MESOSCALE PRECIPITATION DISCUSSION 0256...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
636 PM EDT SUN MAY 21 2017

CORRECTED FOR A COUPLE OF TYPOS

AREAS AFFECTED...SOUTHEAST LA

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

VALID 212230Z - 220130Z

SUMMARY...THUNDERSTORMS CONTINUE TO MASS ACROSS SOUTHEAST LA AHEAD
OF A MESOSCALE WAVE.  HOURLY RAIN RATES UP TO 2.5", WITH LOCAL
AMOUNTS TO 4", REMAIN POSSIBLE OVER THE NEXT FEW HOURS.

DISCUSSION...THE TAIL END OF A FORWARD PROPAGATING CONVECTIVE LINE
IS DROPPING SLOWLY INTO SOUTH-CENTRAL LA AT THIS TIME, AS A
MESOSCALE WAVE NEAR NEW IBERIA IS HOLDING UP FORWARD PROGRESSION. 
THIS REGION IS UNDER THE BASE OF A SHORTWAVE MOVING ACROSS
SOUTHEAST AR AT THIS TIME.  RADAR ESTIMATES CONTINUE TO INDICATE
2.5" AN HOUR RAIN RATES WHERE CELLS MERGE AND SHORT PERIODS OF
TRAINING OCCUR.  MOTION OF THE ACTIVITY HAS BEEN CHAOTIC, AS
THUNDERSTORMS HAVE BEEN MOVING INTO THE REGION FROM THE SOUTH NEAR
ST. MARY PARISH, BUT THE GROUPING HAS BEEN MOVING SOUTHEAST AT 15
KTS.  THIS IS DOUBLING THE STORM BASE INFLOW, AS 850 HPA WINDS ARE
FROM THE EAST-SOUTHEAST AT 15 KTS.  PRECIPITABLE WATER VALUES
REMAIN NEAR 2" AND THE MUCAPE GRADIENT IS ALIGNED
NORTHWEST-SOUTHEAST, HELPING TO EXPLAIN CELL MOTION.  LOW-LEVEL
INFLOW IS FROM THE SOUTHEAST, NOT FROM THE MAIN INSTABILITY POOL
NEAR MARSH ISLAND, IMPORTING MUCAPES OF 1000-3000 J/KG INTO THE
CLUSTER.  EFFECTIVE BULK SHEAR IS AROUND 40 KTS.

THE EXPECTATION IS FOR THE CONVECTIVE PROFILE ALONG THE DIRECTION
OF MOTION TO EVENTUALLY NARROW WITH TIME AS THE MESOSCALE WAVE
SHIFTS SOUTHEAST INTO THE GULF OF MEXICO, WHICH SHOULD CAUSE THE
HEAVY RAIN THREAT TO WANE OVER THE NEXT FEW HOURS.  AS THE
ARKANSAS SHORTWAVE MOVES FARTHER AFIELD, DIVERGENCE ALOFT SHOULD
SOMEWHAT LESSEN WITH TIME.  UNTIL THEN, HOURLY RAIN RATES UP TO
2.5" REMAIN POSSIBLE.  THE MESOSCALE GUIDANCE GENERALLY INDICATES
LOCAL AMOUNTS TO 4" DURING THE NEXT SEVERAL HOURS, WHICH SEEMS
REASONABLE.

ROTH

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

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

LAT...LON   31009039 30198961 29218963 28819069 29409177
            30129204 30749145