509  
FXUS62 KGSP 220011  
AFDGSP  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC  
811 PM EDT SAT APR 21 2018  
   
SYNOPSIS
 
 
DRY HIGH PRESSURE OVER THE EASTERN UNITED STATES WILL MAINTAIN  
CONTROL OVER THE REGION THROUGH EARLY SUNDAY MORNING. PRECIP  
CHANCES GRADUALLY INCREASE DURING THE DAY ON SUNDAY AHEAD OF  
A STRONG AND SLOW-MOVING LOW PRESSURE SYSTEM OVER THE LOWER  
MISSISSIPPI RIVER VALLEY. A COLD RAINFALL WILL BECOME WIDESPREAD  
SUNDAY NIGHT INTO MONDAY AND PERSIST THROUGH EARLY TUESDAY,  
RESULTING IN SIGNIFICANT ACCUMULATIONS. THE REMAINTER OF THE WORK  
WEEK MAY REMAIN UNSETTLED DUE TO A SERIES OF COLD FRONTS.  
 

 
   
NEAR TERM /THROUGH SUNDAY/
 
 
AS OF 800 PM...NO SIGNIFICANT CHANGES NEEDED WITH THIS UPDATE. TEMPS  
FALLING SLIGHTLY MORE SLOWLY THAN FCST, DUE TO INCREASING CIRRUS.  
FOR NOW, I WILL LEAVE MIN TEMPS AS IS, BUT IF THE LOW CLOUDS SEEN ON  
SATELLITE ADVANCE IN A LITTLE FASTER THAN EXPECTED, MIN TEMPS MAY BE  
NEED TO BE BUMPED UP A TAD IN THE WESTERN ZONES.  
 
AS OF 200 PM EDT SATURDAY: TODAY WILL LIKELY BE THE LAST TIME WE'LL  
SEE THE SUN ACROSS MUCH OF THE FA IN QUITE A WHILE. DRY SURFACE HIGH  
PRESSURE HAS SHIFTED OFF THE MID-ATLANTIC COAST, WITH WEAK SURFACE  
SOUTHEASTERLY FLOW AT THE SURFACE AND ONLY HIGH CIRRUS SPILLING INTO  
THE AREA. GRADUAL LOW-LEVEL MOISTENING WILL OCCUR OVERNIGHT AND  
TOMORROW AS A HEALTHY ATLANTIC/GULF MOISTURE TAP SETS UP. AN UPPER  
LOW AND SURFACE LOW PRESSURE SYSTEM WILL EMERGE FROM THE DEEP SOUTH  
TOMORROW, BEGINNING TO APPROACH THE AREA BY THE LATE AFTERNOON. AT  
THE SAME TIME, A WEDGE OF SURFACE HIGH PRESSURE WILL SLIDE DOWN THE  
SPINE OF THE SOUTHERN APPALACHIANS, SETTING THE STAGE FOR WHAT WILL  
LIKELY BECOME A PROLONGED COLD-AIR DAMMING EVENT ACROSS THE AREA IN  
THE SHORT TERM PERIOD. SKIES WILL BECOME PROGRESSIVELY CLOUDIER  
TOMORROW AS DEEPER MOISTURE ENTERS THE CAROLINAS, THOUGH HELD OFF ON  
SPREADING POPS INTO THE FORECAST AREA DURING THIS NEAR-TERM PERIOD  
EXCEPT FOR OUR NORTH GA COUNTIES AND FAR SOUTHWESTERN UPSTATE SC.  
LOWS OVERNIGHT TONIGHT WILL BE WARMER THAN THIS MORNING (THOUGH  
STILL BELOW AVERAGE) DUE TO CLOUD COVER SPREADING IN OVERNIGHT, WITH  
HIGHS TOMORROW EXPECTED TO BE ABOUT A CATEGORY BELOW AVERAGE FOR THE  
SAME REASON.  
 

 
   
SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/
 
 
AS OF 245 PM SATURDAY: THE FIRST PART OF NEXT WEEK STILL LOOKS QUITE  
WET ACROSS THE REGION, AND THIS STILL LOOKS LIKE A TROUBLESOME  
FCST WITH LOTS OF PLACES THAT IT COULD GO WRONG. FORTUNATELY,  
THE MODEL GUIDANCE REMAINS IN GENERALLY GOOD AGREEMENT WITH THE  
OVERALL WET PATTERN, THAT BEING A LARGE UPPER LOW MOVING SLOWLY  
EAST FROM THE MS RIVER VALLEY SUNDAY NIGHT AND THEN LINGERING OVER  
THE TN VALLEY MONDAY AND MONDAY NIGHT. WE SHOULD BE WELL ON OUR  
WAY TO ESTABLISHING A COLD AIR DAMMING REGIME AT SUNSET ON SUNDAY  
WITH LIGHT PRECIP BREAKING OUT FROM SW TO NE TO SEAL THE DEAL IN  
THE EVENING, GIVEN A COMPLEX PARENT SFC HIGH STRETCHING FROM ERN  
ONTARIO TO THE MID-ATLANTIC COAST SUPPLYING THE DRY NE BOUNDARY  
LAYER FLOW AND STRENGTHENING SE MOIST UPSLOPE/ISENTROPIC LIFT OVER  
TOP OF IT. CONFIDENCE IS HIGH THAT THE ENTIRE FCST AREA WILL HAVE  
RAIN MONDAY AND MONDAY NIGHT GIVEN THE OVERWHELMING SUPPORT OF  
VERY STRONG LOW LEVEL ISENTROPIC UPGLIDE, DEEP MOISTURE, AND DEEP  
FORCING AS THE UPPER LOW SPINS OVER THE TN VALLEY. RAINFALL AMOUNTS  
ARE THE IMPORTANT PART OF THE FCST. EXPECT THE HEAVIEST PRECIP TO  
DEVELOP ACROSS MAINLY THE SAVANNAH R BASIN, UPSTATE SC, AND THE  
SE UPSLOPE AREAS INTO THE UPPER FRENCH BROAD R BASIN BY MONDAY  
DAYBREAK, AND THEN PERSIST IN THAT REGION AS WE HAVE SOMETHING OF  
A CONVEYOR BELT OF ATLANTIC MOISTURE FOCUSED ON THAT REGION INTO  
MONDAY NIGHT. A FEW INCHES OF RAIN IS LIKELY, AT LEAST 2-3 INCHES,  
WITH HIGHER AMTS IN THE USUAL UPSLOPE REGION. WHETHER OR NOT THIS  
WILL BE SUFFICIENT TO RESULT IN FLOODING IS QUESTIONABLE AS WE  
MAY LACK THE SFC-BASED CONVECTION TO REALLY JACK UP THE PRECIP  
RATES THAT WOULD CAUSE PROBLEMS ON THE SMALL STREAMS. INSTEAD,  
IT LOOKS MORE LIKE A DURATION-TYPE ISSUE, IN WHICH CASE THE  
RUNOFF WILL PROBABLY NOT PILE UP ENOUGH TO CAUSE PROBLEMS UNTIL  
LATE IN THE DAY ON MONDAY. CONFIDENCE IS NOT QUITE HIGH ENOUGH  
TO ENTERTAIN A WATCH FOR AN EVENT THAT WOULD MAINLY BE IN THE  
4TH FCST PERIOD, SO WE WILL HOLD OFF FOR THE TIME BEING. WIND  
WILL ALSO BE STRONG, BUT MAINLY A PROBLEM ABOVE ABOUT 5K FEET,  
SO A HIGH WIND WATCH WILL ALSO BE TABLED. TEMPS WILL MOST LIKELY  
NOT MAKE IT OUT OF THE 50S EAST OF THE BLUE RIDGE WITH THE COLD  
AIR DAMMING IN PLACE AND EXTENSIVE PRECIP. EXPECT A LOW DIURNAL  
RANGE. TUESDAY AND TUESDAY NIGHT SHOULD SEE THE UPPER LOW START  
TO OPEN UP AND THEN GET KICKED UP THE OH RIVER VALLEY, SO THE  
DEMISE OF THE CAD REGIME BECOMES PROBLEMATIC. GUIDANCE SUGGESTS  
THAT ISENTROPIC LIFT AND WARM ADVECTION WILL TAPER OFF ON TUESDAY  
AND DEEPER MOISTURE WILL GET STRIPPED AWAY AS A DRY SLOT ATTEMPTS  
TO WRAP IN FROM THE SW. HOWEVER, IT IS ONE THING TO REMOVE THE  
REINFORCEMENT FOR THE WEDGE AND ANOTHER THING TO BRING IN SOME  
DESTRUCTIVE AGENT. THE NAM KILLS OFF THE CAD DURING THE DAY AS A  
TRIPLE-POINT LOW MOVES THROUGH, BUT REALLY DOESN'T SHOW MUCH OF  
ANY OTHER DESTRUCTIVE SIGNAL. THE NAM WOULD BE A PROBLEM BECAUSE  
IT WOULD ALLOW A PLUME OF SFC-BASED CAPE GREATER THAN 1000 J/KG TO  
ADVECT INTO THE WRN PIEDMONT IN THE AFTERNOON WITH ENOUGH RESIDUAL  
SHEAR TO ORGANIZE SEVERE THUNDERSTORMS. THE GFS, MEANWHILE, KEEPS  
A LOW LEVEL EASTERLY FLOW LONGER INTO THE DAY WHICH WOULD SUPPORT  
HOLDING ONTO SOME REMNANT COOL POOL IN THE USUAL DAMMING REGION  
INTO THE AFTERNOON. THE GFS SEEMS THE MORE REASONABLE SOLUTION  
AND AGREES WITH THE SREF MEAN MUCAPE. THAT BEING SAID, EVEN THE  
GFS SUGGESTS CAUTION IN THAT ENOUGH DESTABILIZATION COULD OCCUR UP  
INTO, SAY, THE CLT METRO AREA IN THE AFTERNOON WITH SBCAPE UPWARDS  
OF 500 J/KG, WITH A REMNANT WEDGE BOUNDARY HANGING OUT IN THE I-85  
CORRIDOR. AT THIS POINT, WE WILL KEEP THIS IDEA ON THE BACK BURNER,  
AS ONE SHOULD AT THIS TIME OF THE YEAR. THE FORCING AND PRECIP  
SHOULD LIFT OUT TO THE NE IN THE EVENING ENDING ANY THREAT FOR  
HEAVY PRECIP/SEVERE STORMS. THINK WE WILL DEVELOP SOMETHING OF A  
LULL EARLY WEDNESDAY BEFORE THE NEXT SYSTEM APPROACHES, SAVE SOME  
REMAINING CHANCE OF LIGHT PRECIP ON THE TN BORDER.  
 

 
   
LONG TERM /WEDNESDAY THROUGH SATURDAY/
 
 
AS OF 230 PM SATURDAY: A SERIES OF SHORT WAVES WILL MOVE THROUGH THE  
LONG WAVE UPPER TROUGH OVER THE EASTERN CONUS THROUGH THE PERIOD.  
HOWEVER, THERE IS CONSIDERABLE DISAGREEMENT ON TIMING AND LOCATION  
OF THESE SYSTEMS FROM MODEL TO MODEL AND FROM RUN TO RUN. THIS MAKES  
FOR A LOW CONFIDENCE FORECAST EACH DAY. THEREFORE, HAVE GONE WITH A  
MODEL BLEND DUE TO THESE DISCREPANCIES AND THE LOW CONFIDENCE. LOOKS  
LIKE GENERALLY DIURNAL POP TRENDS WEDNESDAY AND THURSDAY, WITH BEST  
CHANCES ACROSS THE MOUNTAINS, AS MOISTURE LINGERS AND A SERIES OF  
WEAK WAVES AND A COLD FRONT CROSS THE AREA. THE GFS SHOWS ANOTHER  
WEAK WAVE AND COLD FRONT MOVING THROUGH ON FRIDAY WITH MAINLY LOW  
END DIURNAL POP WHILE THE ECMWF SHOWS IT ON SATURDAY. HAVE GONE WITH  
THE GFS AND CANADIAN FOR FRIDAY AND SATURDAY GIVEN THEIR AGREEMENT.  
HIGHS START OUT NEAR NORMAL, DROP A FEW DEGREES FOR THURSDAY, THEN  
BOUNCE BACK TO JUST BELOW NORMAL FOR FRIDAY AND SATURDAY. LOWS START  
OUT A FEW DEGREES ABOVE NORMAL THEN REMAIN STEADY NEAR NORMAL  
THROUGH THE END OF THE PERIOD.  
 

 
   
AVIATION /00Z SUNDAY THROUGH THURSDAY/
 
 
AT KCLT AND ELSEWHERE: SOUTHEASTERLY FLOW ATOP THE REGION WILL BRING  
IN MOISTURE AND A GRADUAL INCREASE IN LOWER CLOUDS STARTING DURING  
THE PRE-DAWN HOURS IN THE WEST AND SPREAD NORTH AND EAST INTO  
WESTERN NC DURING THE DAY ON SUNDAY. THE TAFS START OUT WITH JUST  
SOME THIN CIRRUS AND A LIGHT SE WIND. LOW VFR CIGS SHOULD SPREAD IN  
ACROSS THE UPSTATE SITES BY DAYBREAK SUNDAY, WITH MVFR CIGS AT KAND  
BY MIDDAY WITH -SHRA POSSIBLE. THE -SHRA SHOULD SPREAD ACROSS THE  
REST OF THE AREA, EXCEPT PERHAPS NOT QUITE TO KHKY BEFORE THE END OF  
THE 00Z TAF PERIOD. LOWER CIGS AND VSBY WON'T REALLY START BECOMING  
WIDESPREAD ACROSS THE REGION UNTIL SUNDAY EVENING. WINDS WILL START  
TO BACK OUT OF THE NE AT THE UPSTATE SITES, DUE TO DEVELOPING COLD  
AIR DAMMING.  
 
OUTLOOK: CONDITIONS WILL DETERIORATE SUNDAY NIGHT WITH WIDESPREAD  
RAIN OVERSPREADING THE AREA. IFR/MVFR CONDITIONS WILL BE LIKELY THRU  
THE DAY ON MONDAY, WITH SOME IMPROVEMENT ON TUESDAY, AS PRECIP  
LARGELY SHIFTS NE OF THE AREA, BUT WITH SOME MOISTURE LINGERING.  
UNSETTLED WEATHER WILL CONTINUE ON WEDNESDAY AND THURSDAY, AS A  
TROUGH OF LOW PRESSURE PERSISTS ACROSS THE REGION.  
 
CONFIDENCE TABLE...  
 
00-06Z 06-12Z 12-18Z 18-00Z  
KCLT HIGH 100% HIGH 100% HIGH 100% HIGH 100%  
KGSP HIGH 100% HIGH 100% HIGH 100% HIGH 92%  
KAVL HIGH 100% HIGH 100% HIGH 99% HIGH 92%  
KHKY HIGH 100% HIGH 100% HIGH 100% HIGH 100%  
KGMU HIGH 100% HIGH 100% HIGH 100% HIGH 82%  
KAND HIGH 100% HIGH 100% MED 77% HIGH 83%  
 
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  
 

 
   
HYDROLOGY
 
 
DUE TO THE PREDOMINANT ENVIRONMENT, RAIN RATES ASSOCIATED WITH THE  
UPCOMING EVENT ARE GENERALLY NOT EXPECTED TO BE SUPPORTIVE OF FLASH  
FLOODING, WHERE FLOODING BEGINS WITHIN 6 HOURS OF RAINFALL ONSET  
AND/OR INITIAL STREAM RESPONSE. MODEL GUIDANCE HAS CONSISTENTLY  
KEPT 6-HOURLY RAIN ACCUMULATIONS GENERALLY UNDER 1.5", WHICH LESSENS  
THE CONCERN FOR SIGNIFICANT (I.E., FLOODING OF PRIMARY ROADWAYS  
AND STRUCTURES) FLOODING. INSTEAD, A MORE GRADUAL ACCUMULATION  
OF SIGNIFICANT RAINFALL MEANS THAT ANY FLOOD THREAT WILL GRADUALLY  
INCREASE MONDAY AFTERNOON INTO TUESDAY AS WIDESPREAD SOIL SATURATION  
IS REACHED AND PERSISTENT RUNOFF OVER LARGE AREAS CAUSES SIGNIFICANT  
RISES ON AREA STREAMS. LOW-LYING AREAS THAT COMMONLY FLOOD DUE  
TO POOR DRAINAGE AND OVERWHELMED DRAINAGE DITCHES WILL BE MOST  
SUSCEPTIBLE AND SUCH FLOODING MAY PERSIST FOR MANY HOURS DUE TO  
THE STEADY AND PERSISTENT NATURE OF THE RAINFALL.  
 
AS A RESULT, MINOR (I.E., PARKS, FARMLAND, BOAT ACCESS AREAS,  
AND OTHER COMMON FLOOD-PRONE LOW LYING AREAS) FLOODING OF THE  
UPPER FRENCH BROAD RIVER AND ASSOCIATED TRIBUTARIES IS POSSIBLE,  
BUT MINOR FLOODING OF MAINSTEMS IN THE UPSTATE IS ALSO POSSIBLE,  
ESPECIALLY ALONG THE BROAD AND SALUDA RIVERS, WITH CRESTS NOT  
OCCURRING UNTIL TUESDAY AND PERHAPS EARLY WEDNESDAY. STREAMLEVEL  
FORECASTS FOR THESE RIVERS WILL INCORPORATE THE TOTAL FORECASTED  
RAINFALL BEGINNING SUNDAY MORNING AND WILL BE ACCESSIBLE AT  
HTTP:/WATER.WEATHER.GOV/AHPS2/INDEX.PHP?WFO=GSP.  
 

 
   
GSP WATCHES/WARNINGS/ADVISORIES
 
 
GA...NONE.  
NC...NONE.  
SC...NONE.  
 

 
 

 
SYNOPSIS...JMP  
NEAR TERM...ARK/CARROLL  
SHORT TERM...PM  
LONG TERM...RWH  
AVIATION...ARK  
HYDROLOGY...  
 
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