MESOSCALE PRECIPITATION DISCUSSION 0241
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
330 PM EDT FRI AUG 01 2014
 
AREAS AFFECTED...NORTHERN MID-ATLANTIC & SOUTHWEST NEW ENGLAND
STATES 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 011929Z - 020109Z
 
SUMMARY...CAPE-DRIVEN CONVECTION WITHIN A MOIST ENVIRONMENT IS
EXPECTED TO CONTINUE SUPPORTING HOURLY RAIN RATES UP TO 2" AN
HOUR, WHICH WOULD EXCEED FLASH FLOOD GUIDANCE VALUES.  FLASH
FLOODING IS POSSIBLE IN THIS REGION THROUGH 01Z.

DISCUSSION...THUNDERSTORMS WITH HEAVY RAINFALL HAVE DEVELOPED OVER
THE NORTHERN MID-ATLANTIC STATES AND SOUTHWEST NEW ENGLAND, DRIVEN
BY DAYTIME HEATING IN AN AREA OF MINIMAL CLOUD COVER WHICH LED TO
CAPE VALUES OF 1000-2000 J/KG PER THE SPC MESOANALYSIS, WELL ABOVE
THOSE ADVERTISED BY RECENT RAP RUNS AND THE 12Z ARW GUIDANCE.
PRECIPITABLE WATER VALUES OF ~1.50" ARE WITHIN THE AREA, AS
MOISTURE INFILTRATES THE REGION FROM THE SOUTH.  LOW-LEVEL INFLOW
IS RELATIVELY WEAK, WITH 15-20 KTS OF 0-3 KM SHEAR NOTED IN
SURFACE OBSERVATIONS AND REGIONAL VAD WIND PROFILES.

THE EXPECTATION IS FOR THE ACTIVITY TO DRIFT NORTHWEST, FOLLOWING
THE HIGHEST PW PLUME SEEN IN THE MOST RECENT RAP GUIDANCE.  WHILE
THIS ENVIRONMENT DOES NOT MEET THE THRESHOLD FOR WET MULTI-CELLS,
THERE ARE POCKETS OF LOW FLASH FLOOD GUIDANCE ACROSS SOUTHEAST PA,
THE NYC METROPOLITAN AREA, SOUTHERN NY, AND SOUTHEAST NEW ENGLAND
WHICH WOULD BE THREATENED BY THE EXPECTED HOURLY RAIN RATES WITHIN
INDIVIDUAL THUNDERSTORMS.  THERE WAS A SIGNAL FOR 30% EXCEEDANCE
OF THE FLASH FLOOD GUIDANCE ACROSS THIS REGION EARLY ON, WITH THE
THREAT OVER BY 01Z, USING THE 12Z SSEO MEMBERS AS A GUIDE. 
CONSIDERING THE AVAILABLE PRECIPITABLE WATER VALUES, IT SHOULD
FADE SOON AFTER SUNSET, WHICH SUPPORTS THE 01Z TIME HORIZON. 
RAINFALL RATES UP TO 2" ARE POSSIBLE IN THIS REGION, WITH MERGING
CELLS POTENTIALLY YIELDING 2-4" LOCALLY WITHIN A COUPLE HOURS. 
FLASH FLOODING SHOULD CONTINUE TO BE POSSIBLE THROUGH 01Z.

ROTH

ATTN...WFO...GYX...BOX...BTV...OKX...ALY...PHI...BGM...CTP...
LWX...

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

LAT...LON   42797490 43387324 43597138 43117071 42477106 41737242
            41347292 40567365 39677467 39307579 40327828 41447731
            42137626 42797490