MESOSCALE PRECIPITATION DISCUSSION 0686
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
454 PM EST MON DEC 21 2015
 
AREAS AFFECTED...SOUTHEAST LA...FAR SOUTHERN MS...FAR SOUTHWEST AL 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 212200Z - 220400Z
 
SUMMARY...CONVECTION DEVELOPING ALONG THE LA COAST WILL TAP DEEP
MOISTURE TO PRODUCE HEAVY TO EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST LA...FAR SOUTHERN MS AND FAR SOUTHWEST AL THROUGH THE
LATE EVENING HOURS.

DISCUSSION...IR IMAGERY SHOWED AN EXPANDING AREA OF VERY COLD
CLOUD TOPS (COLDER THAN MINUS 75 CELSIUS)JUST SOUTH OF K9F2...OFF
THE LA COAST. THE CONVECTION HAS DEVELOPED ON A LOW LEVEL BOUNDARY
STRETCHING FROM THE SOUTHERN LA COAST ACROSS SOUTHERNMOST MS. THE
LOW LEVEL CONVERGENT FLOW IS TRANSPORTING 1.75 TO 2.00 INCH
PRECIPITABLE WATER AIR (WHICH IS BETWEEN TWO AND THREE STANDARD
DEVIATIONS ABOVE THE MEAN) ALONG THIS BOUNDARY. 

THE LATEST SPC MESOANALYSIS SHOWED THAT THE BEST MLCAPE RESIDES
CLOSE TO THE LOW LEVEL BOUNDARY...STRETCHING FROM SOUTHWEST LA TO
A POINT JUST SOUTH OF KMOB. THE CONVECTION NEAR AND SOUTH OF THE
BOUNDARY IS PRODUCING HOURLY RAINFALL RATES BETWEEN 1.50 AND 2.00
INCHES...BASED ON THE KLIX RADAR...AS WELL AS THE LATEST MRMS
PRECIPITATION INFORMATION.

A SHORT WAVE PASSING NORTH OF THE BOUNDARY AND CONVECTION SHOULD
KEEP THE CONVECTION ALONG THE COAST OF LA INTO SOUTHERN MS BEFORE
00Z. CONVECTION SHOULD THEN FOLLOW THE LOW LEVEL CONVERGENCE AND
BEST INSTABILITY ACROSS SOUTHEAST LA INTO FAR SOUTHERN MS AND FAR
SOUTHWEST AL AFTER 00Z. AT THIS POINT...IT IS NOT CLEAR IF THE
CONVECTION WILL AFFECT ENOUGH LAND AREA ACROSS SOUTHEAST LA AND
FAR SOUTHERN MS TO RESULT IN A FLASH FLOODING THREAT. 

HIGH RESOLUTION GUIDANCE IS SPLIT ON THIS...WITH THE LATEST HRRR
AND RAP SOLUTIONS BRINGING THE CONVECTION ACROSS SOUTHEAST LA AND
SOUTHERNMOST MS INTO SOUTHWEST AL. THESE SOLUTIONS PLACE LOCAL 2
TO 4 INCH RAINFALL AMOUNTS ACROSS THESE AREAS. IT SHOULD BE NOTED
THAT THE 19Z EXPERIMENTAL HRRR WAS MUCH MORE BULLISH WITH
QPF...STRETCHING A 4 TO 6 INCH RAINFALL SWATH ACROSS SOUTHEAST LA
INTO FAR SOUTHERN MS AND SOUTHWEST AL THROUGH 22/04Z.

THE WRF ARW AND NMMB KEEP THE BULK OF THE ACTIVITY
OFFSHORE...ALONG THE BEST INSTABILITY GRADIENT. BASED ON THE
TRENDS IN THE IR IMAGERY...IT IS CONCEIVABLE THAT THE BEST
RAINFALL RATES REMAIN OFFSHORE.  GIVEN THE PROXIMITY TO LAND...THE
FLASH FLOOD IS CONSIDERED POSSIBLE THROUGH THE EVENING HOURS.

HAYES

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

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

LAT...LON   31038765 30678729 30338753 30178781 30048801 
            29918813 29778829 29518855 29278880 29038925 
            28928960 28899008 28929064 28959092 28969095 
            29239152 29639168 30029141 30579034 30898953 
            31018821