Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 240238
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1038 PM EDT THU APR 23 2015

.SYNOPSIS...
COOL AND DRY HIGH PRESSURE CONTINUES ACROSS OUR REGION THROUGH
FRIDAY.  A WARM FRONT BRINGS BETTER CHANCES FOR WIDESPREAD RAINFALL
TO OUR AREA OVER THE WEEKEND. DRY HIGH PRESSURE PREVAILS AT
THE START OF NEXT WEEK...THEN A LOW WILL MOVE ALONG THE GULF COAST
MIDWEEK.

&&

.NEAR TERM /THROUGH FRIDAY/...
AS OF 1035 PM EDT THURSDAY...CONSIDERATION HAS BEEN GIVEN ALL NIGHT
TO POSSIBLE FROST ADVISORY ISSUANCE ACROSS THE I40 CORRIDOR OF THE
NC PIEDMONT.  HOWEVER...LATEST GUIDANCE INDICATES THE AIRMASS WILL
BE TOO DRY.  MOSTLY LIKELY BECAUSE IT WILL REMAIN AT LEAST SOMEWHAT
MIXED AS LLV COLD ADVECTIVE FLOW INCREASES PER NAM H85 WINDS.
TEMPERATURES WILL FALL SOMEWHAT OVER THESE REGIONS AS HIGH LEVEL
CIRRUS REMAINS JUST TO THE SOUTH...HOWEVER NAM GUID STILL FAVORS A
NORTHEASTERN MIGRATION OF THE UPPER JET MAX WHICH WOULD SPREAD SAID
CIRRUS INTO THE REGION BY SUNRISE.  THUS...WILL CONTINUE WITH CURRENT
FREEZE WARNING FOR THE MTNS...WITH NO FROST ADVISORY PLANNED.
OTHERWISE...FCST LOOKS GOOD WITH ONLY SLIGHT TWEAKS TO
TEMPS/DEWS/SKY MADE WITH THIS UPDATE.

PREVIOUS DISCUSSION...

AS OF 225 PM EDT...AN UPPER LEVEL JET AXIS WILL LIFT NE ACROSS THE
FORECAST AREA TONIGHT...WITH ACCOMPANYING THIN CIRRUS THROUGHOUT. AN
UPPER LEVEL RIDGE WILL ALSO BUILD OVER THE SE TONIGHT THROUGH FRI AS
LOW PRESSURE COVERS THE PLAINS AND NEW ENGLAND. UNDER THE
RIDGE...THE SFC HIGH CENTER WILL APPROACH THE SRN/CENTRAL
APPALACHIANS OVERNIGHT. THIS SETUP WILL MAKE FOR A VERY CHALLENGING
FROST/FREEZE FORECAST. COOP MOS AND THE LATEST GUIDANCE SOURCES ARE
A TOUCH COLDER THAN PREVIOUSLY FOR THE MTNS...SO THE CURRENT FREEZE
WARNING SEEMS WELL PLACED THERE. FARTHER EAST...ALONG THE INTERSTATE
40 CORRIDOR...OVERNIGHT MINS HAVE TRENDED SLIGHTLY COLDER...BUT THE
DRY SFC AIR AND LIGHT WINDS/MIXING OVERNIGHT WILL MAKE FROST
FORMATION PROBLEMATIC. ANTICIPATE SOME RESURGENCE IN THE LOW LEVEL
COLD ADVECTION OVERNIGHT...AND DEWPOINT RECOVERY WILL BE MODEST TO
MAINTAIN DEPRESSIONS OF ABOUT 4 OR 5 DEGREES IN MOST AREAS. PATCHY
FROST HAS BEEN ADDED ACROSS NC FOOTHILL AND PIEDMONT SECTIONS
OVERNIGHT...BUT NO FROST ADVISORY WILL BE ISSUED AT PRESENT.

THE SFC HIGH WILL MIGRATE EASTWARD ACROSS THE FORECAST AREA ON
FRIDAY. PROFILES WILL REMAIN GENERALLY DRY...BUT SOME DEGREE OF MID
LEVEL MOISTURE WILL START TO ARRIVE OVER THE RIDGE AXIS INTO THE SRN
APPALACHIANS LATE FRIDAY. TEMPERATURES SHOULD REMAIN 5 DEGREES OR
MORE BELOW CLIMO THROUGHOUT.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
AS OF 220 PM THURSDAY...WARM CONVEYOR BELT MOISTURE/FORCING IN
ADVANCE OF INTENSIFYING GREAT PLAINS CYCLONE WILL TRANSLATE ACROSS
THE AREA LATE SAT NIGHT INTO SUNDAY...WITH PRECIP LIKELY SPREADING
INTO THE WESTERN CWA WITHIN A COUPLE OF HOURS OF MIDNIGHT. POPS WILL
THEREFORE RAMP UP QUICKLY TO CATEGORICAL IN THOSE AREAS BY DAYBREAK
SUNDAY...OVERSPREADING THE REMAINDER OF THE FORECAST AREA THROUGHOUT
SUNDAY MORNING. WITH A LINGERING DRY AIR MASS IN PLACE IN ADVANCE OF
THE PRECIP SHIELD...AGEOSTROPHIC ADJUSTMENT/IN SITU COLD AIR DAMMING
WILL LIKELY OCCUR AS WET BULB EFFECTS RESULT IN ESTABLISHMENT OF
SHALLOW COLD POOL IN RESPONSE TO FALLING PRECIP.

WESTERLY FLOW THROUGH A DEEP LAYER ABOVE THE SURFACE IS GENERALLY
NOT INDICATIVE OF STRONG LOW LEVEL FORCING/HEAVY RAIN THREAT...AND
FORECAST PWATS ARE ONLY 1-2 STANDARD DEVIATION ABOVE CLIMO.
INDEED...A CONSENSUS OF MODEL/ENSEMBLE GUIDANCE ALONG WITH WPC
FORECASTS INDICATES MAYBE UP TO 1.5 INCHES OF RAIN WILL FALL
THROUGHOUT THE EVENT...AND THIS SEEMS REASONABLE. HOWEVER...A
CONSENSUS OF SHORT TERM GUIDANCE INDICATES THAT THERE WILL BE
CONSIDERABLE DESTABILIZATION SATURDAY AFTERNOON IN THE WARM SECTOR
ACROSS THE TENN VALLEY/GEORGIA. WITH DEEP LAYER WESTERLY FLOW
ORIENTED MORE OR LESS PARALLEL TO CAD BOUNDARY/WARM FRONT...THE
POTENTIAL WILL EXIST FOR TRAINING CELLS AND POSSIBLY LOCALIZED FLASH
FLOODING ALONG THE FRONT...THE BIG QUESTION (AS USUAL) BEING EXACTLY
WHERE THAT BOUNDARY WILL SET UP. THERE WILL ALSO BE VERY ROBUST
SHEAR WITHIN THE WARM SECTOR...AS MUCH AS 70 KTS IN THE 0-6KM LAYER
ACROSS THE TENN VALLEY. THEREFORE...SEVERE CONVECTION APPEARS TO BE
A GOOD BET IN THE WARM SECTOR SAT AFTERNOON...WHICH COULD POSSIBLY
AFFECT OUR SOUTHERN PIEDMONT ZONES...DEPENDING UPON THE POSITION OF
THE CAD FRONT...BUT PERHAPS MORE LIKELY ACROSS THE WESTERN NC MTNS
WHICH SHOULD SEE WARM SECTOR AIR BY AT LEAST THE END OF THE DAY.

IN FACT...SOME OF THE SHORT TERM GUIDANCE IS SUGGESTING THAT
ORGANIZED CONVECTION WILL DEVELOP NEAR THE SURFACE LOW ACROSS THE
OHIO VALLEY DURING THE AFTERNOON...AND DROP SOUTHEAST ALONG THE WARM
FRONT INTO THE SOUTHERN APPALACHIANS DURING THE EVENING...WHICH
COULD POSE A MORE ORGANIZED RISK FOR SEVERE WEATHER. THEREFORE...
WHILE POPS WILL LOWER DURING SAT AFTERNOON AS THE WARM CONVEYOR BELT
ZONE SHIFTS EAST...POPS WILL RAMP UP AGAIN FROM THE WEST DURING THE
EVENING.

SURFACE LOW IS EXPECTED TO PASS OVER THE VIRGINIA/NC REGION SAT
NIGHT INTO SUNDAY...WITH ASSOCIATED COLD FRONT EXPECTED TO SWEEP
ACROSS THE AREA DURING THE AFTERNOON. SECONDARY SURFACE LOW
DEVELOPMENT IS ANTICIPATED OFF THE MID-ATLANTIC COAST DURING THIS
TIME...WHICH IS EXPECTED TO RESULT IN VEERING NW/DOWNSLOPE LOW LEVEL
FLOW. THERE IS ENOUGH INSTABILITY INDICATED IN ADVANCE OF THE COLD
FRONT TO SUPPORT LOW POPS THROUGH SUNDAY...BUT OVERALL COVERAGE OF
CONVECTION IS QUITE UNCERTAIN CONSIDERING THE DOWNSLOPE FLOW AND
OVERALL WEAKLY FORCED ENVIRONS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
AS OF 215 PM THURSDAY...THE GFS APPEARS TO HAVE A TREND TOWARD THE
00Z ECMWF WITH ITS SURFACE LOW MOVING ACROSS THE DEEP SOUTH DURING
THE MIDDLE OF THE WEEK. AT THE START OF THE PERIOD...SUNDAY NIGHT...
THINK THAT ANY LINGERING LIGHT PRECIP OVER THE NC MOUNTAINS WILL
QUICKLY DRY UP...SO THAT PERIOD WAS KEPT DRY. THE NEW GUIDANCE SHOWS
A SLOWLY PROGRESSIVE UPPER PATTERN FOR EARLY NEXT WEEK. AN UPPER
RIDGE IS EXPECTED TO BUILD IN FROM THE WEST ON MONDAY WITH THE RIDGE
AXIS REMAINING WEST OF THE FCST AREA THRU DAYBREAK ON TUESDAY. THAT
WILL SUPPORT DRY HIGH PRESSURE RIDGING DOWN AT THE SURFACE INTO
MONDAY NIGHT. WITH THIS PATTERN...IT SEEMS UNLIKELY THAT ANY
APPRECIABLE MOISTURE WILL CROSS OVER THE TOP OF THE RIDGE AND BRING
PRECIP INTO THE FCST AREA UNTIL SOMETIME PERHAPS AFTER SUNRISE ON
TUESDAY. WILL ELIMINATE THE LOW PRECIP CHANCE FOR MONDAY NIGHT AS A
RESULT. THE GFS SOLUTION IS SOMEWHAT SUSPECT FOR MONDAY NIGHT AND
TUESDAY AS IT BLOWS UP A LOW LEVEL PV ANOMALY WHICH KEEPS A
PERSISTENT MCS ALONG THE GULF COAST. ASSUMING THAT IS OVERDONE...
THEN PERHAPS AS THE UPPER RIDGE AXIS MOVES OVERHEAD ON TUESDAY...WE
WILL SEE THE ARRIVAL OF GULF MOISTURE IN THE AFTERNOON WHICH SHOULD
FUEL A CHANCE OF SHOWERS. SO...THE NEW FCST CATCHES UP TO THE OLD
FCST BY AFTERNOON WITH A CHANCE OF PRECIP ACROSS THE ENTIRE AREA BY
THE END OF THE DAY. FROM THAT POINT ONWARD...THE MEDIUM RANGE LOOKS
UNSETTLED AS AN UPPER LOW SLOWLY MOVES EAST ACROSS THE DEEP SOUTH.
THE SOUTHERLY TRACK OF THE SFC LOW SHOULD MAINTAIN A MOIST E/SE FLOW
THAT WILL ENHANCE PRECIP POTENTIAL OVER THE MTNS AND FOOTHILLS
WEDNESDAY AND THURSDAY AND KEEP TEMPS ON THE COOL SIDE OF NORMAL.
PRECIP PROBABILITY WAS LIMITED TO THE CHANCE RANGE FOR NOW BECAUSE
OF RUN-TO-RUN CONSISTENCY ISSUES WITH THE LOW TRACK IN THE GFS.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
AT KCLT AND ELSEWHERE...VFR CONDITIONS WILL PREVAIL THROUGH THE TAF
PERIOD AS HIGH PRESSURE PREVAILS.  WINDS THROUGH THE OVERNIGHT WILL
REMAIN LIGHT AND NORTHWESTERLY/NORTHERLY AT ALL SITES AS SFC RIDGE
APPROACHES FROM THE WEST.  UPPER LEVEL JET MAX WILL SLIDE OVERHEAD
WITH FEW/SCT HIGH LEVEL CIRRUS EXPECTED TO INCREASE FROM THE
SOUTH.  WINDS WILL BECOME CALM OR LIGHT/VRB AS THE SFC RIDGE CENTER
MIGRATES TO THE SE EARLY ON FRIDAY WITH WINDS INCREASING/VEERING
SOUTHERLY NEAR PERIODS END AMIDST INCREASING SCT/BKN HIGH CIRRUS.

OUTLOOK...PRECIPITATION AND ASSOCIATED RESTRICTIONS WILL RETURN FRI
NIGHT THROUGH SAT AS A LOW PRES SYSTEM MOVES EAST FROM THE PLAINS
STATES. A TRAILING COLD FRONT WILL CROSS THE REGION ON SUNDAY...WITH
BRIEF DRY HIGH PRESSURE RETURNING EARLY NEXT WEEK.

CONFIDENCE TABLE...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KHKY       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%

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...FREEZE WARNING UNTIL 9 AM EDT FRIDAY FOR NCZ033-048>053-058-
     059-062>065-501-503-505.
SC...NONE.

&&

$$
SYNOPSIS...DEO
NEAR TERM...CDG/HG
SHORT TERM...JDL
LONG TERM...PM
AVIATION...CDG


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