MESOSCALE PRECIPITATION DISCUSSION 0770
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
650 PM EDT WED AUG 30 2017

AREAS AFFECTED...NORTHERN LA...WESTERN MS...SOUTHEAST AR...EXT
EASTERN TX

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

VALID 302300Z - 310500Z

SUMMARY...TIGHTENING DEFORMATION ZONE ORIENTED PARALLEL TO
HARVEY'S TRACK AND STRONG ISENTROPIC ASCENT LEADING TO FLASH
FLOODING CONCERN THROUGH EARLY OVERNIGHT HOURS.

DISCUSSION...RADAR MOSAIC AND EXPERIMENTAL GOES-16 VISIBLE
MESOSCALE WINDOW LOOPS DEPICT A LOBE OF LOW LEVEL VORTICITY
BEGINNING TO PRESS NORTHEASTWARD ACROSS ALLEN INTO EVANGELINE
PARISH IN CENTRAL LA. 7.3UM WV SHOWS LOW LEVEL NORTHERN STREAM
TROF SHEARING FROM OK TO W IL AND STARTING TO SLIDE UNDER UPPER
LEVEL OUTFLOW CANOPY.  BOTH ACTIONS SHOULD LEAD TO A SW TO NE
ELONGATION OF THE DEFORMATION ZONE FROM E TX TO N MS THROUGH THE
LATE EVENING/EARLY NIGHT HOURS.

THIS COMBINATION OF ACTIONS WILL STEADILY REDUCE THE AMOUNT OF
ASCENT WRAPPING BACK ACROSS THE DEFORMATION ZONE IN CENTRAL LA TO
MAINTAIN BROAD SHIELD OF MODERATE RAIN ACROSS E TX AND SW LA FOR A
FEW MORE HOURS WITH AN ADDITIONAL 1-1.5" OF RAIN TO FALL OVER
ALREADY FLOODED REGIONS GENERALLY W OF THE SABINE RIVER AND S OF
POE.

DPVA FROM THE NE SHIFT OF THE LOW LEVEL VORT CENTER SHOULD ALSO
SUPPORT INCREASED CONVECTIVE ENHANCEMENT ALONG THE BAND ACROSS NE
LA AND CENTRAL MS GIVEN MODEST INSTABILITY FOR 1000-1500 J/KG
MLCAPES AND AND AMPLE MOISTURE UP TO 2" TPW.  THIS AIR WILL LIFT
NW INTERACTING/ASCENDING OVER THE DEFORMATION ZONE PRODUCING
1-2"/HR RATES (DEPENDANT ON THE CONVECTIVE VIGOR). 
ADDITIONALLY...UPPER LEVEL PATTERN WILL CONTINUE TO BE MODERATELY
DIFFLUENT DIRECTLY OVER THE INTERSECTION OF THE SELY FLOW AND
DEFORMATION ZONE TO SUPPORT ADDITIONAL ASCENT/UPPER-LEVEL
EVACUATION WITH FURTHER PERIPHERAL RIGHT ENTRANCE REGION JET
DYNAMICS ACROSS SE AR/NW MS.

THE DEFORMATION ZONE IS NOT EXPECTED TO SHIFT MUCH ACROSS THE
ARKLAMISS DELTA TO SUPPORT A DURATION CAPABLE OF 3-5"/HR RATES. 
WHILE THIS AREA HAS BEEN GENERALLY DRY THROUGH THE LAST FEW WEEKS
AND FFG VALUES ARE RANGE OF 3-4" IN THE 3-6HR TIME PERIOD...THERE
IS A GOOD LIKELIHOOD FOR FLASH FLOODING CONDITIONS TO OCCUR.  THIS
IS SUPPORTED BY NUMEROUS HI-RES CAMS PARTICULARLY RECENT HRRR/ESRL
HRRRV3 AND ASSOCIATED HRRR-TLE SOLUTIONS PROVIDING FAIRLY GOOD
CONFIDENCE THROUGH 05Z.

GALLINA

ATTN...WFO...JAN...LCH...LZK...MEG...SHV...

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

LAT...LON   34369066 33938990 33109043 32689074 32339104
            31929175 31569253 30809314 30759383 31579423
            32779327 33969156