MESOSCALE PRECIPITATION DISCUSSION 0338
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1254 PM EDT SUN JUN 18 2017

AREAS AFFECTED...NORTHEAST OH, NORTHWEST PA, & WESTERN NY

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

VALID 181654Z - 182254Z

SUMMARY...SHOWERS AND THUNDERSTORMS ARE BEGINNING TO FIRE ACROSS
THE REGION NEAR MESOSCALE BOUNDARIES.  HOURLY RAIN RATES UP TO 2",
WITH LOCAL AMOUNTS UP TO 5", ARE EXPECTED.

DISCUSSION...THE LATEST EXPERIMENTAL GOES-16 IMAGERY SHOWS
TOWERING CUMULI ACROSS NORTHEAST OH, NORTHWEST PA, AND OVER LAKE
ERIE WEST OF NY.  THIS IS OCCURRING NEAR A MESOSCALE FRONTAL
WAVE/OUTFLOW BOUNDARY LEFT OVER FROM OVERNIGHT AND EARLY MORNING
THUNDERSTORMS.  A DEEP-LAYERED CYCLONE ACROSS NORTHERN LAKE
MICHIGAN IS PROVIDING THE DIVERGENCE ALOFT.  MLCAPE VALUES ARE
1000-1500 J/KG, WELL AHEAD OF WHAT IS NORMALLY SEEN IN THIS REGION
AT THIS TIME OF DAY.  INFLOW AT 850 HPA IS SOUTHWEST AT 25-35 KTS
PER VAD WIND PROFILES.  GPS PRECIPITABLE WATER DATA INDICATE
VALUES HAVE RISEN TO 1.4-1.8" AS OF 15Z.  EFFECTIVE BULK SHEAR IS
25-35 KTS.

WITH PARTLY CLOUDY SKIES IN THE REGION, MLCAPE VALUES COULD RISE
ANOTHER 500-1000 J/KG WITH TIME.  PRECIPITABLE WATER VALUES SHOULD
CONTINUE TO INCREASE TO NEAR 2", WHICH IS APPROACHING 3 SIGMAS
ABOVE THE MEAN FOR MID JUNE.  THE AVAILABLE BULK SHEAR WORKING IN
CONCERT WITH FAIRLY UNIDIRECTIONAL FLOW UP TO 300 HPA (PER THE 12Z
BUFFALO NY SOUNDING) SHOULD ORGANIZE TRAINING BANDS OF CONVECTION,
WHICH MAY ATTEMPT TO FORWARD PROPAGATE EASTWARD INTO THE
INSTABILITY WITH TIME, MAINLY AFTER 21Z.  RAP MASS FIELDS SHOW A
COUPLE ZONES OF CONVERGENCE MOVING THROUGH THE REGION OVER THE
NEXT SIX HOURS, IMPLYING MORE THAN ONE ROUND OF CONVECTION.  THE
MESOSCALE GUIDANCE BULKED UP THEIR EXPECTATIONS IN THEIR 12Z RUNS,
AND PRESENT A NEARLY UNIFIED MESSAGE OF 3-5" OF RAIN LOCALLY
THROUGH 23Z (HRRR RUNS WERE ON THE LOW END OF THE GUIDANCE). 
HOURLY RAIN RATES UP TO 2", WITH LOCAL AMOUNTS UP TO 5" ARE
POSSIBLE OVER AN AREA WITH MODEST FLASH FLOOD GUIDANCE.  FLASH
FLOODING IS A CONCERN.

ROTH

ATTN...WFO...BGM...BUF...CLE...CTP...PBZ...

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

LAT...LON   43377792 43327750 43317672 42717683 42127791
            41627895 40398192 40468241 41478163 41828112
            42178070 42457960 42787915 43227878