926
FXUS62 KGSP 221921
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
321 PM EDT SAT SEP 22 2018
SYNOPSIS
A MOIST COLD FRONT WILL MOVE IN FROM THE NORTH TOMORROW AND PERSIST
AS A NEARLY STATIONARY FRONT THROUGH TUESDAY. ANOTHER COLD FRONT IS
FORECAST TO PUSH INTO THE FORECAST AREA FROM THE WEST ON WEDNESDAY.
THIS FRONT MAY EVENTUALLY CROSS OUR AREA, BRINGING SHOWERS AND
THUNDERSTORMS BEFORE THE START OF THE WEEKEND.
NEAR TERM /THROUGH SUNDAY/
ISOLATED SHOWERS HAVE FINALLY BEGUN TO INITIATE ACROSS THE HIGH
TERRAIN WITHIN MODESTLY UNSTABLE AIR MASS. FORECAST SOUNDINGS
CONTINUE TO DEPICT EROSION OF THE MID-LEVEL STABLE LAYER THAT HAS
BEEN IN PLACE FOR AT LEAST THE PAST COUPLE PF DAYS, AS THE UPPER
RIDGE BECOMES SUPPRESSED TO THE EAST AND SOUTHEAST. THEREFORE,
CONVECTIVE CHANCES SHOULD BE IMPROVED THIS AFTERNOON, WITH 30-50
POPS ADVERTISED ACROSS MUCH OF THE MOUNTAINS. DESPITE RATHER MODEST
SBCAPE, SUBSTANTIAL MID-LEVEL DRY AIR/RATHER HIGH DCAPE SUGGEST
MICROBURST POTENTIAL WITH A ONE OR TWO OF THE STRONGER UPDRAFTS.
POPS ACROSS THE REMAINDER OF THE NORTHERN TIER WILL GRADUALLY
INCREASE INTO THE CHANCE CATEGORY THIS EVENING, AS FRONTAL BOUNDARY
CURRENTLY EXTENDING FROM THE LOWER MISS VALLEY THROUGH THE TENN
VALLEY INTO THE CENTRAL APPALACHIANS/MID-ATLANTIC BEGINS TO BACK
DOOR ITS WAY ACROSS THE VIRGINIA AND NC PIEDMONT...BUT LIKELY NOT
MAKING ITS WAY INTO OUR NORTHERN ZONES UNTIL AROUND DAYBREAK. MIN
TEMPS WILL REMAIN 5-10 DEGREES ABOVE CLIMO.
POPS AND TEMPS ON SUNDAY WILL DEPEND UPON HOW FAR SOUTH THE FRONTAL
BOUNDARY WILL PUSH BEFORE STALLING (ALWAYS A BIT OF A CRAP SHOOT THIS
TIME OF YEAR). THE MODEL CONSENSUS IS SOMEWHAT STRONG ON THE
BOUNDARY STALLING NEAR THE NC/SC PIEDMONT BORDER, AND THE FORECAST
IS HEDGED IN THIS DIRECTION. THE MAX TEMP FORECAST THEREFORE RANGES
FROM THE LOWER/MID 70S ALONG/NORTH OF I-40 TO THE MID/UPPER 80S
ACROSS THE UPPER SAVANNAH RIVER VALLEY AND THE SC LAKELANDS. IN
TERMS OF POPS, DESPITE THE PRESENCE OF THE BOUNDARY AND ASSOCIATED
LOW LEVEL CONVERGENCE, GUIDANCE DOESN'T SEEM PARTICULARLY EXCITED
ABOUT CONVECTIVE CHANCES SUNDAY AFTERNOON, AS INSTABILITY WILL
REMAIN MODEST. POPS ARE THEREFORE ADVERTISED IN THE 20-40% RANGE
ACROSS MUCH OF THE AREA.
SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/
AS OF 230 PM EDT SATURDAY: A LARGE SURFACE HIGH OVER THE NERN
U.S. WILL FORCE A NORTHEAST FLOW TO BRING A WEDGE OF COLD AIR DOWN
THE EASTERN SIDE OF THE MOUNTAINS. THE COLD AIR WILL BE EXPANDING
SOUTH AND WEST OVER WESTERN NC AND UPSTATE SC AT THE START OF THE
SHORT TERM PERIOD. CLOUD COVERAGE WILL INCREASE THROUGH THE NIGHT.
THE CLOUDS MAY NOT SPREAD FAR ENOUGH BY MORNING TO COVER THE S/SE
THIRD OF THE FORECAST AREA, BUT MAY EVEN EXPAND A BIT MORE ON
MONDAY. POPS MAY BE OVERDONE AT 40S IN THE MOUNTAINS. IT LOOKS COOL
AND STABLE, BUT SOME PATCHES OF -RA MAY BE FOUND OVER THE HIGHER
ELEVATIONS. TEMPS MONDAY MORNING WILL RANGE FROM THE M50S N TO U60S
S. WITH LITTLE OR NO SUN OVER THE NW HALF OR MORE OF THE AREA, TEMPS
WILL MAX OUT IN THE L-M60S ON THE HIGHEST ELEVATIONS, AND U60S-L70S
FOR THE VALLEYS WHICH WILL BE UNDER THE CLOUDS. THE SE HALF OF THE
AREA WILL BE IN THE 70S - BELOW NORMALS FOR LATE SEPT. THERE HASN'T
BEEN A BELOW NORMAL DAY (AVG TEMP) AT GSP ALL MONTH, AND NO DAYS
LESS THAN 2F ABOVE NORMAL AT CLT. THIS WILL BE A CHANGE.
LONG TERM /TUESDAY THROUGH SATURDAY/
AS OF 245 PM EDT SATURDAY: LITTLE OR NO CHANGES TO THE FIRST
HALF OF THE NEW WEEK. BUT A FORECAST CHALLENGE EXISTS IN THE SECOND
HALF OF THE WEEK. TUES WILL START OUT PRETTY CLOUDY WITH THE WEDGE
STILL IN PLACE. HAVE BEEN JUST A LITTLE PESSIMISTIC ON CLOUDS
BREAKING UP/CLEARING OUT OF THE MOUNTAINS. BUT, SOME DECENT SUN
SHOULD BE SEEN OVER THE SE HALF BY THE END OF THE AFTERNOON. HAVE
CONTINUED TO CARRY SCT POPS ON TUES, BUT DROPPED THEM TO 30 OR LESS
BY LATER TUES NIGHT. LOW LEVEL FORCING/UPGLIDE IS STILL POSSIBLE TO
GENERATE A SHOWER OR TWO, ESP IF THE SUN CAN BREAK THROUGH AND
DESTABILIZE THINGS.
A FRONT STARTS TO PUSH IN FROM THE NORTHWEST AS UPPER HEIGHTS LOWER
IN THE MID-WEEK. LIKELY OR EVEN WIDESPREAD SHOWERS AND THUNDERSTORMS
ARE ON TAP FOR WED-THURS. THE FRONT LOOKS LIKE IT SHOULD RUN THROUGH
MOST OF THE AREA ACCORDING TO ALL GUIDANCE. BUT... THE GFS IS MUCH
LESS PROGRASSIVE WITH THE FRONT VS ECMWF. THE EC WOULD BLOW IT ON
THROUGH AND MAKE THE LATTER HALF OF THE WEEK ALMOST RAIN-FREE. DUE
TO THE SPREAD IN THE SOLUTIONS, CONFIDENCE IS VERY LOW FOR LATE WEEK
AND NEXT WEEKEND. THIS IS THE MAIN REASON FOR THE SEEMINGLY
CONTINUOUS 20-30 POPS IN THE LATTER HALF OF THE EXTENDED. TEMPS LOOK
LIKE THEY WILL RISE MUCH ABOVE NORMAL BEFORE THE FRONT ON WED WITH
GOOD SW FLOW. THE SHOULD DROP BACK NEAR NORMAL OR JUST SLIGHTLY
ABOVE NORMAL FOR THE REST OF THE PERIOD.
AVIATION /20Z SATURDAY THROUGH THURSDAY/
AT KCLT AND ELSEWHERE: CONVECTIVE COVERAGE (MAINLY IN THE FORM OF
SHOWERS) IS EXPECTED TO STEADILY INCREASE THROUGH THE AFTERNOON,
WARRANTING VCSH AT KAVL BEGINNING AT 20Z. STEERING CURRENTS WILL
TEND TO TAKE CELLS THAT DEVELOP TOWARD THE NE, AND IT THEREFORE
APPEARS THAT KHKY IS THE ONLY OTHER TERMINAL THAT WILL HAVE MUCH OF
A CHANCE OF A SHOWER OVER AT LEAST THE NEXT 12 OR SO HOURS. A VCSH
IS INCLUDED THERE DURING THE EVENING, MAINLY AS A NOD TO A BACK DOOR
FRONT THAT WILL BE APPROACHING THE AREA FROM THE NORTH AND NE
TONIGHT. FROPA SHOULD OCCUR AT KHKY BY DAYBREAK, TURNING WIND TO
LIGHT NE AND LIKELY RESULTING IN DEVELOPMENT OF IFR CIGS, WITH CIGS
RESTRICTIONS EXPECTED TO CONTINUE THROUGH THE MORNING, ALTHOUGH
GRADUAL IMPROVEMENT IS LIKELY. OTHERWISE, OTHER THAN VALLEY FOG AND
LOW STRATUS ADVERTISED IN THE MTN VALLEYS (INCLUDING KAVL), VFR IS
FORECAST THROUGH THE PERIOD. IT IS UNCERTAIN AS TO HOW FAR SOUTH AND
WEST THE BACKDOOR FRONT WILL PUSH BEFORE STALLING. LOW CIGS CANNOT
BE RULED OUT AT SOME POINT TOMORROW FROM KGMU/KGSP TO KCLT, BUT
CONFIDENCE IS LOW AT THIS POINT.
OUTLOOK: ANTICIPATE COVERAGE OF SHOWERS AND STORMS TO INCREASE AGAIN
SUNDAY AFTERNOON IN ASSOCIATION WITH BACK DOOR COLD FRONT. WEAK COLD
AIR DAMMING MAY SUPPORT RESTRICTIONS AND OCCL SHOWERS/LIGHT RAIN
LATE SUNDAY INTO THE EARLY PART OF THE WORK WEEK. A COLD FRONT
MOVING IN FROM THE WEST WILL THEN BRING HIGHER CHANCES OF SHOWERS
AND STORMS DURING THE LATTER HALF OF THE WEEK.
CONFIDENCE TABLE...
19-01Z 01-07Z 07-13Z 13-18Z
KCLT HIGH 100% HIGH 100% HIGH 83% HIGH 86%
KGSP HIGH 100% HIGH 100% HIGH 100% HIGH 100%
KAVL HIGH 100% HIGH 96% MED 76% HIGH 97%
KHKY HIGH 100% HIGH 100% MED 70% HIGH 82%
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:
WWW.WEATHER.GOV/GSP/AVIATION
GSP WATCHES/WARNINGS/ADVISORIES
GA...NONE.
NC...NONE.
SC...NONE.
SYNOPSIS...CARROLL
NEAR TERM...JDL/PM
SHORT TERM...MMD
LONG TERM...MMD
AVIATION...JDL
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