MESOSCALE PRECIPITATION DISCUSSION 0191
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
205 PM EDT FRI MAY 05 2017

AREAS AFFECTED...FAR SOUTHEASTERN NY/LONG ISLAND/CT/RI/SOUTHERN
MA...

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

VALID 051805Z - 052105Z

SUMMARY...MESOSCALE BANDING WITHIN A BROADER PRECIPITATION SHIELD
HAS FOCUSED AN INTENSE BAND OF LOW-TOPPED CONVECTION ACROSS THE
NEW YORK CITY METRO AREA. WHILE TRANSIENT IN NATURE...THE
POTENTIAL FOR THIS TO PERSIST INTO DOWNSTREAM LOCATIONS IS
POSSIBLE WHICH COULD SPREAD THE FLASH FLOOD THREAT INTO LOWER
COASTAL NEW ENGLAND.

DISCUSSION...WHILE LACKING IN INSTABILITY AS MUCAPE VALUES HAVE
GENERALLY NOT EXCEEDED 250 J/KG...IMPRESSIVE LOW-LEVEL WARM
ADVECTION PUMPING 1.50 INCH PLUS PWATS INTO THE UPPER MID-ATLANTIC
COAST HAS MADE UP FOR THESE LIMITATIONS. LOCALLY INTENSE HEAVY
RAINFALL HAS FALLEN ACROSS EAST-CENTRAL NJ UP TOWARD THE NEW YORK
CITY METRO AREA WITH HOURLY OBSERVATIONS REACHING THE 1 TO 1.25
INCH RANGE. THIS HAS LED TO A NUMBER OF FLASH FLOOD
REPORTS...ESPECIALLY ACROSS THE MORE SUSCEPTIBLE URBANIZED LOCALES.

ON A BROADER SENSE...HOURLY RATES HAVE GENERALLY STAYED IN THE
0.25 TO 0.50 INCH RANGE WHILE SOME MORE IMPRESSIVE MESOSCALE BANDS
HAVE TRIPLED THESE AMOUNTS. WITH LIMITED FORECASTABILITY IN SUCH
OCCURRENCES DOWN THE ROAD...THE POTENTIAL STILL EXISTS FOR FURTHER
FLARE-UPS AS THE SYSTEM LIFTS TOWARD CT/RI/MA. MESOSCALE MODELS
SUPPORT A BROAD SWATH OF 1 TO 2 INCH AMOUNTS DURING THE NEXT 6
HOURS...PARTICULARLY OVER CT/RI. THIS IS SUPPORTED BY THE 12Z
NSSL-WRF/HRW-ARW/RECENT RUNS OF THE HRRR. WHILE SUCH FORECAST
AMOUNTS SHOULD NOT BE A FLASH FLOOD ISSUE DURING A 6-HOUR
SENSE...ANY MORE INTENSE DOWNPOURS LIKE WHAT WAS SEEN FROM
EAST-CENTRAL NJ INTO SOUTHEASTERN NY MAY CAUSE SOME LOCALIZED
ISSUES.

RUBIN-OSTER

ATTN...WFO...ALY...BOX...OKX...

ATTN...RFC...NERFC...

LAT...LON   42437130 42027082 41367082 41167139 40747226
            40167305 40447382 41437338 42167260