MESOSCALE PRECIPITATION DISCUSSION 0139
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
553 PM EDT THU JUN 27 2013
 
AREAS AFFECTED......PORTIONS OF PA/NY/NJ... 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 272152Z - 280252Z
 
SUMMARY...FLASH FLOODING IS POSSIBLE ACROSS PORTIONS OF EASTERN
PENNSYLVANIA...SOUTHERN NEW YORK...AND NORTHWEST NEW JERSEY THIS
EVENING.

DISCUSSION...A MESOSCALE CONVECTIVE VORTEX (MCV) OVER NORTHWEST
PENNSYLVANIA CONTINUES TO DRIFT SLOWLY EASTWARD INTO A
CONDITIONALLY UNSTABLE AND PROGRESSIVELY MOIST ENVIRONMENT.  RADAR
AND SATELLITE IMAGERY...ALONG WITH VAD WIND PROFILER DATA AND RAP
ANALYSES...SUGGEST THAT THE MCV EXTENDS THROUGH THE MAJORITY OF
THE TROPOSPHERE AND THUS IS WELL ORGANIZED...WITH THE ATMOSPHERE
NEAR AND AHEAD OF ITS LOCATION RESPONDING QUICKLY THROUGH DEEP
ASCENT TO MAINTAIN BALANCE.  REGIONAL AND IN PARTICULAR THE STATE
COLLEGE WSR-88D SUGGEST THAT A MESOBETA-SCALE LOW EXISTS NEAR
STATE COLLEGE AND CONNECTS WITH MULTIPLE DEEP-LAYER TROUGHS WHICH
ARE ENHANCING AND MAGNIFYING ASCENT.  A LESS VIGOROUS MCV EXISTS
OVER EASTERN MARYLAND WHICH IS ACCELERATING NORTHWARD TOWARD THE
RISK AREA.  THE ATMOSPHERE WITHIN THE RISK AREA IS ALSO EXTREMELY
MOIST...WITH PRECIPITABLE WATER VALUES NEARING 1.75 INCHES...AND A
RELATIVELY MOIST BOUNDARY LAYER AND DEEP TROPOSPHERIC PROFILES
FAVORING BACK-BUILDING CELL DEVELOPMENT GIVEN THE RELATIVELY WEAK
WIND PROFILES AND SLOWLY CHANGING CONDITIONS.  

WHILE HI-RESOLUTION MODEL GUIDANCE DISAGREE ON THE EXACT PLACEMENT
OF ADDITIONAL HEAVY RAIN...CONSENSUS EXISTS THAT THE OVERALL
CONDITIONS ARE VERY FAVORABLE FOR HEAVY RAIN TO GRADUALLY EXPAND
ACROSS THE RISK AREA THROUGH THE EVENING AS THE MCV CONTINUES TO
DRIFT SLOWLY EASTWARD INTO THE PROGRESSIVE MOIST AND UNSTABLE
ENVIRONMENT.  RAINFALL RATES ARE EXPECTED TO AVERAGE 0.5 TO 0.75
INCHES PER HOUR WITHIN THE HEAVIER AND MORE PERSISTENT CORES. 
HOWEVER...AREAS IMMEDIATELY AHEAD OF THE MESOBETA-SCALE LOW AND
WITHIN THE DEEPER MOISTURE COULD RECEIVE RAINFALL RATES OF 1 TO 2
INCHES PER HOUR...POSSIBLY LASTING FOR 2 HOURS OR MORE.  FFG
VALUES ARE GENERALLY HIGH ACROSS EASTERN PENNSYLVANIA BEFORE
FALLING SLIGHTLY OVER PARTS OF NEW YORK AND NEW JERSEY. 
HOWEVER...THE ACCUMULATION OF LOCALLY HIGH RAINFALL RATES WITH
TRAINING CELLS AND POSSIBLY LOCAL ENHANCEMENT FROM UPSLOPE
FLOW...FAVORS A LOW PROBABILITY...25 TO 40 PERCENT...OF EXCEEDING
1 TO 3-HOURLY FLASH FLOOD GUIDANCE SOMEWHERE WITHIN THE RISK AREA.

JAMES

ATTN...WFO...OKX...ALY...PHI...BGM...CTP...

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

LAT...LON   41027717 41407770 41987763 42627630 42607492 41827419
            40637450 39847539 39797613 40007660 40477691 41027717