MESOSCALE PRECIPITATION DISCUSSION 0786
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1037 PM EDT WED SEP 06 2017

AREAS AFFECTED...EASTERN LONG ISLAND/SOUTHEAST NEW ENGLAND

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

VALID 070236Z - 070836Z

SUMMARY...AN INCOMING FRONTAL WAVE SHOULD LEAD TO HOURLY RAIN
TOTALS UP TO 1.5" WITH LOCAL AMOUNTS TO 3".

DISCUSSION...A FALL-LIKE SCENARIO IS PLAYING OUT WITH AN UPPER
TROUGH ACROSS THE GREAT LAKES AND A BROADLY DIFFLUENT/ DIVERGENT
AREA ALOFT DOWNSTREAM FROM OFFSHORE THE MID-ATLANTIC COAST INTO
SOUTHERN NEW ENGLAND.  A SURFACE WAVE APPEARS TO LIE
EAST-SOUTHEAST OF CAPE MAY BASED ON 02Z OBSERVATIONS. 
PRECIPITABLE WATER VALUES OF 1.5-1.8" EXIST ACROSS THE REGION PER
GPS DATA.  MU CAPE VALUES ARE CURRENTLY IN THE 100-700 J/KG RANGE
WITH THE GRADIENT SLOWLY SHARPENING OVER THE PAST FOUR HOURS.  THE
FLOW WITH HEIGHT IS NEARLY UNIDIRECTIONAL FROM THE SURFACE UP TO
100 HPA.  THE 850 HPA FLOW IS 30 KTS FROM THE SOUTH-SOUTHWEST,
SIMILAR TO THE VALUE OF THE EFFECTIVE BULK SHEAR.

THE OKX/UPTON NY AND CHH/CHATHAM MA SOUNDINGS ARE SATURATED WITH
WET BULB ZERO HEIGHTS IN THE 12,000-13,000 FOOT RANGE, WHICH
SHOULD ALLOW PRECIPITATION TO BE PARTICULARLY EFFICIENT -- 500+
J/KG MU CAPE SHOULD BE ENOUGH FOR THUNDERSTORMS WITH HEAVY
RAINFALL.  RAP MASS FIELDS SHOW BOUNDARY LAYER MOISTURE
CONVERGENCE INCREASING WITH TIME.  MU CAPE TRENDS DOWN SOUTH NEAR
AND AHEAD OF A FRONTAL WAVE ARE UPWARD, AND THE THINKING IS THAT
THE VALUES ACROSS THIS REGION SHOULD INCREASE WITH TIME.  CELL
TRAINING IS THE MAIN CONCERN PER THE BULK SHEAR AND UNIDIRECTIONAL
FLOW PRESENT, THOUGH OCCASIONAL MERGERS COULD AUGMENT RATES.  THE
GUIDANCE THAT APPEARS BEST WITH THE EXPECTED RAINFALL PATTERN ARE
00Z HRRR, 18Z CANADIAN REGIONAL, AND 21Z HRRR EXPERIMENTAL WHICH
SHOW LOCAL AMOUNTS IN THE 2-3" RANGE.  THE 18Z SSEO NEIGHBORHOOD
PROBABILITIES SUGGEST THAT THE HEAVY RAIN THREAT WOULD LIE IN THE
04Z-09Z TIME FRAME.  HOURLY RAIN TOTALS TO 1.5" ARE ANTICIPATED. 
WITHIN URBAN AREAS, THIS COULD BE ENOUGH FOR HEAVY RAIN-RELATED
ISSUES.

ROTH

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

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

LAT...LON   42857088 42617052 42427080 42297062 42067050
            42207022 42227008 41926978 41846984 41186996
            41067131 40577322 41807247