MESOSCALE PRECIPITATION DISCUSSION 0156
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
218 PM EDT SAT APR 30 2016
 
AREAS AFFECTED...SOUTHEAST LA...SOUTHERN MS...SOUTHWEST AL 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 301815Z - 010000Z
 
SUMMARY...EXTREMELY HEAVY RAINFALL RATES WILL MOVE ACROSS
SOUTHEAST LA AND INTO SOUTHERN MS/AL THROUGH THE MID TO LATE
AFTERNOON HOURS.

DISCUSSION...VERY DIFFLUENT FLOW ALOFT ASSOCIATED WITH A SPLIT
UPPER JET STRUCTURE AND A SHORTWAVE LIFTING ACROSS THE LOWER MS
VALLEY CONTINUES TO FOSTER VERY INTENSE THUNDERSTORM ACTIVITY OVER
SOUTHEAST LA AND INTO PORTIONS OF SOUTHWEST MS. THE GOES-OTD
ALGORITHM SHOWS NUMEROUS OVERSHOOTING TOPS OVER ST MARTIN...IBERIA
AND ST MARY PARISHES ALONG WITH EXTREME LIGHTNING DENSITY STRIKE
ACTIVITY.

THIS IS INDICATIVE OF VERY STRONG FORCING/UPDRAFT ACTIVITY...AND
AS A RESULT...VERY INTENSE RAINFALL RATES AS HIGH AS 3 INCHES/HR
HAVE BEEN OBSERVED  WHICH CONTINUES TO GRADUALLY ADVANCE EAST
TOWARD SOUTHEAST LA.

THE INSTABILITY PARAMETERS ARE QUITE CONDUCIVE FOR THE CONVECTION
TO MAINTAIN ITS STRENGTH AS IT APPROACHES THE GREATER NEW ORLEANS
AREA OVER THE NEXT COUPLE OF HOURS. RAP ANALYSES SHOW MLCAPE
VALUES OF 1500 TO 2000 J/KG ALONG WITH A NARROW AXIS OF 1.5 TO
1.75 INCH PWATS. THE LATEST GOES-14 SRSO VIS IMAGERY DOES INDICATE
A RELATIVELY CONFLUENT LOW LEVEL CU FIELD OUT AHEAD OF THE CURRENT
ACTIVITY WHICH IS FEEDING NORTH OUT OF THE GULF OF MEXICO...AND
THIS YIELD SOME ADDITIONAL CONFIDENCE THAT THE CURRENT ACTIVITY
WILL MAINTAIN ITSELF THIS AFTERNOON.

THE LATEST WRF-ARW...WRF-NMMB AND TO A LESSER EXTENT THE NSSL-WRF
ARE DOING A VERY POOR JOB IN HANDLING THE CURRENT CONVECTION OVER
SOUTHERN LA...AS THOSE MODELS ARE PLACING THE HEAVIER TOTALS THIS
AFTERNOON MUCH TO FAR TO THE NORTH. THE HRRR OUTPUT HAS BEEN
BETTER BUT IS ALSO PERHAPS A BIT TOO FAR NORTH. BASED ON THE
RAINFALL RATES AND RELATIVELY SLOW FORWARD PROPAGATION...RAINFALL
AMOUNTS OF UP TO 3 TO 5 INCHES CAN BE EXPECTED...WITH POSSIBLY
SOME HEAVIER AMOUNTS WHERE OVERSHOOTING TOPS PERSIST. 

THE ACTIVITY WILL ADVANCE THROUGH SOUTHEAST LA AND SHOULD IMPACT
AREAS OF SOUTHERN MS AND POSSIBLY SOUTHWEST AL LATER THIS
AFTERNOON. THIS INCLUDES THE GULFPORT AND BILOXI MS AREAS WHERE
VERY HEAVY RAINS OCCURRED TWO MORNINGS AGO. BETWEEN EXTREMELY
HEAVY RAINFALL RATES...AND THE WET ANTECEDENT CONDITIONS...FLASH
FLOODING WILL CERTAINLY BE POSSIBLE AND ESPECIALLY OVER THE MORE
URBANIZED AREAS.

ORRISON

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

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

LAT...LON   31688928 31528834 31138770 30598760 30068788 
            29718864 29208943 28999002 28939077 29069139 
            29179194 29519218 29789212 30389150 31409062