MESOSCALE PRECIPITATION DISCUSSION 0394
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
210 PM EDT TUE JUL 03 2018

AREAS AFFECTED...I-95 CORRIDOR FROM DC TO BOSTON...ADJACENT AREAS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 031809Z - 040000Z

SUMMARY...ISOLATED TO SCATTERED THUNDERSTORMS ARE EXPECTED TO
DEVELOP IN THE I-95 CORRIDOR THIS AFTERNOON...AND PERHAPS INTO
ADJACENT INLAND PORTIONS OF THE NORTHEAST SUCH AS THE BERKSHIRES
AND POCONOS. ALTHOUGH COVERAGE OF STORMS MAY NOT BE
WIDESPREAD...STORM MOTIONS ARE VERY LIKELY TO BE SLOW AND WILL
CONTRIBUTE TO A THREAT OF LOCALIZED FLASH FLOODING ACROSS THE
REGION. GREATEST CHANCES OF FLASH FLOODING ARE EXPECTED TO EXIST
FROM NORTHERN NEW JERSEY INTO WESTERN CONNECTICUT...INCLUDING THE
LOWER HUDSON VALLEY AND NORTH SIDE OF THE NYC METRO AREA.

DISCUSSION...IN WHAT MAY BE A PREVIEW OF THINGS TO COME THIS
AFTERNOON...KOKX RADAR SHOWED AN ISOLATED THUNDERSTORM INITIATING
OVER NEWARK NJ AROUND 1630Z...AND THEN PROPAGATING ONLY AROUND 5
MILES TO THE SOUTHWEST OVER THE NEXT 60 MINUTES. DUAL POL RAIN
RATES WERE ESTIMATED TO BE APPROACHING 2 IN/HR. THIS ISOLATED
STORM WAS NOW SITUATED IN AN ENHANCED CUMULUS FIELD PER GOES-16
VISIBLE SATELLITE...STRETCHING ALONG THE I-95 CORRIDOR FROM DC TO
NYC METRO AREA...AND THEN EXTENDING UP INTO SOUTHERN NEW ENGLAND.
HI-RES MODELS CONTINUE TO DEVELOP ISOLATED TO SCATTERED CONVECTION
IN THIS SAME CORRIDOR ON THE NORTHERN PERIPHERY OF A STRONG
INSTABILITY AXIS THROUGH THE AFTERNOON HOURS...ROUGHLY IN THE BEST
OVERLAP OF INSTABILITY AND HIGH PW VALUES. CIRA BLENDED TPW VALUES
WERE AROUND 1.8 INCHES...ABOVE THE 95TH PERCENTILE PER CSFR
CLIMATOLOGY FOR THIS TIME OF YEAR. PWATS OF THIS MAGNITUDE...IN
COMBINATION WITH THE STRONG INSTABILITY...SHOULD BE MORE THAN
SUFFICIENT TO MAINTAIN A RISK OF 2 IN/HR RAIN RATES THROUGH THE
AFTERNOON.

OF MOST CONCERN ARE THE EXTREMELY SLOW STORM MOTIONS. 850-300MB
MEAN WINDS FALL BELOW 10 KNOTS THIS AFTERNOON AND CORFIDI VECTORS
(FORWARD AND BACKWARD PROPAGATING) BOTH FALL TO AROUND 0-5 KNOTS.
THIS IS ENTIRELY CONSISTENT WITH THE POSITION OF THE REGION NEAR
THE CENTER OF THE MID-UPPER LEVEL RIDGE AXIS. THEREFORE...THERE IS
POTENTIAL FOR STATIONARY OR NEARLY STATIONARY STORMS INTO THE
AFTERNOON HOURS...AND THE POTENTIAL FOR CONVECTION TO ANCHOR ON
OUTFLOW BOUNDARIES OR LOW-LEVEL GRADIENTS (SUCH AS A SEA BREEZE).
THIS WILL PROMOTE THE POTENTIAL FOR HEAVY RAIN RATES TO PERSIST
LONGER THAN AN HOUR.

OVERALL...CONVECTION SHOULD REMAIN ISOLATED TO SCATTERED SO THE
PROBABILITY OF FLASH FLOODING AT ANY ONE LOCATION SHOULD REMAIN
SOMEWHAT LOW. HOWEVER...THERE IS HIGH CONFIDENCE THAT SOME
CONVECTION SHOULD DEVELOP...AND GIVEN THE SLOW STORM MOTIONS THIS
ALSO MEANS THAT SOME LOCALIZED FLASH FLOODING APPEARS LIKELY. THE
POTENTIAL FOR STORMS TO ANCHOR IN PLACE FOR AWHILE IN A HIGHLY
URBANIZED REGION (AND ADJACENT AREAS OF TERRAIN SUSCEPTIBLE TO
FLASH FLOODING) SUGGESTS THAT A COUPLE INSTANCES OF MORE
SIGNIFICANT FLASH FLOODING CANNOT BE RULED OUT.
THEREFORE...DESPITE THE LOWER OVERALL PROBABILITY OF FLASH
FLOODING IN A GIVEN LOCATION...VIGILANCE IS RECOMMENDED GIVEN THE
POTENTIAL FOR MORE SIGNIFICANT IMPACTS IN CONCENTRATED AREAS. THE
GREATEST CHANCES OF 2+ IN/HR RAIN RATES ON THE 12Z HREF AND RECENT
RUNS OF THE HRRR TIME-LAGGED ENSEMBLE ARE FOCUSED FROM NRN NJ INTO
WRN CT...INCLUDING THE LOWER HUDSON VALLEY AND NRN PORTIONS OF THE
NYC METRO AREA. THIS WOULD TEND TO BE THE AREA OF GREATEST
CONCERN...ALTHOUGH ALL PORTIONS OF THE DISCUSSION AREA WILL HAVE
SOME CHANCE OF FLASH FLOODING.

LAMERS

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

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

LAT...LON   42677084 41937126 41367273 40617388 39637460
            39497546 38987612 38797709 39307745 40197621
            41687530 42577355