MESOSCALE PRECIPITATION DISCUSSION 0400
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1209 PM EDT WED JUL 04 2018

AREAS AFFECTED...EASTERN PA, NORTHERN NJ, SOUTHEAST NY, & WESTERN
CT

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

VALID 041608Z - 042208Z

SUMMARY...A WEAK WAVE OF LOW PRESSURE IN NORTHERN PA IS EXPECTED
TO HELP LEAD TO HOURLY RAIN TOTALS UP TO 2.5" WITH LOCAL AMOUNTS
UP TO 4" OVER THE NEXT SEVERAL HOURS.

DISCUSSION...THE LATEST SPC MESOANALYSIS ALONG WITH GOES-16
"VEGGIE BAND" IMAGERY SHOW NEW CONVECTION THAT IS BECOMING
SURFACE-BASED ACROSS SOUTHEAST NY NEAR A STATIONARY FRONT TO THE
EAST OF A WEAK FRONTAL WAVE AND PARENT MCV NEAR THE CENTRAL NY/PA
BORDER.  ANOTHER SHORTWAVE IS EVIDENT IN WATER VAPOR IMAGERY
MOVING ACROSS NORTHERN AND WESTERN NJ.  ML CAPE NEAR AND SOUTH OF
THE BOUNDARY IS UP TO 1000-2000 J/KG.  INFLOW AT 850 HPA IS WEAK,
BECOMING WEST-SOUTHWESTERLY AT 10-15 KTS.  PRECIPITABLE WATER
VALUES OF 1.8-2.1" EXIST HERE, WHICH IS CLOSE TO TWO SIGMAS ABOVE
EARLY JULY AVERAGES.

THE MESOSCALE GUIDANCE SIGNAL AT 16Z IS ALREADY TOO FAR TO THE
NORTH WITH CONVECTION PER RECENT RADAR/SATELLITE IMAGERY, SO HAVE
USED A SLIGHTLY SOUTHWARD ADJUSTED AREA FROM WHAT IS ADVERTISED TO
COMPENSATE.  THE EXPECTATION IS FOR CONVECTION TO EXPAND IN
COVERAGE AS CIN CONTINUES ERODING DURING THE NEXT HOUR OR TWO. 
STORMS SHOULD BE SLOW MOVING AS THE MEAN 850-400 HPA WIND IS ~5
KTS PER NAM FORECASTS.  THE 12Z HREF PROBABILITIES OF 1"+ SPIKE
SHORTLY AND PEAK 20-22Z ACROSS EASTERN PA, SOUTHEAST NY, AND CT. 
ALTHOUGH CONVECTION IS EXPECTED TO BE PULSE IN NATURE, THE
UPSTREAM MCV MAY LEAD TO SOME ATTEMPTS AT MORE ORGANIZED STRUCTURE
ACROSS EASTERN PA AND NEAR THE NY/NJ BORDER LIKE SEEN EARLIER THIS
MORNING NEAR THE WESTERN PA/NY BORDER.  GIVEN THE AVAILABLE
MOISTURE, HOURLY RAIN TOTALS TO 2.5" WITH LOCAL AMOUNTS TO 4" ARE
POSSIBLE THIS AFTERNOON, WHICH WOULD LEAD TO ISSUES IN URBAN AREAS.

ROTH

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

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

LAT...LON   42097364 41977241 41257312 40737331 40537386
            40257453 40137510 40297580 40507627 41377712
            41957595 42087488