MESOSCALE PRECIPITATION DISCUSSION 0751
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
618 PM EDT FRI OCT 21 2016
 
AREAS AFFECTED...EASTERN L.I. & SOUTHEAST NEW ENGLAND 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 212217Z - 220417Z
 
SUMMARY...SHOWERS AND DEVELOPING THUNDERSTORMS ARE BEGINNING TO
SHOW A TRAINING SIGNATURE FROM EASTERN LONG ISLAND INTO CENTRAL
MA.  HOURLY RAIN RATES UP TO 2", WITH LOCAL AMOUNTS UP TO 3",
COULD LEAD TO FLASH FLOODING IN URBAN AREAS AND WHERE DRAINAGE IS
POOR DUE TO LEAF FALL.

DISCUSSION...A SURFACE LOW IS APPROACHING 40N 71W PER SATELLITE
DATA AND SURFACE OBSERVATIONS, MOVING GENERALLY NORTHWARD.  IN
ADVANCE OF THE LOW, INSTABILITY HAS SLOWLY INCREASED, WITH
MESOANALYSES FROM SPC SHOWING MLCAPES APPROACHING 500 J/KG AND
MUCAPES SLIGHTLY ABOVE 500 J/KG IN EASTERN LONG ISLAND, SOUTHEAST
CT, AND RI.  PRECIPITABLE WATER VALUES ARE NEAR 2" ALONG THE COAST
AND ABOVE 1.5" IN SOUTHERN NEW ENGLAND PER RAP MASS FIELDS. 
LOW-LEVEL INFLOW (850 HPA LEVEL) IS CONVERGENT AND ON THE ORDER OF
15-30 KTS PER VAD WIND PROFILES.  BULK SHEAR IN THE 0-6 KM LAYER
IS 25-35 KTS ACROSS THE REGION, JUST SHY OF THE THRESHOLD FOR
MESOCYCLONES.  THE LIGHTNING DETECTION NETWORK SHOWS EMBEDDED
THUNDERSTORMS, WHILE VISIBLE SATELLITE IMAGERY JUST PRIOR TO DUSK
HINTED AT OVERSHOOTING TOPS WITHIN THE SHOWER/THUNDERSTORM AREA.

THERE IS A GOOD SIGNAL IN THE MESOSCALE/CAM GUIDANCE FOR LOCAL
AMOUNTS IN THE 1.5-3" RANGE.  HOURLY RAIN RATES HAVE RECENTLY BEEN
SEEN UP TO 1.5", WHICH COULD INCREASE SOMEWHAT FURTHER TO 2" IF
INSTABILITY BUILDS FURTHER.  WITH TIME, THE LOW SOUTH OF NEW
ENGLAND MOVES INLAND, AND CONVERGENCE WITHIN THE TROWAL BECOMES
BETTER DEFINED, WHICH COULD CONTINUE TO ACT AS A FOCUS FOR
CONVECTION AS MUCAPES ARE FORECAST TO REMAIN AT OR ABOVE 500 J/KG.
 HOWEVER, THE TROWAL IS EXPECTED TO MOVE REMAIN MOBILE ENOUGH,
MOVING TO THE NORTH AND NORTHEAST WITH TIME, TO PREVENT
SIGNIFICANT ISSUES.  WHILE FLASH FLOOD GUIDANCE IS MODEST, IN THE
2.5" IN THREE HOUR RANGE, RECENT LEAF FALL FROM FALL FOLIAGE COULD
CREATE POOR DRAINAGE IN SOME AREAS.  WITH THE SHOWERS AND
THUNDERSTORMS EXPECTED TO MOVE INTO THE BOSTON METRO AREA, URBAN
AREAS COULD ALSO EXPERIENCE ISSUES WITH THESE SORT OF RAIN RATES.

ROTH

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

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

LAT...LON   43877032 43736999 43117054 42787069 42667046 
            42397080 42117058 42147019 41896986 41266991 
            41187030 41247070 41087160 40937188 40877235 
            41357248 42357236 43467149