Graphic for MPD #0768

MESOSCALE PRECIPITATION DISCUSSION 0768
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
212 PM EDT WED AUG 30 2017

AREAS AFFECTED...FAR EAST TX...WEST AND NORTH LA...WEST-CENTRAL MS

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

VALID 301810Z - 302310Z

SUMMARY...FLASH FLOODING IS LIKELY TO CONTINUE WITH PERSISTENT
HEAVY RAIN BANDS FROM FAR EAST TEXAS INTO WESTERN LOUISIANA...WITH
ADDITIONAL HEAVY RAIN BANDS POSSIBLY DEVELOPING FROM NORTHEAST
LOUISIANA INTO WEST-CENTRAL MISSISSIPPI.

DISCUSSION...THE DISTRIBUTION OF THE RAIN BANDS IN THE IMMEDIATE
VICINITY OF T.S. HARVEY HAS NOT CHANGED APPRECIABLY IN THE PAST
6-12 HOURS...WITH AN ARCING HEAVY RAIN BAND PERSISTING TO THE WEST
AND NORTHWEST OF THE LOW-LEVEL CIRCULATION CENTER. THIS IS
ASSOCIATED WITH A DEFORMATION ZONE AND ENHANCED CONVERGENCE IN THE
LOW LEVELS...AND HAS A HISTORY OF PRODUCING SIGNIFICANT RAINFALL
IN SHORT PERIODS OF TIME. DUAL POL RAIN RATES OBSERVED BY THE NWS
RADAR NETWORK HAVE GENERALLY BEEN PEAKING IN THE 1.5-2.0 IN/HR
RANGE OVER THE PAST COUPLE HOURS...AND THAT IS CONSISTENT WITH
MRMS RAIN RATES AS WELL.

NON-OPERATIONAL GOES-16 IR CHANNELS REVEAL A LACK OF DEEP
CONVECTION IN THIS REGION AS OF 18Z...AND THAT MAY BE KEEPING THE
RAIN RATES LOWER THAN THE PROLIFIC NUMBERS OBSERVED LAST NIGHT
WHEN DEEP CONVECTION WAS PRESENT. AND THE GOES-16 MICROPHYSICS AND
CONVECTION RGBS GENERALLY CONFIRM WEAKER CONVECTION AS WELL.

THEREFORE RAIN RATES WILL LIKELY BE DEPENDENT UPON WHETHER DEEPER
CONVECTION CAN BE RE-ESTABLISHED CLOSER TO THE CIRCULATION CENTER.
HOWEVER...EVEN PERSISTENCE OF ONGOING RATES OF 1-2 IN/HR WOULD BE
LIKELY TO CONTINUE FLASH FLOODING GIVEN THE VERY SLOW MOVEMENT OF
THE RAIN BANDS AND ALREADY SATURATED SOILS. HOWEVER...AN INCREASE
IN THE RAIN RATES WOULD HAVE THE POTENTIAL TO INCREASE THE
SEVERITY OF THE ONGOING FLOODING...OR LEAD TO NEW AREAS OF
FLOODING AS EVENING APPROACHES. THE HRRR IS NOT KEEN ON THIS
HAPPENING...WITH SIMULATED IR BRIGHTNESS TEMPERATURES REMAINING
RELATIVELY CONSTANT THROUGH 00Z. NEVERTHELESS...HIGH-RES MODEL
GUIDANCE IS IN GOOD AGREEMENT WITH A HEAVY RAIN SIGNAL OVER THE
NEXT 6 HOURS FROM 18-00Z...WITH THE PROBABILITY MATCHED MEAN FROM
THE HREF YIELDING AN ADDITIONAL 4-6 INCHES OF RAIN. THIS IS A
HIGH-CONFIDENCE SIGNAL IN CONTINUED HEAVY RAIN AND FAIRLY
WIDESPREAD FLOODING.

FURTHER TO THE NORTHEAST...MODELS ALSO SHOW THE DEVELOPMENT OF
HEAVY RAIN AS CONVERGENT RAIN BANDS CONTINUE TO DEVELOP IN THE
EASTERN SEMI-CIRCLE OF HARVEY'S CIRCULATION. THESE RAIN BANDS ARE
NARROWER...ONLY 10-20 MILES WIDE AS OF 18Z...AND THUS MAY PROVIDE
A MORE TRANSIENT OR LOCALIZED FLASH FLOODING THREAT.
HOWEVER...TRAINING OF THE RAIN BANDS FOR AN HOUR OR TWO MAY
PRODUCE RAIN RATES HIGH ENOUGH TO GENERATE FLASH FLOODING...AND
THE SPATIAL COVERAGE OF HEAVY RAIN IN THIS AREA MAY EXPAND AS
HARVEY DRIFTS CLOSER.

LAMERS

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

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

LAT...LON   32959151 32929079 32799031 32259018 31809046
            31459073 31299098 31469143 31589188 31399232
            30839286 30109355 29949390 29979436 30139475
            30539504 31169506 31999470 32379408 32769302
            32949223


Last Updated: 212 PM EDT WED AUG 30 2017