MESOSCALE PRECIPITATION DISCUSSION 0519
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
419 PM EDT THU JUL 20 2017

AREAS AFFECTED...SOUTHERN NY...NORTHERN/EASTERN PA...NORTHERN NJ

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

VALID 202015Z - 210015Z

SUMMARY...STRONG THUNDERSTORMS PRODUCING RAINFALL RATES OF UP TO
2.5 INCHES/HR WILL ADVANCE SOUTHEAST TOWARD THE POCONOS AND WILL
LIKELY IMPACT THE NYC METROPOLITAN AREA OVER THE NEXT 2 TO 3 HOURS.

DISCUSSION...A RATHER PROGRESSIVE BAND OF THUNDERSTORMS ASSOCIATED
WITH A LONG-LIVED MCS IS MOVING ACROSS SOUTHERN NY AND WILL BE
ADVANCING SOUTHEAST ACROSS AREAS OF NORTHERN AND NORTHEAST PA AND
EVENTUALLY PARTS OF FAR SOUTHEAST NY AND NORTHWEST NJ OVER THE
NEXT 2 TO 3 HOURS.

THE ENVIRONMENT AHEAD OF THE MCS IS FAVORABLE FOR ADDITIONAL
INTENSIFICATION AND ORGANIZATION OF THE CONVECTION. A DOWNSTREAM
INVERTED SURFACE TROUGH IS ENHANCING SURFACE CONVERGENCE OVER
SOUTHEAST NY AND NORTHEAST PA...AND MEANWHILE MLCAPE VALUES HAVE
INCREASED TO AS MUCH AS 1500 J/KG VIA STRONG DIURNAL HEATING.
ADDITIONALLY...A MOIST ENVIRONMENT IS IN PLACE CHARACTERIZED BY
1.5 TO 1.75 INCHES. ALREADY OBSERVED RAINFALL RATES COMING OUT OF
SOUTHERN NY ARE AS HIGH AS 2.5 INCHES/HR AND IN LINE WITH AREAS
WHERE A COUPLE OF OVERSHOOTING TOPS WERE NOTED AS SUGGESTED BY
EXPERIMENTAL GOES-16 DATA.

THESE RAINFALL RATES ARE CAUSING AREAS OF FLASH FLOODING...AND AS
THE CONVECTION ADVANCES SOUTHEAST OVER SOME OF THE HIGHER TERRAIN
OF NORTHEAST PA/NORTHWEST NJ INVOLVING THE POCONOS...THERE WILL BE
SOME ELEVATED CONCERNS FOR FLASH FLOODING HERE THROUGH THE
LATE-AFTERNOON AND EARLY EVENING HOURS. ADJACENT AREAS OF
SOUTHEAST NY NEAR THE CATSKILLS AND ALSO DOWN INTO THE MORE
URBANIZED NYC METROPOLITAN AREA SHOULD ALSO MONITOR THIS
CONVECTIVE EVOLUTION WHERE ENHANCED RUNOFF AND RESULTING FLASH
FLOODING MAY OCCUR GIVEN THE INTENSE RAINFALL RATES. THE
PROGRESSIVE NATURE OF THE MCS ACTIVITY SHOULD PRECLUDE AMOUNTS
FROM BEING TOO HEAVY...BUT STORM TOTALS OF 2 TO 3 INCHES IN JUST
ONLY A 1 TO 2 HOUR PERIOD WILL BE POSSIBLE.

ORRISON

ATTN...WFO...ALY...BGM...CTP...OKX...PHI...

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

LAT...LON   42577505 42137379 41587351 40737384 40337432
            40177480 40237579 40667688 41407788 41967804
            42037688 42497575