MESOSCALE PRECIPITATION DISCUSSION 0136
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
317 PM EDT SAT MAY 12 2018

AREAS AFFECTED...SOUTHWEST PA...NORTHERN WV...WESTERN MD...EASTERN
OH

CONCERNING...HEAVY RAINFALL

VALID 121917Z - 122330Z

SUMMARY...DEVELOPING LINEAR CONVECTIVE SEGMENTS MAY TRAIN THROUGH
SUSCEPTIBLE GROUND CONDITIONS/TERRAIN TO POSE LOCALIZED FLASH
FLOODING RISK THROUGH EARLY EVENING BEFORE BECOMING OUTFLOW
DOMINANT.

DISCUSSION...GOES-16 1-MINUTE MESO VISIBLE IMAGERY AND REGIONAL
RADARS DEPICT TWO DISTINCT WEST TO EAST ORIENTED LINES OF TCU
BUILDING TO FULL CBS AT THIS TIME.  SPC MESO DISCUSSION 391
HIGHLIGHTS THE FRONTAL ZONE AND ENVIRONMENTAL CONDITIONS FOR THIS
DEVELOPMENT WELL WITH 1250+ J/KG CAPES.  FROM THE FLOODING
PERSPECTIVE THERE ARE A FEW ADDITIONAL FACTORS THAT MAY AID
LOCALIZED FLASH FLOODING ISSUES OVER THE NEXT 3-4 HOURS.  THE MAIN
REASON IS LOWERED FFG VALUES ACROSS COMPLEX TERRAIN MAINLY ACROSS
SOUTHWEST PA WHERE 1 HR FFG VALUES ARE AROUND 1"/HR AND 3HR VALUES
ARE ABOUT 1.5" (WITH LOWER VALUES FURTHER SOUTH OVER N WV/W MD) AS
WELL IN URBAN CENTERS.  THE 18Z PBZ SOUNDING HIGHLIGHTS THE OTHER
MAJOR FACTOR...WHERE UNIDIRECTIONAL FLOW REGIME IS IN PLACE THAT
IS GENERALLY PARALLEL TO THE ORIENTATION OF THE CONVECTION.  GIVEN
THE LENGTH OF THE CONVECTIVE LINES (LIKELY TO EXPAND DOWNSTREAM AS
WELL) THIS FLOW REGIME AND CONVECTIVE DEVELOPMENT/ ORIENTATION
POSES A GOOD LIKELIHOOD FOR CELLS TO TRAIN.

SURFACE TDS IN THE HIGH 50S/LOWER 60S AND MODEST INCREASING 9-7H
MST FLUX JUST AHEAD OF STRONG DPVA FROM THE MCV CROSSING NW OH
SHOULD SUPPORT SUFFICIENT FLUX THOUGH OVERALL MOISTURE SOUNDING
(REPRESENTED WELL BY PBZ) SHOWS SOME POCKETS OF DRIER AIR/DEW
POINT DEPRESSIONS IN THE MAIN UPDRAFT REGIONS.  IT IS THIS DRY AIR
THAT WILL BE THE ULTIMATE LIMITING FACTOR MAINLY IN REDUCING THE
TIME FRAME FOR THE TRAINING LINEAR CLUSTERS AS THEY WILL BECOME
INCREASINGLY LIKELY TO GENERATE COLD POOLS AND DISRUPT THE IDEAL
TRAINING ENVIRONMENT.

STILL FLUX AND PWS UP TO 1.25" WITH INCREASING CONVECTIVE COVERAGE
WITHIN THE TRAINING LINES SUGGEST OF LOCALIZED RAINFALL TOTALS OF
1.25-1.75" EXCEEDING FFG VALUES THROUGH 00Z.  THIS IS SUPPORTED BY
THE 12Z NMMB/NSSLWRF/NAMCONEST/17Z HRRR/.

GALLINA

ATTN...WFO...CTP...LWX...PBZ...RLX...

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

LAT...LON   40737958 40707824 40717725 40657680 40597661
            40377661 40137677 39927703 39687736 39207778
            39197851 39177928 39227998 39408099 39678145
            40208119 40668017