Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 182344
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
744 PM EDT SAT APR 18 2015

.SYNOPSIS...
A VERY MOIST WARM FRONT WILL LIFT NORTHEAST ACROSS THE AREA TONIGHT
INTO SUNDAY. A COLD FRONT WILL THEN CROSS THE AREA ON
MONDAY...USHERING IN SEASONABLE TEMPERATURES AND DRY CONDITIONS.

&&

.NEAR TERM /THROUGH SUNDAY/...
AS OF 730 PM EDT SATURDAY...ONLY WEAK INSTABILITY LEFT ACROSS THE
AREA WITH A DIMINISHING TREND SEEN IN THE INTENSITY OF THE SHOWERS
AND FEW THUNDERSTORMS OUT THERE. EVEN THAT THOUGH IS A BIT OF A
MISNOMER SINCE WE HAVE NOT SEEN ANY CLOUD-TO-GROUND LIGHTNING IN ANY
OF THE ACTIVITY SO FAR THIS EVENING. EXPECT A CONTINUED DIMINISHING
TREND FOR THE NEXT COUPLE OF HOURS...RAMPING UP AGAIN AFTER 06Z AND
ESPECIALLY AFTER 09Z AS THE WARM FRONT LIFTS NORTH AND ISENTROPIC
UPGLIDE INCREASES SIGNIFICANTLY. OVERALL POP TRENDS STILL LOOK OKAY
BUT WILL LIKELY NEED TO REFINE THEM WITH THE LATE EVENING UPDATE...
MOST LIKELY TO LOWER THEM SOMEWHAT THROUGH THE EARLY OVERNIGHT
PERIOD BUT RAISE THEM TOWARD SUNRISE...REALLY SEE NO REASON TO NOT
GO 100 PERCENT AS THE DEEPER MOISTURE PUSHES IN. TEMPERATURES LOOK
PRETTY GOOD FOR THE MOST PART AND HAVE ONLY UPDATED HOURLY TRENDS
BASED ON OBS.

AS OF 430 PM EDT SATURDAY...ONLY MINOR TWEAKS TO THE NEAR TERM
FORECAST AT THIS TIME. UPDATED HOURLY TEMP/DEWPOINT/SKY TRENDS TO
LINE UP WITH OBSERVATIONS. NO CHANGES TO POPS AT THIS TIME AS LATE
AFTERNOON AND EVENING TRENDS LOOK GOOD PER RADAR AND HIRES
REFLECTIVITY FORECASTS...BUT SOME OF THE LATTER BRING MORE
WIDESPREAD PRECIPITATION INTO NORTHEAST GEORGIA AND THE UPSTATE
BETWEEN 03-06Z. WILL CONTINUE TO WATCH TRENDS BUT MAY NEED TO BUMP
UP POPS FOR THAT TIMEFRAME IN SUBSEQUENT UPDATES.

AS OF 300 PM EDT SATURDAY...PRIMARY FCST CONCERN THROUGH THE NEAR
TERM PERIOD WILL BE THE POTENTIAL FOR HEAVY RAIN ACROSS MUCH OF THE
AREA ON SUNDAY.  AS FOR TODAY...EXPECTING CONVECTION TO CONTINUE TO
INITIATE...ESPECIALLY ACROSS THE EASTERN UPSTATE AND NC PIEDMONT
WHERE BEST DIFFERENTIAL HEATING HAS OCCURRED.  LATEST SPC MESO
ANALYSIS INDICATES ROUGHLY 500K/KG SBCAPE SPREAD ACROSS THESE ZONES
WITH FURTHER EXPANSION EXPECTED ALONG WITH ADDITIONAL
DESTABILIZATION.

BEYOND THAT...CONVECTION WILL DIMINISH THIS EVENING WITH HEATING
LOSS MAKING WAY FOR INCOMING SHORTWAVE AND ASSOCIATED SURFACE WARM
FRONT.  THUS...FCST FEATURES CHANCE POPS ACROSS MUCH OF THE AREA
THIS AFTERNOON TO ACCOUNT FOR ISOLATED/SCATTERED THUNDERSTORMS...
WITH FURTHER INCREASES ACROSS NORTHEAST GA...SOUTHWEST NC...AND THE
WESTERN UPSTATE OVERNIGHT.  EXPECTING THE UPPER VORT MAX TO PASS
THROUGH THE REGION AFTER MIDNIGHT WITH ENHANCED H85 LLJ OUT AHEAD.
NEARLY 40-50KTS SOUTHERLY H85 FLOW WILL HELP PROMOTE ENHANCED
UPSLOPING ALONG THE SOUTHWEST NC...AND WESTERN NC ESCARPMENT.
FURTHERMORE...ANTECEDENT CONDITIONS FROM PREVIOUS DAYS RAINFALL OVER
THE UPSTATE AND PORTIONS OF NORTHEAST GA WILL YIELD ENHANCED
FLOODING CONCERNS AS A RESULT OF NEARLY 2-4 INCHES OF QPF IN THE
FCST.  THUS...OPTED TO ISSUE FLOOD WATCH FOR THESE ZONES STARTING
TONIGHT AT MIDNIGHT RUNNING THROUGH 8PM SUNDAY.

IN ADDITION...EXPECTING CONVECTION ALONG THE WARM FRONT AS IT SWEEPS
THROUGH ON SUNDAY.  MODELS INDICATE NEARLY 500-1000J/KG SBCAPE WITH
MODEST SHEAR AT 30-40KTS BULK.  THUS...FCST FEATURES CAT POPS
THROUGH THE DAY ON SUNDAY WITH HEAVY RAIN MENTION DUE TO WIDESPREAD
SHOWERS/THUNDERSTORMS.  NEW SPC DAY2 OUTLOOK PLACES THE REGION IN
THE SLIGHT RISK FOR SEVERE WEATHER...WHICH SEEMS TO FIT WELL WITH
THE FCST.  THEREFORE EXPECTING ADDITIONAL THREATS OF DAMAGING
WINDS...AND PERHAPS ANY ISOLATED TORNADO ON SUNDAY.  TEMPERATURES
THROUGH THE PERIOD WILL REMAIN ABOVE NORMAL.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
AS OF 215 PM SATURDAY...PRECIP CHANCES WILL BE WINDING DOWN SUNDAY
EVENING...AS WARM CONVEYOR BELT ZONE SHIFTS INTO THE MID-ATLANTIC.
LOW LEVEL MOISTURE WILL LINGER THROUGH THE OVERNIGHT WITHIN DEEP SW
FLOW...AND TOKEN SLIGHT/LOW CHANCE POPS WILL BE RETAINED THROUGH
MONDAY MORNING.

A POTENTIALLY VERY INTERESTING SITUATION THEN EVOLVES FOR MONDAY
AFTERNOON...AS HEIGHT FALLS AND ATTENDANT FRONTAL ZONE OVERSPREAD
THE REGION FROM THE TENN VALLEY. IT SEEMS THAT THE MODELS ARE
DEPICTING AN INCREASINGLY WELL-FORCED FRONT WITH EACH SUCCESSIVE
RUN...SWINGING A FAIRLY SUBSTANTIVE SHORT WAVE TROUGH ACROSS THE
AREA DURING THE AFTERNOON. IN RESPONSE...THE LOW LEVEL FLOW IS MORE
BACKED THAN IN EARLIER RUNS...RESULTING IN BETTER SHEAR PROFILES AS
WELL AS IMPROVED FRONTAL CONVERGENCE. SHORT TERM MODELS CONTINUE TO
DEPICT ROBUST BUOYANCY...AS ADVECTION OF ELEVATED MIXED LAYER AIR
INTO THE SOUTHEAST SUPPORTS MID-LEVEL LAPSE RATES OF AROUND 7 C/KM.
BASED UPON A MODEL AND ENSEMBLE CONSENSUS...CAPE OF AROUND 2000 J/KG
APPEARS LIKELY ACROSS MUCH OF THE AREA DURING THE AFTERNOON. A
CONSIDERABLE DEGREE OF DRY MID-LEVEL AIR...MODEST FORCING...AND DEEP
LAYER SHEAR IN EXCESS OF 40 KTS MAY LEAD TO MORE IN THE WAY OF
SCATTERED/DISCRETE CELLS AS OPPOSED TO LINEAR ORGANIZATION. VERY DRY
AIR ALOFT AND A RELATIVELY DRY/DEEP BOUNDARY LAYER WOULD SUPPORT
DAMAGING WINDS AND LARGE HAIL AS THE PRIMARY THREATS.

POPS LINGER INTO MONDAY EVENING...AS THE MAIN UPPER TROUGH AXIS
LINGERS TO OUR WEST...AND FRONTAL BOUNDARY TAKES A BIT OF TIME
PUSHING EAST OF THE AREA. THE REMAINDER OF THE PERIOD WILL BE DRY
AND SEASONABLY MILD...AS CP HIGH PRESSURE OVERSPREADS THE AREA IN
THE WAKE OF THE FRONT.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
AS OF 230 PM SATURDAY...WEDNESDAY THROUGH SATURDAY...A LARGE MID
LEVEL CLOSED LOW IS EXPECTED TO ROTATE OVER THE U.S./CANADA BORDER
FROM THE GREAT LAKES EAST...AS A RIDGE REMAINS ACROSS THE SOUTHERN
PLAINS. THE HEIGHTS ACROSS THE CWA SHOULD PERSIST GENERALLY ZONAL
THROUGH THE MEDIUM RANGE. THE PERIOD SHOULD BEGIN DRY WITH POPS
INCREASING TO CHC TO SCHC THURSDAY AND FRIDAY. THE GREATEST CHC FOR
RAIN WILL OCCUR ON SATURDAY AS SFC LOW PRESSURE SYSTEM ORGANIZES
OVER THE SOUTHERN MISSISSIPPI RIVER VALLEY AND TRACKS EAST...WITH A
WARM FRONT RUNNING ACROSS THE SOUTHERN APPALACHIANS. I WILL
INDICATES SOLID CHC POPS WITH LIKELY ACROSS THE MTN RIDGES. AS THE
CASE OVER THE PAST SEVERAL DAYS...THE GFS IS MUCH WETTER THAN THE
ECMWF...I WILL USE A BLEND. TEMPERATURES THROUGH THE PERIOD SHOULD
RANGE WITHIN SEVERAL DEGREES OF NORMAL.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
AT KCLT...EXPECT VFR THIS EVENING AND EARLY OVERNIGHT IN ADVANCE OF
THE WARM FRONT LIFTING NORTH ON SUNDAY. CIGS WILL START LOWERING
BETWEEN 04-06Z AS THE COLUMN SATURATES...WITH MVFR CIGS/VSBY AND
VCSH. BY 10Z OR SO...EXPECT WIDESPREAD SHRA ACROSS THE REGION WITH
CIGS LOWERING TO IFR DUE TO THE ABUNDANT MOISTURE. WILL LIKELY SEE
SOME PERIODS OF +SHRA BUT LEFT TAF AS SHRA FOR NOW. KEPT TREND OF
INTRODUCING PROB30 TSRA AT 16Z AS CONVECTION DEVELOPS IN THE WARM
SECTOR. EXPECT THAT CIGS AND VSBY MAY BE UP AND DOWN DEPENDING ON
HOW HEAVY THE RAIN IS BUT OVERALL GENERALLY IFR CONDITIONS EXPECTED
ONCE THE SHRA OVERSPREADS.

ELSEWHERE...GENERALLY SIMILAR TREND TO KCLT THOUGH WITH LESS
CONFIDENCE IN TSRA AT KAVL/KHKY AND MORE CONFIDENCE AT UPSTATE TAFS.
HAVE KEPT TREND OF VCTS RATHER THAN PROB30 TSRA AT UPSTATE TAFS FOR
SUNDAY MIDDAY. AS THE WARM FRONT APPROACHES OVERNIGHT...SHOULD SEE
ONLY MINOR TIMING DIFFERENCES AS FOR EXAMPLE CIGS LOWER FIRST AT
KAND AND SPREADING EAST FROM THERE. AGAIN...BEST TSRA CHANCES REMAIN
ACROSS THE UPSTATE CLOSER TO THE BETTER INSTABILITY AND AT SOME
POINT MAY HAVE TO GO PREVAILING TSRA DEPENDING ON DESTABILIZATION
TRENDS.

OUTLOOK...A COLD FRONT WILL SWEEP THROUGH THE REGION ON MONDAY WITH
STRONG TO SEVERE THUNDERSTORMS BEING A POSSIBILITY.  RESTRICTIONS
WILL BE LIKELY AT TIMES ON MONDAY FOR ALL SITES.  HIGH PRESSURE WILL
BUILD IN BEHIND THE COLD FRONT LEADING TO LOWER PRECIP/RESTRICTION
CHANCES.

CONFIDENCE TABLE...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       HIGH 100%     LOW   52%     HIGH  93%     MED   70%
KGSP       HIGH  93%     HIGH 100%     HIGH 100%     HIGH  80%
KAVL       HIGH 100%     MED   76%     HIGH 100%     HIGH 100%
KHKY       HIGH 100%     MED   64%     HIGH 100%     MED   70%
KGMU       HIGH  93%     HIGH 100%     HIGH  93%     HIGH  80%
KAND       HIGH  85%     MED   64%     HIGH  93%     HIGH  90%

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...FLOOD WATCH FROM MIDNIGHT EDT TONIGHT THROUGH SUNDAY EVENING
     FOR GAZ010-017-018-026-028.
NC...FLOOD WATCH FROM MIDNIGHT EDT TONIGHT THROUGH SUNDAY EVENING
     FOR NCZ033-035>037-049-050-053-056-057-062>065-068>072-
     501>510.
SC...FLOOD WATCH FROM MIDNIGHT EDT TONIGHT THROUGH SUNDAY EVENING
     FOR SCZ001>010.

&&

$$
SYNOPSIS...CSH
NEAR TERM...CDG/TDP
SHORT TERM...JDL
LONG TERM...NED
AVIATION...TDP



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