MESOSCALE PRECIPITATION DISCUSSION 0591
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1103 PM EDT FRI JUL 28 2017

AREAS AFFECTED...NORTHERN WV/SOUTHERN PA/MD/NORTHERN
VA/DC/DE/SOUTHERN NJ...

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 290301Z - 290731Z

SUMMARY...WIDESPREAD HEAVY RAINFALL ACROSS THE MID-ATLANTIC
STRETCHING BACK INTO THE CENTRAL APPALACHIANS REMAINS A CONTINUED
FOCUS FOR FLASH FLOODING. SLOW-MOVING PRECIPITATION BANDS IN THE
PRESENCE OF HIGHLY ANOMALOUS PRECIPITABLE WATER VALUES WILL ALLOW
HOURLY RAINFALL RATES TO APPROACH THE 2 TO 3 INCH RANGE IN SOME
AREAS. FLASH FLOODING WILL REMAIN LIKELY GIVEN THE POTENTIAL FOR
SUCH EXTREME RATES.

DISCUSSION...REGIONAL RADAR/SATELLITE IMAGERY ACROSS THE
MID-ATLANTIC SHOWED A COUPLE ITEMS OF NOTE. FIRST...ACROSS WESTERN
PA...INTENSE CONVECTION HAS ERUPTED IN ADVANCE OF THE SLOW-MOVING
UPPER TROUGH CURRENTLY MOVING INTO EASTERN OH. IN ADDITION TO THE
STRONGER DYNAMICAL FORCING...THIS BAND OF HEAVY RAINFALL APPEARS
TO BE MAKING THE MOST OF THE 250-500 J/KG OF MUCAPE WHICH HAS EVEN
SPARKED SOME LIGHTNING STRIKES. FARTHER EAST...A MESOSCALE
CIRCULATION WAS NOTED IN THE KLWX RADAR AS IT MOVES EAST-SOUTHEAST
AWAY FROM CHARLESTOWN WV. DUAL-POL HOURLY RAINFALL RATES ARE IN
THE 2 TO 3 INCH RANGE WITH THIS PARTICULAR BAND AND WILL INCREASE
THE POTENTIAL FOR FLASH FLOODING IN DOWNSTREAM LOCATIONS. WHILE
CONVECTIVE ACTIVITY HAS GENERALLY BEEN MORE LOCALIZED IN
NATURE...EFFICIENT WARM RAIN PROCESSES CONTINUE TO DOMINATE IN THE
PRESENCE OF A 2 TO 2.25 INCH PRECIPITABLE WATER AIR MASS AND
WET-BULB ZERO HEIGHTS IN THE 13 TO 14 KFT RANGE. HEAVY RAINFALL
WILL REMAIN WIDESPREAD AND EXPAND EASTWARD AS THE UPSTREAM
SHORTWAVE MOVES INTO THE CENTRAL APPALACHIANS OVERNIGHT.

SOME VARIABILITY IN THE HIGH-RESOLUTION SOLUTIONS IS NOTED BUT
THERE IS A DECENT CONSENSUS ACROSS THE BOARD. THERE ARE MANY SUCH
MODELS SUGGESTING A WIDESPREAD 3 TO 5 INCH TOTAL IN A 6-HOUR
PERIOD WHICH APPEARS QUITE REASONABLE GIVEN PAST HISTORY. FLASH
FLOODING WILL REMAIN LIKELY THROUGH THE OVERNIGHT HOURS. WILL
RE-ASSESS THE SITUATION BEYOND 0730Z.


RUBIN-OSTER

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

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

LAT...LON   41147810 40897688 40567605 40187560 39367487
            38607461 37687480 37387538 37467608 37987726
            38197773 38327804 38837965 39198049 39608081
            40468053 41127944