MESOSCALE PRECIPITATION DISCUSSION 0521
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1132 AM EDT SUN SEP 27 2015
 
AREAS AFFECTED...PORTIONS OF THE CENTRAL GULF COAST 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 271532Z - 272132Z
 
SUMMARY...THUNDERSTORMS WITH HEAVY RAIN ARE MOVING INTO AND
DEVELOPING ACROSS THE REGION. WITH RAIN RATES UP TO 2" POSSIBLE,
FLASH FLOODING IS CONSIDERED POSSIBLE.

DISCUSSION...A DEEP-LAYERED CYCLONE IS CENTERED ROUGHLY 100 MILES
OFFSHORE CAMERON PARISH LA.  AROUND ITS EAST SIDE, THUNDERSTORMS
HAVE BEEN TRAINING/MERGING NEAR THE COAST OF PLAQUEMINES PARISH LA
AS THEY MOVE NORTHEAST/FORWARD PROPAGATE AT 15 KTS.  IN ADDITION,
SHORT PERIODS OF TRAINING HAVE OCCURRED IN THE EASTERLY FLOW TO
THE EAST OF NEW ORLEANS, WITH NEW ACTIVITY FORMING JUST OFFSHORE
THE AL BARRIER ISLANDS.  PRECIPITABLE WATER VALUES ARE NEAR 2" PER
GPS VALUES, 1.5-2 SIGMAS ABOVE THE NORM FOR LATE SEPTEMBER. 
INFLOW AT 850 HPA IS ~20 KTS (AT A RIGHT ANGLE AND SIMILAR IN
MAGNITUDE TO THE MEAN 850-400 HPA WIND) IS FUNNELING INTO THE
REGION FROM THE NORTHERNMOST GULF OF MEXICO, KEEPING MUCAPES IN
THE REGION IN THE 500-1500 J/KG RANGE.

THE EXPECTATION IS FOR MLCAPES TO INCREASE ACROSS SOUTHERNMOST
AL/MS OVER THE NEXT SEVERAL HOURS, WHICH WOULD FUEL NEW ACTIVITY
NEAR/JUST INLAND OF THE GULF COAST.  MEANWHILE, ACTIVITY ACROSS
SOUTHEAST LA SHOULD SPREAD INTO SOUTHERNMOST AL/MS WITH TIME WHILE
LOW-LEVEL INFLOW SLOWLY INCREASES.  THE 06Z GFS-BASED
GALVEZ-DAVISON INDEX (A MEASURE OF CONVECTIVE COVERAGE) APPEARS TO
BACK THESE IDEAS UP, SHOWING VALUES NEAR AND ABOVE 40 SPREADING
ACROSS SOUTHEAST LA INTO SOUTHERN MS AND SOUTHERN AL BY 21Z. 
WHILE FLASH FLOOD GUIDANCE VALUES ARE HIGH (~4"/3 HOURS), HOURLY
RAIN RATES UP TO 2" ARE POSSIBLE WHERE CELLS MERGE/TRAIN AND A FEW
URBAN AREAS LIE WITHIN THIS REGION (NEW ORLEANS, MOBILE, AND
PENSACOLA), WHICH COULD EXHIBIT PROBLEMS WITH RAIN RATES AS LOW AS
1-1.5" AN HOUR.  THE CAM GUIDANCE GENERALLY ADVERTISES LOCAL 3-4"
AMOUNTS BETWEEN PENSACOLA FL AND SOUTH-CENTRAL LA OVER THE NEXT
SEVERAL HOURS, WHICH COULD FALL QUICKLY.  FLASH FLOODING IS
CONSIDERED POSSIBLE, PARTICULARLY IN URBAN AREAS.

ROTH

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

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

LAT...LON   31088887 31068810 30778704 30728692 30498697 
            30248716 30138801 30188882 30138880 29978873 
            29768879 29408912 29368909 29208900 29058902 
            28938922 28948950 29188970 29208976 29148992 
            29009020 29079030 29329057 29529064 30169034 
            30828952