MESOSCALE PRECIPITATION DISCUSSION 0508
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
452 AM EDT SUN JUL 22 2018

AREAS AFFECTED...EASTERN LONG ISLAND & SOUTHERN NEW ENGLAND

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

VALID 220851Z - 221451Z

SUMMARY...AN ENHANCED ATMOSPHERIC RIVER IS EXPECTED TO IMPACT
EASTERN LONG ISLAND AND MUCH OF SOUTHERN NEW ENGLAND FROM ~10Z
ONWARD.  HOURLY RAIN TOTALS OF 1"+ AND LOCAL AMOUNTS OF 2"+ ARE
POSSIBLE THROUGH 15Z.

DISCUSSION...AN OCCLUDED CYCLONE IS MOVING NORTHWARD THROUGH
SOUTHEAST PA AS OF 08Z.  TO ITS EAST, A WARM FRONT IS APPROACHING
LONG ISLAND AND SOUTHERN NEW ENGLAND.  THE SYSTEM'S WARM CONVEYOR
BELT IS STREAMING IN FROM THE WESTERN ATLANTIC, WHICH HAS A
RESERVOIR OF 500-1500 J/KG OF MU CAPE TO THE SOUTH OF THE WARM
FRONT; THE 250 J/KG MU CAPE CONTOUR IS OVER LONG ISLAND AND PAST
NANTUCKET MA AT THIS TIME.  A CONVECTIVE BAND IS SEEN ON SATELLITE
NEAR THE 72TH MERIDIAN AND IS MOVING GENERALLY NORTHWARD.
PRECIPITABLE WATER VALUES OF 1.7-2.2" RESIDE ACROSS THE DISCUSSION
REGION PER GPS DATA.  INFLOW AT 850 HPA IS SOUTH-SOUTHEAST AT
30-50 KTS, WITH EFFECTIVE BULK SHEAR TO MATCH NEAR THE JUST
OFFSHORE.

THE INGREDIENTS ARE THERE FOR 1"+ AN HOUR RAIN RATES WITHIN THE
NEXT COUPLE HOURS, EVEN IF NOT ADVERTISED BY THE 00Z HREF
PROBABILITIES.  OFFSHORE, SUCH AMOUNTS HAVE BEEN ESTIMATED BY THE
OKX RADAR DURING THE PAST HOUR.  RAP FORECAST FIELDS SHOW MU CAPE
VALUES EXCEEDING 1000 J/KG BY 11Z ACROSS LONG ISLAND/CT WITH THE
500 J/KG CONTOUR NEARING BOSTON AT THAT TIME.  AS THE ATMOSPHERE
IS SATURATED, 500 J/KG OF MU CAPE IS SUFFICIENT FOR 1"+ AN HOUR
RAINFALL.  FROM 09Z-15Z, BOUNDARY LAYER MOISTURE CONVERGENCE IS
FORECAST TO REMAIN QUASI-STATIONARY FROM EASTERN LONG ISLAND
ACROSS EASTERN CT INTO EAST-CENTRAL MA, WHICH RAISES THE PROSPECT
FOR CELL TRAINING.  IN AREAS OF CELL TRAINING, 1"+ AN HOUR RAIN
RATES SHOULD BE REALIZED.  WHILE THE MESOSCALE GUIDANCE KEEPS
AMOUNTS UNDER 2", LOCAL AMOUNTS OF 2"+ WOULD NOT BE SURPRISE. 
THESE AMOUNTS WOULD BE MOST PROBLEMATIC IN URBAN AREAS AND
LOCATIONS WITH POOR DRAINAGE.

ROTH

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

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

LAT...LON   42867084 42557060 42357086 42337077 42247060
            42017050 41877039 41827038 41827026 41867018
            41937016 41987027 42077030 42147021 42066998
            42016993 41706983 41196993 41187021 41287050
            41147112 41037162 40787234 40807266 41297259
            42167223 42587168