MESOSCALE PRECIPITATION DISCUSSION 0518
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
932 AM EDT FRI SEP 25 2015
 
AREAS AFFECTED...COASTAL SC/NC 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 251331Z - 251931Z
 
SUMMARY...SHOWERS AND THUNDERSTORMS MOVING ASHORE FROM THE
ATLANTIC COULD LEAD TO RAIN RATES UP TO 2.5" IN AREAS OF TRAINING.
 FLASH FLOODING IS POSSIBLE.

DISCUSSION...CONVERGENT LOW-LEVEL FLOW BETWEEN A DEEP LAYERED LOW
IN GA/SC AND A BLOCKING RIDGE IN THE VICINITY OF MD IS DRAWING IN
ANOMALOUS MOISTURE FROM THE ATLANTIC INTO THE CAROLINAS. 
SATELLITE AND RADAR IMAGERY SHOWS CONVECTION BUBBLING NEAR AND
JUST OFFSHORE THE UPPER SC AND MUCH OF THE NC COAST AT THIS TIME. 
RADAR ESTIMATES SHOW SPOTTY MAXIMA IN THE 4-6" RANGE, WHICH SEEM
TO BE CONFIRMED BY RECENT COCORAHS REPORTS IN GEORGETOWN COUNTY SC
AND CARTARET COUNTY NC.  PRECIPITABLE WATER VALUES ARE 1.9-2.3" IN
THIS REGION, 2-3 SIGMAS ABOVE THE MEAN FOR LATE SEPTEMBER. 
LOW-LEVEL INFLOW HAS BEEN MODEST AS OF LATE, ROUGHLY 10-20 KTS, ON
PAR WITH THE MEAN 850-400 HPA WINDS BUT AT A RIGHT ANGLE;
DIRECTIONAL WIND SHEAR EXISTS WITHIN AREA SOUNDINGS. THE 0-6 KM
BULK SHEAR INDICATED BY GFS/NAM/RAP MASS FIELDS IS LEADING TO
FORWARD PROPAGATION OF ORGANIZED CELLS OFFSHORE, ROUGHLY TO THE
NORTH AT 10-15 KTS.  MUCAPES OFFSHORE ARE IN THE 1000-2000 J/KG
RANGE WITHIN RECENT SPC MESOANALYSES AND A RECENT SAB BRIEFING
CONCERNING SATELLITE-DERIVED VALUES (MENTIONED IN THE 1259Z SPENES
MESSAGE).

THE RAP GUIDANCE INDICATES A MODEST INCREASE IN THE 850 HPA INFLOW
ACROSS THE REGION TOWARDS THE 25 KT RANGE WITH TIME, WHICH ALONG
WITH THE BULK SHEAR SHOULD CONTINUE CELL ORGANIZATION NEAR THE
COAST.  CIN IN THE REGION ALLOWS FOR THE MUCAPE TO BE TAPPED UNTIL
AT LEAST 17Z, WHEN AREA CIN IS EXPECTED TO ERODE (THOUGH WITH
EXPECTED CLOUD COVER IT COULD EXCEED RAP EXPECTATIONS).  SHORT
PERIODS OF CELL TRAINING ARE THE MAIN THREAT FROM A FLASH FLOOD
PERSPECTIVE AS THE AVAILABLE MOISTURE SUPPORTS HOURLY RAIN RATES
UP TO 2.5"; SO FAR, OBSERVATIONS HAVE BEEN CAPPED AT 1.75".  THE
CAM GUIDANCE ADVERTISES LOCAL AMOUNTS IN THE 3-4" RANGE, WHICH
SHOULD FALL QUICKLY.  THE RETURN PERIOD OF 3-4" OF RAIN WITHIN
THREE HOURS IS A DECADE, NEAR ONE OF THE FLASH FLOOD THRESHOLDS. 
SOILS ARE SLOWLY SATURATING IN THIS AREA, WITH FLASH FLOOD
GUIDANCE VALUES SLOWLY FALLING OFF.  HOWEVER, SOME OF THIS REGION
IS MARSHY, WHICH COULD MITIGATE FLASH FLOODING.  STILL, FLASH
FLOODING IS CONSIDERED POSSIBLE WHERE CELLS TRAIN.  

ROTH

ATTN...WFO...AKQ...ILM...MHX...RAH...

ATTN...RFC...SERFC...

LAT...LON   36667592 35977549 35497537 35207553 34997609 
            34707630 34507656 34507723 33957769 33717800 
            33597874 33347904 33467968 34397913 35367807 
            36227693