MESOSCALE PRECIPITATION DISCUSSION 0254
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 PM EDT SUN MAY 21 2017

AREAS AFFECTED...SOUTHERN LA & VICINITY

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

VALID 211838Z - 212258Z

SUMMARY...THUNDERSTORMS ARE BEGINNING TO EXPAND IN COVERAGE ACROSS
SOUTHERN AND CENTRAL LA.  HOURLY RAIN RATES UP TO 2.5", WITH LOCAL
AMOUNTS UP TO 5", ARE POSSIBLE OVER PARTIALLY SATURATED SOILS.

DISCUSSION...AN OUTFLOW BOUNDARY HAS ACTED AS A CATALYST FOR
CONVECTIVE GROWTH ACROSS CENTRAL AND SOUTHERN LA AS OF LATE.  THIS
IS OCCURRING SOUTHEAST OF A SHORTWAVE SEEN IN THE CONVECTIVE
PATTERN ACROSS NORTHEAST LA.  A SEA BREEZE FRONT ACROSS SOUTHWEST
LA IS SLOWLY PUSHING TOWARDS THE AREA FROM THE WEST.  PRECIPITABLE
WATER VALUES ARE NEAR 2", MLCAPES SHOW A NICE GRADIENT ACROSS THE
AREA OF 1500-3000 J/KG, AND EFFECTIVE BULK SHEAR IS ~40 KTS. 
INFLOW AT 850 HPA IS ALONG THE BOUNDARY, OUT OF THE EAST-SOUTHEAST
AT 20-30 KTS PER VAD WIND PROFILES IN THE REGION.

THE CONVECTIVE EVOLUTION IN THIS REGION IS BEING HINTED AT BY
ACTIVITY NEAR LAKE CHARLES AT THIS TIME.  THE LOW-LEVEL FLOW AND
MEAN 850-400 HPA ARE COUNTERBALANCING EACH OTHER TO SOME DEGREE,
WHICH COULD LEAD TO SOME WEST-NORTHWEST TO EAST-SOUTHEAST TRAINING
ACROSS SOUTH-CENTRAL AND SOUTHEAST LA OVER THE NEXT FEW HOURS,
WHICH WOULD EFFECTIVELY DOUBLE TO INFLOW IN A STORM-RELATIVE
SENSE.  CELL TRAINING UNDER THE BASE OF THE CONVECTIVE SHORTWAVE
IS ALSO POSSIBLE ACROSS PORTIONS OF CENTRAL LA AND SOUTHWEST MS. 
THE AMOUNT OF BULK SHEAR AVAILABLE COULD ALLOW FOR MESOCYCLONES TO
FORM IN THIS ENVIRONMENT.  THE AVAILABLE MOISTURE AND INSTABILITY
FAVOR HOURLY RAIN RATES UP TO 2.5".  THE MESOSCALE GUIDANCE HAS A
GOOD SIGNAL FOR LOCAL AMOUNTS OF 5" OVER THE NEXT SEVERAL HOURS. 
SINCE LOCALLY HEAVY RAINS FELL HERE YESTERDAY, SOILS ARE PARTIALLY
SATURATED.  FLASH FLOOD ISSUES ARE MORE LIKELY WITHIN URBAN AREAS.

ROTH

ATTN...WFO...JAN...LCH...LIX...SHV...

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

LAT...LON   32349091 31339039 30939018 29178900 29098997
            29829133 30299283 30509298 31219322 31879216