498  
FOUS30 KWBC 240825  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
424 AM EDT MON APR 24 2017  
   
..VALID 12Z MON APR 24 2017 - 12Z TUE APR 25 2017
 
   
..REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL
 
 
MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
105 SE HXD 70 SE HXD 45 SE HXD 15 NNW NBC 25 SW CAE 20 W CAE  
15 E AVL 15 WNW ROA 20 S W99 20 ESE W99 20 S OKV 10 NW OMH  
15 SSW CHO 15 NE LYH DAN AVC NGU 15 NE NTU 35 ENE FFA 40 E HSE.  
 
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
95 SE CHS 40 S CHS 15 SSE MMT 35 NNE CUB 10 S EXX 10 NE GSO  
15 NNE RWI 30 NE NKT 50 SSE MRH.  
 
MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
45 SSE MYR 40 SW MYR 45 WSW MYR UDG 10 NNW 45J 15 ESE HRJ OAJ  
10 S NCA 20 E SUT 35 S SUT 45 SSE MYR.  
 
...SOUTHERN-CENTRAL APPALACHIANS / SOUTHEASTERN U.S. / MID  
ATLANTIC...  
 
A HEAVY RAINFALL EVENT WILL CONTINUE TODAY...FOCUSED MAINLY ACROSS  
PORTIONS OF NORTH AND SOUTH CAROLINA. OVERALL AN IMPRESSIVE  
SETUP...WITH BOTH CONDITIONS ALOFT AND IN THE LOWER LEVELS  
CONDUCIVE TO HEAVY CONVECTIVE RAINFALL. IN THE UPPER LEVELS THE  
CLOSED LOW MOVING SOUTHEASTWARD WILL CONTINUE TO TAKE ON MORE OF A  
NEGATIVE TILT RESULTING IN INCREASED ASCENT OVER NC/SC. ALSO NOTE  
IMPRESSIVE DIFFLUENCE AT 250 MB...WHICH SHOULD AID IN THE  
MAINTENANCE OF CONVECTION THROUGH THE DAY. IN THE LOWER LEVELS A  
STRONG COASTAL FRONT EXTENDS ALONG THE CAROLINA COAST...WITH  
INSTABILITY ALONG/SOUTH AND JUST NORTH OF THIS BOUNDARY. STRONG  
EASTERLY FLOW INTO AND OVER THIS BOUNDARY WILL HELP MAINTAIN A  
STRONG CONVERGENCE AXIS...AND ALSO HELP MAINTAIN INSTABILITY INTO  
THE BOUNDARY. PRECIPITABLE WATER VALUES WILL CONTINUE TO INCREASE  
TO AROUND CLIMATOLOGICAL RECORD VALUES ACROSS PORTIONS OF SC/NC AS  
MOISTURE FROM THE TROPICAL WAVE OFF OF FLORIDA IS WRAPPED WEST  
INTO THE CLOSED LOW.  
 
THE EXPECTATION IS THAT THE HEAVY CONVECTIVE LINE MOVING ACROSS SC  
AS OF 06Z WILL CONTINUE SLOWLY MOVING EAST INTO THE MORNING.  
HOWEVER AS THE CLOSED LOW DIVES SOUTHEAST AND BECOMES INCREASINGLY  
NEGATIVELY TILTED...EXPECT THE NORTH/SOUTH CONVERGENCE AXIS  
RESPONSIBLE FOR THIS CONVECTIVE LINE TO SLOW ITS EASTWARD  
PROGRESSION. THIS SHOULD ALLOW FOR CONTINUED CONVECTIVE  
REGENERATION OVER EASTERN SC THIS MORNING...WITH THE ACTIVITY  
SLOWLY MOVING EASTWARD INTO SOUTHERN NC WITH TIME. THE COMBINATION  
OF PROLONGED SYNOPTIC SUPPORT AND ONLY SLOW EASTWARD PROGRESSION  
OF THE CONVERGENCE AXIS...AND WIND FIELDS CONDUCIVE TO SOME  
SOUTHWARD BACKBUILDING OF CONVECTION INTO THE INSTABILITY...BOTH  
POINT TOWARDS THE LIKELIHOOD OF TRAINING CONVECTION. WHICH GIVEN  
THE VERY HIGH PWAT VALUES...WILL LEAD TO SOME IMPRESSIVE RAINFALL  
TOTALS INTO THIS EVENING.  
 
SOME MODEL DIFFERENCES EXIST WITH REGARDS TO THE EXACT AXIS OF  
HIGHEST QPF AMOUNTS. AT LEAST SOME OF THE DIFFERENCES ARE RELATED  
TO EXACTLY HOW MUCH CONVECTION IS ABLE TO PROPAGATE EASTWARD WITH  
TIME. THE HIGH RES ARW PROPAGATES THE MOST...PUSHING THE HEAVIEST  
TOTALS MORE INTO SOUTHERN NC. THE HIGH RES NMMB ON THE OTHER HAND  
IS THE LEAST PROGRESSIVE SOLUTION...KEEPING THE HEAVIEST AMOUNTS  
OVER CENTRAL SC. BASED ON RECENT RADAR TRENDS THINK THE SLIGHTLY  
MORE PROGRESSIVE NSSL WRF AND HIGH RES ARW ARE PROBABLY CLOSER TO  
TO BEING CORRECT. WPC QPF GENERALLY FOLLOWED A MULTI MODEL  
BLEND...ALTHOUGH WAS WEIGHTED TOWARDS THE HIGH RES CONSENSUS.  
THOUGHT MUCH OF THE NON CAM GUIDANCE (GFS/NAM/ECMWF) MAY HAVE BEEN  
A BIT TOO FAR INLAND WITH THEIR HIGHEST AMOUNTS...AND PROBABLY A  
BIT TOO FAR SOUTHWEST...AS THEY MAY BE FAILING TO CORRECTLY  
PROPAGATE CONVECTION SLOWLY EASTWARD. LOOKING AT AREAL AVERAGED  
AMOUNTS MAXING IN THE 4-6" RANGE ALONG THE NC/SC BORDER NEAR THE  
COAST. WOULD ANTICIPATE SOME ISOLATED 6-10" TOTALS IN THIS  
VICINITY AS WELL WHERE TRAINING AND CELL MERGERS END UP MAXIMIZED.  
WILL CONTINUE WITH A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS  
THIS AREA WITH RAINFALL RATES AND DURATION LIKELY ENOUGH TO RESULT  
IN SOME FLASH FLOOD ISSUES TODAY...WITH SOME SIGNIFICANT ISSUES  
POSSIBLE. THE FACT THAT THE CLOSED LOW WILL REMAIN MOVING WILL  
PREVENT RAINFALL TOTALS FROM BEING EVEN MORE EXTREME AND CAPS THIS  
EVENT IN THE MODERATE RISK CATEGORY.  
 
THIS MODERATE RISK AREA WILL BE SURROUNDED BY A SLIGHT RISK. THE  
SLIGHT RISK ACCOUNTS FOR CONVECTION FURTHER SOUTHWEST ACROSS SC  
CLOSER TO THE CLOSED LOW...AS WELL AS PORTIONS OF EAST CENTRAL NC  
WHERE CONVECTION SHOULD EVENTUALLY INCREASE IN COVERAGE...BOTH  
ALONG THE COASTAL FRONT AND AHEAD OF THE MAIN CONVECTIVE AXIS AS  
IT MOVES EAST. THE SLIGHT RISK AREA GENERALLY FOLLOWS THE EDGE OF  
EXPECTED ISOLATED TO SCATTERED 2-3" AMOUNTS IN THE 12Z-12Z DAY 1  
PERIOD. EXTENDED A MARGINAL RISK FURTHER NORTH INTO PORTIONS OF  
THE SOUTHERN APPALACHIANS. IN GENERAL RAINFALL INTENSITY SHOULD BE  
ON A DECREASE ACROSS THIS AREA AFTER 12Z AS THE SYSTEM BEGINS TO  
CUTOFF A BIT FROM THE FLOW AND WE START TO LOSE THE BETTER  
MOISTURE AND DYNAMICS. HOWEVER THE COMBINATION OF SATURATED SOILS  
AND CONTINUED LIGHT TO MODERATE RAINFALL WILL LIKELY LEAD TO SOME  
CONTINUING WATER ISSUES...ESPECIALLY INTO FAVORED UPSLOPE AREAS.  
 
CHENARD  
 

 
 
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