MESOSCALE PRECIPITATION DISCUSSION 0248
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
415 PM EDT SAT MAY 20 2017

AREAS AFFECTED...PORTIONS OF LA, MS, & AL

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

VALID 202014Z - 210214Z

SUMMARY...THUNDERSTORMS WITH TRAINING SEGMENTS WITHIN A CONVERGENT
LOW-LEVEL FLOW REGIME ARE CAPABLE OF HOURLY RAIN RATES TO 2.5",
AND LOCAL AMOUNTS UP TO 3-5".

DISCUSSION...THUNDERSTORMS CONTINUE TO DEVELOP BETWEEN THE
ATCHAFALAYA BASIN IN LA AND WESTERN AL WITHIN A REGION OF
CONVERGENT 850 HPA INFLOW OF 15-20 KTS.  TWO BOUNDARIES APPEAR TO
BE RESPONSIBLE TO THE ORIENTATION OF THE ACTIVITY: ONE MOVING 20
KTS ACROSS CENTRAL MS AT THIS TIME AND A LEADING OUTFLOW BOUNDARY
MOVING EAST AT 10 KTS INTO WESTERN AL.  PRECIPITABLE WATER VALUES
ARE 1.75-2" HERE, AND MLCAPES ARE 1000-3000 J/KG.  BULK SHEAR IN
THE 0-6 KM LAYER IS 25-45 KTS, WHICH HAS SO FAR LED TO OCCASIONAL
TRAINING BANDS AS THE WIND FLOW IS FAIRLY UNIDIRECTIONAL FROM THE
SOUTH-SOUTHWEST.  RECENT STRUCTURES IN SOUTHWEST MS ON RADAR IMPLY
MESOCYCLONES MAY BE PRESENT.

THE EXPECTATION IS FOR THE TWO BOUNDARIES TO MERGE OVER THE NEXT
SEVERAL HOURS AS THEY CONTINUE MOVING INTO SOUTHERN MS AND CENTRAL
AL, WITH HEAVY RAIN AREAS SHIFTING MORE SOUTHERLY WITH TIME AS
INSTABILITY ERODES MORE INLAND FROM ONGOING CONVECTION.  HOURLY
RAIN RATES UP TO 2.5" ARE POSSIBLE WHERE CELLS MERGE OR TRAIN,
WHICH HAS BEEN OCCASIONALLY NOTED IN RADAR IMAGERY AS OF LATE. 
THE MESOSCALE GUIDANCE SHOWS A CONSISTENT SIGNAL FOR LOCAL AMOUNTS
UP TO 3-5".  FLASH FLOOD GUIDANCE VALUES ARE HIGH HERE, SO BELIEVE
HEAVY RAIN ISSUES WOULD BE MORE CONCENTRATED IN URBAN AREAS.

ROTH

ATTN...WFO...BMX...HUN...JAN...LCH...LIX...MEG...MOB...OHX...

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

LAT...LON   35698736 35328648 33688606 31658618 30708724
            30489003 30399124 30439279 31679102 33139000
            34528893