MESOSCALE PRECIPITATION DISCUSSION 0496
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
113 PM EDT MON JUL 17 2017

AREAS AFFECTED...EASTERN PA...NORTHWEST NJ...CENTRAL/EASTERN
NY...NORTHWEST MA...VT...NH...WESTERN ME

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

VALID 171710Z - 172310Z

SUMMARY...SHOWERS AND THUNDERSTORMS WILL EXPAND IN COVERAGE AND
BECOME MORE ORGANIZED THIS AFTERNOON WITH INTENSE RAINFALL RATES.
SOME FLASH FLOODING IS POSSIBLE.

DISCUSSION...STRONG DIURNAL HEATING IN CONJUNCTION WITH A
RELATIVELY MOIST BOUNDARY LAYER HAS BEEN ALLOWING FOR A STEADY
DESTABILIZATION OF THE AIRMASS SOUTH AND EAST OF A
QUASI-STATIONARY FRONTAL ZONE AND AHEAD OF A WAVE OF LOW PRESSURE
NOTED OVER FAR WESTERN NY. MLCAPE VALUES HAVE RISEN TO GENERALLY
BETWEEN 1000 AND 2000 J/KG...WITH PWATS RUNNING AROUND 1.4 TO 1.5
INCHES. MEANWHILE...AN UPPER TROUGH IS SEEN DRIFTING EAST OVER THE
LOWER GREAT LAKES REGION WHICH IS FOSTERING SOME MODEST UPPER
FORCING OVER THE REGION AND SUPPORTING ROUGHLY 30 KTS OF EFFECTIVE
BULK SHEAR. THEREFORE...CONDITIONS ARE BECOMING FAVORABLE FOR
CONVECTION TO DEVELOP AND EXPAND IN COVERAGE AS PER SPC MCD #1329.

IN FACT...EXPERIMENTAL GOES-16 10.3 MICRON/IR IMAGERY SHOWS
CONVECTIVE INITIATION WELL UNDERWAY NOW ACROSS PORTIONS OF EASTERN
PA AND STRETCHING NORTH UP ACROSS CENTRAL NY. THIS ACTIVITY IS
EMBEDDED WITHIN DEEP LAYER SOUTHWEST FLOW AND IS ALLOWING FOR SOME
SW/NE ORIENTED CONVECTIVE BANDS/CLUSTERS TO BEGIN SETTING UP. THE
CELLS ARE CAPABLE OF PRODUCING RAINFALL RATES OF 1.5 TO 2
INCHES/HR AND THIS WILL LOCALLY APPROACH THE 1-HOUR FFG VALUES
ACROSS THE REGION.

A CONSENSUS OF THE 12Z HIRES GUIDANCE INCLUDING THE ARW/NMMB AND
NSSL-WRF...ALONG WITH RECENT HRRR/HRRRX GUIDANCE SUGGESTS AN
UPTICK IN CONVECTIVE ORGANIZATION THIS AFTERNOON AND AN EXPANSION
OF CONVECTION OFF TO THE EAST-NORTHEAST WITH TIME. THIS WILL ALLOW
CONVECTION TO GRADUALLY BEGIN OVERSPREADING AREAS OF VT/NH AND
WESTERN ME...AND EVENTUALLY AREAS FARTHER SOUTH INCLUDING
SOUTHEAST NY...NORTHWEST NJ AND FAR WESTERN MA LATER IN THE
AFTERNOON. RAINFALL AMOUNTS AS PER THE HIRES MODEL CONSENSUS IS ON
THE ORDER OF 2 TO 3 INCHES WITH ISOLATED HEAVIER AMOUNTS. GIVEN
THAT THESE TOTALS MAY FALL OVER ONLY A 1 TO 2 HOUR PERIOD...AND
WITH HIGHER TERRAIN/OROGRAPHICS INVOLVED...THERE WILL BE A THREAT
FOR SOME POCKETS OF FLASH FLOODING. WILL CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...ALY...BGM...BOX...BTV...BUF...CAR...CTP...GYX...
LWX...OKX...PHI...

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

LAT...LON   46346963 46226883 45676882 44886936 43977050
            43217152 42427289 41897357 40827441 40237505
            39807589 39717663 39797718 40117734 40667700
            41387679 41797753 42387723 43187631 43997481
            44487382 44797296 45017220 45257146 45587094
            46107029