MESOSCALE PRECIPITATION DISCUSSION 0213
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
929 AM EDT FRI MAY 12 2017

AREAS AFFECTED...SOUTHERN LA

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

VALID 121328Z - 121858Z

SUMMARY...THUNDERSTORMS ARE BEGINNING TO BROADEN AND SHOW TRAINING
CHARACTERISTICS NEAR I-10.  WITH HOURLY RAIN RATES UP TO 2.25",
AND LOCAL AMOUNTS UP TO 5" EXPECTED, FLASH FLOODING IS POSSIBLE.

DISCUSSION...AN OUTFLOW BOUNDARY MOVING IN FROM THE NORTH APPEARS
TO BE STALLING IN SOUTHWEST LA.  A DEW POINT DISCREPANCY NEAR I-10
APPEARS TO BE STARTING TO ACT AS A FOCUS FOR CONVECTION AND
BROADEN THE PRECIPITATION FOOTPRINT ACROSS SOUTHERN LA. 
DIVERGENCE ALOFT IS PLENTIFUL BETWEEN THE SUBTROPICAL AND SOUTHERN
STREAM JET STREAMS. NO CIN EXISTS IN THIS REGION; MLCAPES OF
1000-2000 J/KG ARE BEING ADVECTED IN BY WEST-SOUTHWEST INFLOW OF
20-25 KTS (PER VAD WIND PROFILE INFORMATION).  THE WIND FLOW IS
NEARLY UNIDIRECTIONAL WITH HEIGHT WHICH IS LIKELY LEADING TO THE
TRAINING PATTERN.  PRECIPITABLE WATER VALUES ARE 1.5-1.8" PER GPS
DATA AND NEARBY RADIOSONDE INFORMATION.  BULK SHEAR IN THE 0-6 KM
LAYER IS 35-50 KTS.

THE AMOUNT OF BULK SHEAR SUPPORTS MULTICELLS AND POSSIBLE
MESOCYCLONES, WHICH COULD ENHANCE THE HEAVY RAIN RISK.  IN THE
SHORTER TERM, CELL MERGERS BETWEEN THE LESS ORGANIZED CONVECTION
ALONG THE DEW POINT GRADIENT AND CONVECTION MOVING DOWN FROM THE
CONVECTIVE ARC COULD LEAD TO MERGERS.  HOURLY RAIN RATES UP TO
2.25" ARE ANTICIPATED.  THE MESOSCALE GUIDANCE HAS A FAIRLY
COHESIVE SIGNAL HERE, INDICATING LOCAL AMOUNTS IN THE 2-4" RANGE. 
BELIEVE 5" IS A BETTER UPPER BOUND FOR LOCAL AMOUNTS AS CELL
MOTION HAS BEEN EAST ~25 KTS LATELY AND THE BAND IS CLOSE TO 75 NM
LONG.  FORWARD PROGRESSION COULD SPEED UP TO 35 KTS IF CONVECTION
ORGANIZES PER PROPAGATION VECTORS, SO THERE IS UNCERTAINTY
REGARDING THE UPPER BOUND OF RAINFALL HERE.  THIS AMOUNT OF RAIN
COULD LEAD TO FLASH FLOODING IN AN AREA THAT HAS RECEIVED HEAVY
RAINFALL OVER THE PAST COUPLE WEEKS, PARTICULARLY IN URBAN AREAS
NEAR I-10 AND I-12.

ROTH

ATTN...WFO...LCH...LIX...MOB...

ATTN...RFC...LMRFC...

LAT...LON   30868924 30478861 29678906 29649202 30199294
            30799100