MESOSCALE PRECIPITATION DISCUSSION 0486
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
234 PM EDT THU JUL 13 2017

AREAS AFFECTED...SRN NY/LONG ISLAND INTO SRN NEW ENGLAND

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

VALID 131834Z - 140000Z

SUMMARY...SCATTERED FLASH FLOODING WILL BE POSSIBLE ACROSS
PORTIONS OF SRN NY/LONG ISLAND INTO SRN NEW ENGLAND THROUGH 00Z.
RAINFALL RATES OF 1-2 IN/HR WITH MAXIMUM RAINFALL TOTALS UP TO 4
INCHES ARE EXPECTED.

DISCUSSION...RELATIVELY CLEAR SKIES OVER PORTIONS OF SRN NY INTO
CT EARLIER TODAY HAS ALLOWED FOR A MAXIMUM IN MLCAPE TO DEVELOP
WITH AN ESTIMATED 1000-2000 J/KG IN PLACE BASED ON THE 17Z SPC
MESOANALYSIS PAGE. DEWPOINTS IN THE LOW TO MID 70S PRESENT SOUTH
OF A STRONG...SLOW MOVING BACK DOOR FRONT ACROSS THE REGION IS
COINCIDENT WITH PRECIPITABLE WATER VALUES OF 1.6 TO 1.8 INCHES PER
RECENT GPS SITE OBSERVATIONS. BACK TO THE WEST...A WELL DEFINED
MCV NEAR BGM CONTINUES TO STEADILY TRACK ESEWD ALONG THE FRONTAL
BOUNDARY AND EXTRAPOLATION FORECASTS THE VORTEX TO REACH SERN NEW
ENGLAND BY 00Z.

ENHANCED LOW LEVEL MOISTURE TRANSPORT AND LIFT AHEAD OF THE MCV
SHOULD CONTINUE TO SUPPORT RAINFALL RATES OF 1-2 IN/HR IN THE
VICINITY OF THE FRONTAL BOUNDARY. IN ADDITION...UNIDIRECTIONAL WLY
FLOW IN THE WARM SECTOR IS ALREADY SUPPORTING TRAINING CELLS
ACROSS PORTIONS OF SRN CT AND RI. BOTH OF THESE FORCING MECHANISMS
WILL CONTRIBUTE TO A FLASH FLOOD THREAT THROUGH THE EARLY EVENING
HOURS. WHILE THERE A POSSIBILITY THAT INSTABILITY IS SIGNIFICANTLY
REDUCED WITH ONGOING CONVECTION ACROSS CT AND RI BEFORE THE MCV
ARRIVES SUCH THAT RAINFALL RATES ARE BELOW FLASH FLOOD
CONCERNS...THE HISTORY OF THE MCV HAS SUPPORTED 1-2 IN/HR RATES
WITH NEAR WEAK INSTABILITY EARLIER THIS MORNING OVER UPSTATE NEW
YORK...SUCH THAT HIGH PRECIPITABLE WATER VALUES MAY MAKE UP FOR A
LACK OF GREATER INSTABILITY.

GIVEN FLASH FLOOD GUIDANCE IS A LITTLE HIGHER ACROSS SRN NEW
ENGLAND AND ESPECIALLY EASTERN LONG ISLAND COMPARED TO UPSTATE NEW
YORK...THE GREATEST CONCERN FOR FLASH FLOODING THIS
AFTERNOON/EARLY EVENING WILL BE ACROSS PORTIONS OF ERN CT/MA/RI
WHICH RECEIVED HEAVY RAINFALL OVER THE PAST 24-36 HOURS AND ARE
LIKELY MORE SUSCEPTIBLE TO RUNOFF ISSUES.

OTTO

ATTN...WFO...ALY...BGM...BOX...OKX...PHI...

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

LAT...LON   42407217 42387071 42016997 41436990 40967061
            40697196 40607385 41187471 41767470 42237422
            42347343