEAR THE SFC WARM FRONT AND WITHIN A BELT OF UPPER JET
DIVERGENCE. HOWEVER...THE CONVECTION IS TRACKING QUICKLY
EAST...OUTPACING THE MOST SUPPORTIVE ENVIRONMENT. AS A
RESULT...RECENT IR SATELLITE IMAGES INDICATED A STEADY WARMING TREND
OF CLOUD TOP TEMPERATURES...LIGHTNING COUNTS WERE ALSO DECREASING.
THESE WEAKENING TRENDS MATCH UP WELL WITH THE CONVECTION ALLOWING
MODELS...WHICH INDICATES THAT THE DEEP CONVECTION WILL STEADY WEAKEN
AS IT TRACKS EAST ALONG THE I-20 CORRIDOR. A LARGE AREA RAIN IS
FORECAST TO OCCUR ACROSS THE CWA THIS MORNING...PRIMARILY WEST OF I-
26. I WILL INDICATE THE RAINFALL TO FALL FROM SHRA WITH SOME
EMBEDDED THUNDERSTORMS. SEVERE WEATHER ASSOCIATED WITH THE MORNING
ACTIVITY APPEARS MINIMAL.

BY MID DAY...THE BULK OF THE DECAYING CONVECTION IS EXPECTED TO PUSH
EAST OF THE REGION. DURING THE AFTERNOON...THE PRIMARY CHALLENGE
WILL BE THE PLACEMENT OF A WEAK IN-SITU AREA OF COLD AIR DAMMING.
BASED OFF THE LATEST NAM...I WILL INDICATE THAT THE NC FOOTHILLS AND
NRN PIEDMONT WILL SEE LIGHT NE WINDS AND THICK AND PERSISTENT CLOUD
COVER. HOWEVER...AREAS TO THE SOUTH AND WEST MAY EXPERIENCE DEEP
SOUTHWEST FLOW AND DIURNALLY WARMING TEMPERATURES. IN FACT...THE NAM
SHOWS A HORSESHOE SHAPED AREA OF SB CAPE ACROSS THE MTNS AND
PORTIONS OF THE UPPER SAVANNAH RIVER VALLEY AND SC UPSTATE EAST OF I-
85. IN ADDITION...IMPRESSIVE SHEAR VALUES DEVELOP WITHIN THE
UNSTABLE AREAS...PEAKING DURING THE MID EVENING HOURS. HEILICITY
BETWEEN SFC-1KM IS FORECAST TO RANGE AROUND 300 M2/S2...EHI VALUES
FROM 2.5-4...AND SWEAT VALUES GENERALLY AROUND 350. HOWEVER...CIN
VALUES FROM THE NAM SOUNDING RANGE FROM 30 TO 50 J/KG THIS EVENING.
MODELS INDICATE THAT CONVECTION COVERAGE WILL BE AT A MINIMUM DURING
THE LATE AFTERNOON AND EVENING.

TONIGHT...A WELL DEVELOPED SFC LOW WILL SLIDE EAST ACROSS
KY/TN...CROSSING THE SOUTHERN APPALACHIANS AROUND MIDNIGHT. AT THE
MID LEVELS...THE CLOSED LOW FROM THIS MORNING WILL DEVELOP INTO A
POSITIVE TILTED TROF AND WILL RIPPLE EAST OVER THE SFC LOW. NEAR TERM
MODELS INDICATE THAT A MCS WILL LIKELY DEVELOP AHEAD OF THE LOW
CENTER...STEERING FLOW SHOULD RESULT IN THE SYSTEM TO TRACK EAST-
SOUTHEAST TONIGHT. BASED ON EXPERIENCE...I ANTICIPATE THAT THE MCS
SPEED WILL BE GREATER THAN THE MODEL CONSENSUS. THIS ADJUSTMENT
WOULD PLACE THE MCS OVER THE NC MTNS DURING THE EARLY MORNING HOURS
SUNDAY. BY THAT TIME...THE GREATEST SHEAR AND INSTABILITY VALUES
HAVE ENDED...LEAVING WEAK TO MODERATE VALUES ACROSS THE MTNS AND
SC/GA LINE. THIS WILL LIKELY SUPPORT THE MCS REACHING THE MTNS...BUT
STEADILY WEAKENING AS THE ACTIVITY APPROACHES THE FOOTHILLS.
HOWEVER...LOW WBZ AND LINGERING SHEAR AND INSTABILITY WOULD LIKELY
YIELD SEVERAL SEVERE WIND AND HAIL EVENTS AS THE CONVECTION ARRIVES.
WE WILL CONTINUE TO HIGHLIGHT IN THE HWO.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
AS OF 330 AM SATURDAY...THE SHORT TERM BEGINS WITH TRYING TO TIME
THE EXIT OF THE SURFACE LOW OFF THE CAROLINA COAST. SHORT TERM
GUIDANCE IN PRETTY GOOD AGREEMENT WITH EAST-WEST ELONGATED TROUGH
STRETCHING WESTWARD INTO OUR AREA FROM THE CENTER OF THE SURFACE LOW
IN THE ATLANTIC AT THE START OF THE SHORT TERM. SPLIT FLOW PATTERN
CONTINUES ALOFT...WITH SHORTWAVE ALSO PUSHING OFFSHORE AND MEAN
TROUGH TAKING OVER. STROGN DOWNSLOPING COMPONENT ON SUNDAY WILL LEAD
TO HIGHS INCREASING TO WELL ABOVE NORMAL VALUES ACROSS THE UPSTATE
AND PORTIONS OF THE PIEDMONT BEFORE THE ACTUAL COLD FRONT PUSHES
THROUGH...THOUGH ESPECIALLY THE NORTHERN MOUNTAINS WILL BE ON THE
COOL SIDE. MIDLEVEL MOISTURE HANGS ON ENOUGH TO KEEP SOME CLOUDINESS
AND PERHAPS SOME LINGERING SHOWERS ACROSS THE AREA THROUGH THE EARLY
PART OF THE DAY...BUT WE SHOULD START SEEING A CLEARING TREND
OVERNIGHT SUNDAY NIGHT INTO MONDAY.

SHOULD SEE A PRETTY MARKED DIFFERENCE IN HIGHS ON MONDAY BEHIND THE
FRONT WITH DEEP-LAYER CAA ACROSS THE AREA...THOUGH THE MAGNITUDE OF
THE CAA DECREASES THROUGH THE DAY. ANOTHER SHORTWAVE AXIS PUSHES
DOWN THE UPPER TROUGH...STILL FIRMLY IN PLACE ACROSS THE EASTERN
PORTION OF THE COUNTRY...LATE MONDAY INTO EARLY TUESDAY. THIS TROUGH
MANAGES TO FINALLY PUSH THE DEEP UPPER LOW FARTHER OUT INTO THE
ATLANTIC BUT WILL USHER IN COOLER HIGH PRESSURE FOR THE BEGINNING OF
THE EXTENDED.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
AS OF 345 AM SATURDAY...HIGH PRESSURE CENTERED ACROSS THE NORTHERN
PLAINS AND SOUTHERN CANADA WILL BE RIDGING INTO THE SOUTHEAST AS WE
START THE EXTENDED...WITH THE AFOREMENTIONED UPPER SHORTWAVE DIGGING
AROUND THE WESTERN PERIPHERY OF THE LOW OVER THE NORTHWEST ATLANTIC.
MEANWHILE...A CUTOFF LOW OVER THE SOUTHERN PLAINS WILL BEGIN TO
SLIDE EAST AS THE WAVETRAIN FINALLY PROGRESSES. HEIGHT FALLS AHEAD
OF THE UPPER LOW WILL INDUCE CYCLOGENESIS ACROSS THE NORTHERN
GULF...SPREADING MOISTURE INTO THE LOWER MISSISSIPPI VALLEY AND
SOUTHEAST THROUGH THE MIDDLE OF THE WORK WEEK. BOTH THE GFS AND
ECMWF HAVE BEEN TRENDING THE SURFACE LOW FARTHER SOUTH WITH
SUBSEQUENT RUNS AND FOR THIS MORNING IT IS LITTLE DIFFERENT...WITH
THE LOW PUSHING EAST ACROSS THE FLORIDA PANHANDLE UNTIL WEDNESDAY
NIGHT AND ONLY LIFTING SLIGHTLY INTO SOUTH GEORGIA. THE ECMWF HAS
SPED THE SYSTEM UP WITH A DEEPER NORTH-SOUTH ELONGATED TROUGH PHASED
WITH THE GREAT LAKES TROUGH...WHEREAS THE GFS HAS TRENDED A LITTLE
SLOWER AND TAKES A LITTLE LONGER TO PHASE THE FORMER CUTOFF LOW WITH
THE LARGER SYSTEM. IN ANY CASE...CLOUDY AND COOL CONDITIONS WILL
PREVAIL THROUGH THE MIDDLE OF THE WEEK...WITH HIGHS ESPECIALLY
WEDNESDAY A GOOD 10 DEGREES BELOW SEASONAL NORMALS. VERY HARD TO
TELL AT THIS POINT...WITH THE DIFFERENCES IN THE MODELS PLUS LACK OF
RUN-TO-RUN CONTINUITY...WHEN THE PRECIP WILL MOVE OUT. IN GENERAL
HAVE CONTINUED TREND OF MID-RANGE POPS...WITH A FEW LIKELIES SOUTH
ON WEDNESDAY...WITH A SLOW DRYING TREND THURSDAY INTO THURSDAY
EVENING. MORE DIVERGENCE IN EXTENDED GUIDANCE AT THE END OF THE
PERIOD BUT OVERALL EXPECT A DRYING TREND TOWARD THE END OF THE WORK
WEEK WITH TEMPERATURES TRENDING BACK TOWARDS CLIMO.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
AT KCLT...LOW CONFIDENCE FORECAST...WITH MODEL GUIDANCE QUITE
DIVERGENT. OBSERVATIONS SHOW IFR CIG RESTRICTION...BUT VSBY HAS
IMPROVED BEHIND DEPARTING RAINFALL. MODEL TIME HEIGHTS SUPPORT BRIEF
CIG IMPROVEMENT TO LOW VFR THIS EVENING BEFORE LOWERING TO AT LEAST
MVFR LATE TONIGHT INTO SUNDAY MORNING. GUIDANCE SUPPORTS AN MVFR
VSBY LATE THIS EVENING...BUT SUGGEST A RETURN TO VFR TOWARD DAWN...
OSTENSIBLY DUE TO NE WINDS THAT SET UP OVERNIGHT AS A COLD FRONT
ARRIVES FROM THE N. CONVECTIVE CHANCES APPEAR BEST OVERNIGHT AS THE
FRONT APPROACHES.

ELSEWHERE...CONFIDENCE IS LIMITED AS TRENDS DEPART FORM PREVIOUS
FORECAST...AND MODEL SOLUTIONS DIVERGE. OBSERVED CIG RESTRICTIONS
MAY IMPROVE THIS EVENING BASED ON MODEL TIME HEIGHTS...BUT PROBABLY
NOT AS MUCH AS A MODEL BLEND WOULD SUGGEST. VSBY IMPROVEMENT HAS
BEEN NOTED AS RAINFALL MOVES OFF TO THE EAST...AND THIS TREND IS
EXPECTED OT CONTINUE. TIME HEIGHTS SHOW CIGS LOWERING AGAIN TOWARD
DAWN...AND REMAINING LOW INTO SUNDAY MORNING. VSBY RESTRICTIONS
APPEAR TO RETURN AT KHKY LATE THIS EVENING...THEN END AS A BACK DOOR
COLD FRONT ARRIVES BEFORE DAWN VEERING THE WIND TO THE NE. MODEL
DATA SUPPORTS KAND HAVING A DAYBREAK VSBY RESTRICTION...BUT MODEL
WINDS REMAIN UP...FAVORING VFR. CONVECTIVE CHANCES NOW APPEAR TO BE
BEST OVERNIGHT AHEAD OF THE COLD FRONT.

OUTLOOK...A COLD FRONT WILL SWEEP ACROSS THE REGION SUNDAY
NIGHT...FOLLOWED BY DRY HIGH PRESSURE THROUGH TUESDAY. ANOTHER LOW
PRESSURE SYSTEM MAY TRACK ACROSS THE DEEP SOUTH WEDNESDAY AND
THURSDAY.

CONFIDENCE TABLE...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       MED   75%     HIGH  83%     LOW   31%     MED   66%
KGSP       HIGH 100%     HIGH  87%     HIGH 100%     HIGH  80%
KAVL       HIGH  83%     LOW   56%     MED   64%     MED   60%
KHKY       MED   75%     HIGH 100%     MED   64%     MED   73%
KGMU       HIGH 100%     HIGH  87%     HIGH  93%     MED   70%
KAND       HIGH 100%     HIGH  81%     HIGH  93%     LOW   53%

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...TDP
NEAR TERM...JAT/NED
SHORT TERM...TDP
LONG TERM...TDP
AVIATION...JAT


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