MESOSCALE PRECIPITATION DISCUSSION 0393...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
500 PM EDT MON JUL 27 2015
 
AREAS AFFECTED...PORTIONS OF WV AND VA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 272037Z - 280237Z
 
CORRECTED FOR TYPO IN THE SUMMARY SECTION

SUMMARY...THUNDERSTORMS ARE BECOMING MORE CONCENTRATED ACROSS
CENTRAL VA.  FLASH FLOODING REMAINS POSSIBLE OVER THE NEXT SEVERAL
HOURS.

DISCUSSION...AFTERNOON CONVECTION ACROSS THE AREA HAS BEEN
INCREASING IN COVERAGE OVER THE PAST COUPLE HOURS WITHIN A MOIST
ENVIRONMENT THAT CONTAINS PRECIPITABLE WATER VALUES OF 1.75-2". 
RADAR ESTIMATES HAVE INDICATED HOURLY RAIN RATES UP TO 2", WHICH
BEFITS THE MOISTURE AVAILABLE.  LIGHT SOUTHWEST FLOW IS SEEN AT
850 HPA, WHILE THE MEAN WIND IS FROM THE NORTHWEST AT 15 KTS. 
MLCAPES ARE IN THE 1500-2500 J/KG RANGE.  BULK SHEAR IS IN THE
20-25 KT RANGE, WHICH APPEARS TO BE ORGANIZING AREA CONVECTION
OUTSIDE THE EXPECTED PULSE MODE.  WHILE THIS COMBINATION WOULD
NORMALLY FOSTER FORWARD PROPAGATING CONVECTION, MUCH OF THE
ACTIVITY HAS BEEN DIFFICULT TO BUDGE FROM THE BLUE RIDGE MOUNTAINS.

AS A COLD POOL FORMS AT THE SURFACE, EXPECT CONVECTION TO BECOME
DISLODGED FROM THE TOPOGRAPHY AND MIGRATE SOUTHEAST AT 15 KTS. 
ACTIVITY NEAR THE DC/BALTIMORE AREAS SHOULD FADE FIRST AS A COLD
POOL FROM EARLY CONVECTION PUSHES AN OUTFLOW BOUNDARY SOUTHWARD. 
THE MESOSCALE GUIDANCE HAS GENERALLY SHOWN A NORTHWARD BIAS WITH
THE PLACEMENT OF THE THUNDERSTORM ACTIVITY SO FAR, WITH THE 12Z
SPC WRF AND 12Z WRF4NSSL CONSIDERED THE LEAST USEFUL.  THE
GUIDANCE SUGGESTS LOCAL AMOUNTS OF 2-4" ARE POSSIBLE IN THIS
REGION, WHICH COULD FALL QUICKLY WITH HOURLY RAIN RATES UP TO 2". 
THE AVAILABLE INSTABILITY SUPPORTS AT LEAST FOUR MORE HOURS OF
CONVECTIVE ACTIVITY, WITH THE 12Z SSEO NEIGHBORHOOD PROBABILITIES
OF EXCEEDING THREE HOUR FLASH FLOOD GUIDANCE SUGGESTING PROBLEMS
LINGER THROUGH 05Z.  FLASH FLOODING IS POSSIBLE, PARTICULARLY
ACROSS THE TERRAIN AND WITHIN URBAN AREAS.  

ROTH

ATTN...WFO...AKQ...LWX...PBZ...PHI...RLX...RNK...

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

LAT...LON   39707875 39027699 39097573 39047533 38647495 
            37747535 36967606 36527747 37177908 38578066 
            39067999