MESOSCALE PRECIPITATION DISCUSSION 0652
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1121 PM EDT MON AUG 07 2017

AREAS AFFECTED...SOUTHEAST VA & NORTHEAST NC

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

VALID 080320Z - 080920Z

SUMMARY...SHOWERS AND THUNDERSTORMS ARE INCREASING IN COVERAGE
NEAR A STATIONARY FRONT.  HOURLY RAIN RATES TO 2.5" WITH LOCAL
AMOUNTS TO 4" ARE POSSIBLE.

DISCUSSION...TWO WAVES OF LOW PRESSURE -- ONE NEAR DANVILLE VA AND
ANOTHER IN THE LOWER CHESAPEAKE BAY -- ALONG WITH DIVERGENCE ALOFT
SPONSORED BY A SHORTWAVE MOVING ACROSS WV ARE LEADING TO AN
INCREASE IN THUNDERSTORM COVERAGE ACROSS SOUTHEAST VA. 
PRECIPITABLE WATER VALUES ARE 2-2.5" PER RECENT GPS DATA. 
EFFECTIVE BULK SHEAR IS ~30 KTS PER SPC MESOANALYSES.  INFLOW AT
850 HPA IS 35-40 KTS FROM THE WEST-SOUTHWEST PER VAD WIND
PROFILES.  MU CAPE VALUES IN THE REGION ARE 500-1000 J/KG WHICH
HAVE BEEN SUFFICIENT SO FAR TO INCREASE CELL COVERAGE WITHIN THIS
MOIST ATMOSPHERE.

THE MESOSCALE GUIDANCE HAS A GOOD SIGNAL FOR HEAVY RAIN -- THE
QUESTION IS FOR HOW LONG AND HOW FAR INTO NORTHEAST NC IT COULD
SAG.  NORMALLY THIS DEGREE OF INSTABILITY CAN BE ERODED IN A FEW
HOURS, BUT RAP MASS FIELDS SUGGEST SOME SOUTHWARD EROSION WITH
TIME WHICH SHOULD SHIFT THE BAND TO THE SOUTH OVER THE NEXT
SEVERAL HOURS.  WARM ADVECTION AHEAD OF THE WAVE NEAR DANVILLE --
WHICH IS EXPECTED TO REACH NORFOLK VA 09-10Z -- COULD MAINTAIN
INSTABILITY IN THE WARM SECTOR WELL INTO THE NIGHT.  THE MESOSCALE
GUIDANCE SHOWS A GOOD SIGNAL FOR LOCAL AMOUNTS UP TO 3-4", WHICH
APPEARS REALISTIC.  THE AVAILABLE MOISTURE AND INSTABILITY COULD
LEAD TO HOURLY RAIN TOTALS UP TO 2.5" IN AREAS OF CELL TRAINING.

ROTH

ATTN...WFO...AKQ...MHX...RAH...RNK...

ATTN...RFC...MARFC...OHRFC...SERFC...

LAT...LON   37707566 37527556 36857587 36247574 35807674
            36077863 36367965 36838039 36997887 37327706
            37597619