MESOSCALE PRECIPITATION DISCUSSION 0845
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1106 AM EDT SAT SEP 30 2017

AREAS AFFECTED...SUFFOLK COUNTY NY

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

VALID 301505Z - 302005Z

SUMMARY...SHOWERS AND THUNDERSTORMS SHOWING LITTLE MOVEMENT ARE
SLOWLY DEVELOPING ACROSS SUFFOLK COUNTY NY.  HOURLY RAIN RATES UP
TO 2" WITH LOCAL AMOUNTS TO 4" ARE POSSIBLE.

DISCUSSION...A SURFACE LOW SOUTH OF LONG ISLAND ALONG WITH AN
INVERTED TROUGH/TROWAL LIKE FEATURE NEAR THE WESTERN PORTION OF
SUFFOLK COUNTY ARE LEADING TO THE SLOW DEVELOPMENT OF SHOWERS AND
THUNDERSTORMS.  MU CAPE IN THE REGION IS ~500 J/KG.  EFFECTIVE
BULK SHEAR IS ~25 KTS PER SPC MESOANALYSES.  PRECIPITABLE WATER
VALUES OF 0.75-0.85".  THE ENVIRONMENT IS COLD ALOFT WITHIN THE
SOUTHEAST PORTION OF AN UPPER LEVEL LOW WITH WET BULB ZERO HEIGHTS
IN THE 6000-6500 FOOT RANGE.  HOWEVER, STORMS ACROSS SOUTHEAST NEW
ENGLAND AND IN LONG ISLAND SOUND HAVE BEEN EFFICIENT, SHOWING RAIN
TOTALS OF 2"+ AN HOUR BASED ON RADAR ESTIMATES.

THE CONCERN IS FOR INCREASING COVERAGE OF THUNDERSTORMS SHOWING
MINIMAL EAST TO SOUTHEAST MOVEMENT OVER URBAN AREAS.  THERE IS A
SIGNAL IN THE MESOSCALE GUIDANCE FOR LOCAL AMOUNTS IN THE 2" RANGE
THROUGH ~20Z, THOUGH THE GUIDANCE HAS SHOWN ESTIMATES THAT HAVE
UNDERPERFORMED SO FAR.  THE WORRY IS THAT A MESOCYCLONE COULD FORM
HERE SIMILAR TO WHAT OCCURRED IN THE EASTERN BOSTON SUBURBS
EARLIER THIS MORNING AND MOVE DOWN THE INVERTED TROUGH AS IT
SLOWLY PIVOTS EASTWARD ACROSS LONG ISLAND AS THE LOW PULLS AWAY
FROM THE REGION.  WITH BOUNDARY LAYER CONVERGENCE REMAINING FAIRLY
STRONG UNTIL ~20Z, BELIEVE LOCAL AMOUNTS UP TO 4" ARE POSSIBLE.

ROTH

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

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

LAT...LON   41167235 41167219 41077222 41077207 41117190
            41107180 41077183 40817244 40647295 40697330
            40917354 40997309 41047260