MESOSCALE PRECIPITATION DISCUSSION 0354
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 PM EDT WED JUN 21 2017

AREAS AFFECTED...SOUTHEAST LA...SOUTHERN MS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 211900Z - 220030Z

SUMMARY...OUTER BAND OF TS CINDY TAKING SHAPE OVER LIKELY TO TRACK
THROUGH SATURATED AREAS ALONG THE CENTRAL GULF COAST WITH TRAINING
HIGHLY EFFICIENT RAIN PRODUCING CELLS LEADING TO LIKELY FLASH
FLOODING.. 

DISCUSSION...EXPERIMENTAL GOES-16 VIS/IR INDICATE CONVECTIVE
GROWTH BETWEEN SW TIP OF MS THROUGH THE MS RIVER DELTA. 
IMPRESSIVE 1500+ J/KG MLCAPES (GIVEN MOIST LAPSE RATES) ARE
AVAILABLE TO FEED THESE EXPANDING CONVECTIVE CELLS.  ELONGATED
SHORTWAVE ROTATING ALONG THE EASTERN PERIPHERY WITH ASSOCIATED
SPEED MAX THROUGH THE MID-LEVELS IS ENHANCING PRESSURE FALLS ALONG
THE CONVERGENCE BAND TO FOCUS INCREASED CONFLUENCE ALONG THE BAND
TO SUPPORT STRONG UPDRAFTS AND TAP THE AVAILABLE INSTABILITY
ALOFT.  PROFILES CONTINUE TO BE QUITE SATURATED THROUGH THE CLOUD
LAYER FOR SIGNIFICANT WARM PROCESS RAINFALL GENERATION...GIVEN
TPWS UP TO 2.6" WITHIN THE BAND AND STRONG MST CONVERGENCE
FLUX...RAIN RATES UP TO 3"/HR ARE POSSIBLE WITH THESE CELLS
PARTICULARLY IF THEY CONTINUE TO GROW TO MID-UPPER LEVELS (SUB
-70C ON EIR). 

TO COMPOUND MATTERS...THE STRENGTHENED CONFLUENT LOW LEVEL
FLOW/FLUX OF 40-50KT SFC TO 70KT 850MB WILL SUPPORT UPSTREAM
PROPAGATION VECTORS (15-20KT) BACK OPPOSITE TO THE 40-45KT MEAN
CLOUD BARING FLOW...SUPPORTING A TRAINING PROFILE.  THERE IS SOME
ANGULAR SPLIT OF THE PROPAGATION VECTOR (PER RAP/HRRR) THAT
SUGGEST SOME SLOW NE MOVEMENT OF THE BAND SHOULD OCCUR MOVING THE
FOCUS FROM SE LA TOWARD S MS WITH TIME...OBVIOUSLY THIS IS THE
LARGEST UNKNOWN/UNCERTAINTY AT THIS TIME.  HOWEVER...IF THE BAND
IS QUITE SLOW LIFTING SUCH AS THE NAM-CONEST/NMMB  (THOUGH ALREADY
TOO FAR NE FROM REALITY)...THOSE TYPES OF MAGNITUDES (4-7" BY 00Z)
ARE POSSIBLE.  MOST HI-RES GUIDANCE IS ALREADY TOO FAR NORTH AND
EAST OF THE CURRENT DEVELOPMENT BUT ONE CAN DERIVE SOME
INFORMATION/EXPECTATION FROM THE GUIDANCE JUST SHIFTING THE
MAGNITUDES SW BY A COUNTY OR SO. 

EVEN WITH MODEST RAINFALL OF 3-4" TOTALS SUGGESTED BY EVEN THE
MOST CONSERVATIVE RAINFALL HRRR AND HRRRX...THIS WOULD EXCEED FFG
VALUES IN THE AREA OF CONCERN MAINLY SOUTH AND EAST NEARER THE
COASTAL COUNTIES...AND THERE IS FAIRLY GOOD CONFIDENCE THAT FLASH
FLOODING IS LIKELY OVER THE NEXT 6HRS...WITH POSSIBLE MAGNITUDES
CAPABLE OF LIFE-THREATENING FF WHICH IS A BIT MORE UNCERTAIN (WITH
DEVELOPMENT TRENDS TOWARD IT).

GALLINA

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

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

LAT...LON   32319063 32088951 31428865 30348832 29948840
            29818870 29428891 29298889 28928898 28828933
            28958958 29098988 29069009 29479038 30529155
            31079151