MESOSCALE PRECIPITATION DISCUSSION 0496
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
954 PM EDT FRI JUL 20 2018

AREAS AFFECTED...COASTAL SECTIONS OF NORTHEAST SC/NC

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 210153Z - 210753Z

SUMMARY...SHOWERS AND THUNDERSTORMS ARE SLOWLY MOVING ASHORE FROM
THE ATLANTIC EAST OF A LOW PRESSURE AREA.  HOURLY RAIN TOTALS TO
2.5" WITH LOCAL AMOUNTS TO 4" ARE EXPECTED.

DISCUSSION...A COUPLE AREAS OF LOW PRESSURE EXIST ALONG AN OLD
FRONTAL BOUNDARY STRADDLING THE COAST OF THE CAROLINAS SOUTH OF
MYRTLE BEACH AT 01Z, WITH A COMPANION CIRCULATION IS JUST EAST OF
WILMINGTON.  ALOFT, A STRONG SHORTWAVE IS APPROACHING FROM
NORTHERN GA AND THE WESTERN CAROLINAS PER WATER VAPOR IMAGERY. 
RECENT RADAR TRENDS SHOW A SLOW INCREASE TO SHOWER AND
THUNDERSTORM ACTIVITY NEAR THE BOUNDARY, WITH INSTABILITY TRENDS
ALSO GOING UPWARD.  PRECIPITABLE WATER VALUES OF 2.2-2.5" PER AREA
SOUNDINGS AND GPS DATA.  WET BULB ZERO VALUES ARE NEAR 15,000
FEET, WHICH SHOULD CONTINUE TO SUPPORT WARM RAIN PROCESSES. 
INFLOW IN THE LOW-LEVELS (SURFACE AN 850 HPA) IS SOUTH TO
SOUTHEAST AT 15-25 KTS PER SURFACE OBSERVATIONS, VAD WIND
PROFILES, AND RAP MASS FIELDS (STRONGER THAN THE MEAN 850-400 HPA
WIND).  EFFECTIVE BULK SHEAR OF 20+ KTS LIES JUST OFF THE COAST. 
ML CAPE VALUES ARE CURRENTLY 500-1000 J/KG.

THE EXPECTATION IS FOR SHOWERS AND THUNDERSTORMS TO CONTINUE TO
INCREASE IN COVERAGE OVER THE NEXT SEVERAL HOURS AS THE SHORTWAVE
ALOFT NEARS.  THE WARM FRONT EAST OF THE LOW SHOULD PIVOT A LITTLE
FARTHER NORTHWARD.  RAP MASS FIELDS SHOW SOME INCREASE IN THE
LOW-LEVEL INFLOW, WHICH COULD ALLOW THE BULK SHEAR TO INCREASE AS
WELL.  BOTH RAP AND GFS MASS FIELDS SUGGEST ADDITIONAL INCREASE IN
THE AVAILABLE INSTABILITY.  SHORT CURVED BANDS COULD BRIEFLY SET
UP AND TRAIN DURING THIS TIME FRAME.  HOURLY RAIN TOTALS TO 2.5"
SHOULD BE POSSIBLE WITHIN THIS SATURATED AIR MASS.  THE MESOSCALE
GUIDANCE AGREES ON LOCAL AMOUNTS OF 4" BY 08Z.  THIS DEGREE OF
RAIN COULD CAUSE ISSUES WHERE HEAVY RAIN FELL EARLIER AND WITHIN
ANY URBAN AREAS.  SINCE THE HEAVY RAIN IS EXPECTED NEAR/ALONG THE
COASTLINE, UNCERTAINTY IN THE CONVECTIVE EVOLUTION IS HIGHER THAN
USUAL SINCE THE HEAVY RAIN COULD ALTERNATIVELY SLIP OR OCCUR JUST
OFFSHORE.

ROTH

ATTN...WFO...ILM...MHX...

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

LAT...LON   35907557 35257539 35117584 34777626 34537650
            34587676 34547708 34287754 33947777 33827794
            33837832 33937835 34477816 35287682