MESOSCALE PRECIPITATION DISCUSSION 0761
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
232 PM EDT TUE AUG 29 2017

AREAS AFFECTED...SOUTHEAST TX...EXT SOUTHWEST LA

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

VALID 291845Z - 300045Z

SUMMARY...CONVERGENCE/DEFORMATION ZONE WILL CONTINUE TO FOCUS
CONVECTION WITH NEGLIGIBLE MOTIONS/BACKBUILDING CONTINUING ONGOING
FLASH FLOOD THREAT.

DISCUSSION...GOES-E/16 VISIBLE IMAGERY LOOP DEPICTS SHORT-TERM
TIGHTENING OF THE FLOW GRADIENT ALONG THE GOLDEN TRIANGLE COASTAL
REGION OF TX AS A LOBE OF THE INNER CORE HAS LIFTED NORTH AS THE
OVERALL CENTER ELONGATES N-S. AS SUCH WSR-88D FROM LCH/HGX HAVE
DENOTED RECENT UPTICK IN RAIN RATES NEAR THE CHAMBERS COAST AND
ACROSS ORANGE AND S JASPER/NEWTON COUNTIES.  GIVEN THE AMOUNT OF
FLUX AND DEEP MOISTURE AVAILABLE IN THE COLUMN THE POTENTIAL FOR
3+"/HR RATES FOR A HOUR OR SO WILL CONTINUE FLASH FLOODING THREAT
ALONG IMMEDIATE COAST.

LARGER SCALE PATTERN PER GOES-16 SUITE OF 3 WV CHANNELS DEPICT A
LOBE OF SHORTWAVE ENERGY/SPEED MINIMA OVER SOUTHERN TX DESCENDING
SOUTH POTENTIALLY INCREASING STRETCHING OF THE LARGER SCALE HARVEY
VORTEX FURTHER SW WITH TIME POTENTIALLY SLOWING EASTWARD
PROGRESSION OF THE CENTER AS A WHOLE FOR THE NEXT 5-6HRS. 
LAND/SEA DIFFERENCES OF LATENT HEAT AVAILABILITY HAS LEAD TO AN
EFFECTIVE FRONTAL BOUNDARY ALONG THE COAST FROM GALVESTON BAY
ALONG THE UPPER TX/LA COAST...SOUTH OF WHICH AMPLE INSTABILITY IS
AVAILABLE TO ASCEND ISENTROPICALLY BACK WEST TOWARD THE
DEFORMATION/CONVERGENCE ZONE AROUND THE SABINE RIVER. THIS APPEARS
IN LINE WITH CURRENT 12Z ARW AND NAM-CONEST RUNS WHICH (ACCOUNTING
FOR NAM-CONEST TYPICAL OVER ESTIMATION) WOULD SUGGEST AN
ADDITIONAL 4-6" ACROSS THE GOLDEN TRIANGLE KEEPING FLASH FLOODING
LIKELY.  THERE IS SOME UNCERTAINTY IN WESTWARD EXTENT
(NSSL-WRF/HRRRV3)...WHICH AT THIS TIME THAT MAY PLACE THE HOUSTON
METRO WITHIN THE RAINFALL MAXIMA THROUGH 00Z WHILE NOT FAVORED
BASED ON OBSERVATIONS IT CANNOT BE RULED OUT.

FURTHER EAST (LA)...DISTANCE FROM THE SOURCE AS WELL AS SOME DRY
AIR ENTRAINMENT FROM THE DRY SLOT ACROSS THE CENTRAL LA COAST
SHIFTING NW THEN WEST IS LIKELY TO INHIBIT THE EXTREME RATES
NEARER THE COAST IN SE TX/FAR SW LA (REDUCED CONVECTIVE
VERTICALITY...DEPTH OF RAINFALL GENERATION) ACROSS CENTRAL LA.
ALSO VISIBLE IMAGERY SHOWS SHALLOW CU BANDS IN THE EASTERN
HEMISPHERE OF HARVEY'S LLC ARE NUMEROUS AND ILL-FOCUSED.  AS SUCH
ANY CELLS THAT DEVELOP NEAR THE CENTRAL LA COAST SHOULD BE
ISOLATED AND INTERMITTENT  STILL POSE SOME RISK BUT DURATION WILL
BE QUITE SHORT AND SHOULD ONLY AFFECT ALREADY ONGOING FLOODING
ISSUES...AND AS SUCH IS NOT INCORPORATED IN THE AREA OF CONCERN AT
THIS TIME.  

GALLINA

ATTN...WFO...HGX...LCH...

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

LAT...LON   31089379 30639317 30089276 29869244 29559247
            29569284 29629376 29209465 29119503 29299524
            30069537 30849478