MESOSCALE PRECIPITATION DISCUSSION 0325
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
122 PM EDT THU JUN 15 2017

AREAS AFFECTED...WESTERN NY & WESTERN PA

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

VALID 151721Z - 152321Z

SUMMARY...THUNDERSTORMS CAUSED BY DAYTIME ARE STARTING TO BUILD
ACROSS THE REGION. HOURLY RAIN RATES TO 2", WITH LOCAL AMOUNTS OF
4", ARE POSSIBLE.

DISCUSSION...MESOSCALE ANALYSIS AND SATELLITE IMAGERY REVEAL THREE
BOUNDARIES -- ONE ACROSS SOUTHWEST NY WHICH WAS RECENTLY
REINFORCED BY A THUNDERSTORM CLUSTER, A SLOWLY EASTWARD-MOVING
WARM FRONT IN EASTERN PA NEAR THE ALLEGHENY MOUNTAINS, AND A LAKE
BREEZE FRONT IN FAR NORTHWEST PA AND FAR WESTERN NY.  PRECIPITABLE
WATER VALUES ARE 1.5-1.75" PER 14Z GPS DATA.  INFLOW AT 850 HPA IS
CONVERGENT AT 15 KTS PER VAD WIND PROFILES ACROSS WESTERN PA AND
WESTERN NY, NEAR THE MAGNITUDE OF THE MEAN 850-400 HPA WIND. 
MLCAPES ARE 500-1000 J/KG AND RISING.  UPSTREAM EFFECTIVE BULK
SHEAR IN EASTERN OH IS 25 KTS.  A SUBTLE SHORTWAVE ALOFT APPEARS
TO BE CROSSING WESTERN LAKE ERIE PER WATER VAPOR IMAGERY.

ANOTHER 500-1000 J/KG OF MLCAPE IS EXPECTED TO BUILD DUE TO
DAYTIME HEATING.  WITH INCREASINGLY CONGESTED CUMULI ALONG THE
ALLEGHENY MOUNTAINS OF PA, THUNDERSTORMS ARE EXPECTED TO SOON
STRETCH THROUGH THE LENGTH OF THE MOUNTAIN CHAIN AND BUILD ALONG
THE SURFACE BOUNDARY IN SOUTHWEST NY.  INSTABILITY FIELDS INDICATE
THAT THUNDERSTORMS SHOULD REACH THE APEX OF THEIR COVERAGE BY 00Z,
WHICH WAS ALSO REFLECTED IN THE 06Z SSEO NEIGHBORHOOD
PROBABILITIES OF EXCEEDING FLASH FLOOD GUIDANCE, WITH THE
STRONGEST SIGNAL IN WESTERN NY. 

AS STORMS ARE ALSO FORMING IN PA, IT IS POSSIBLE THAT THE
INSTABILITY POOL COULD ERODE MORE THAN FORECAST WHICH COULD
THEORETICALLY SHIFT MAXIMUM AMOUNTS SOUTH OF WHAT IS CURRENTLY
SEEN IN THE MESOSCALE GUIDANCE.  LOW-LEVEL INFLOW AND EFFECTIVE
BULK SHEAR ARE EXPECTED TO INCREASE WITH TIME, ORGANIZING
CONVECTION.  HEAVY RAIN CORES SHOULD MOVE EAST-SOUTHEAST AT 15-20
KTS IN SOUTHWEST NY/NORTHWEST PA NEAR THE TOP OF THE INSTABILITY
POOL, WHILE NORTHEAST MOTION AT 15-20 KTS IS EXPECTED IN WESTERN
PA.  CELL MERGERS ARE POSSIBLE, PARTICULARLY IN THE NORTHERN
PORTION OF THE AREA, ALONG WITH BRIEF PERIODS OF CELL TRAINING. 
THERE WAS A STRONG SIGNAL FOR LOCAL AMOUNTS UP TO 4", WITH THE WRF
NSSL SHOWING LOCAL AMOUNTS UP TO 8" THROUGH 23Z.  WITH HOURLY RAIN
RATES UP TO 2", LOCAL AMOUNTS TO 4" APPEAR MORE PROBABLE. 

ROTH

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

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

LAT...LON   43017758 42857657 41957607 41247752 39837804
            39827935 41197978 41397985 41587996 42147996
            42757881