MESOSCALE PRECIPITATION DISCUSSION 0903
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
828 AM EDT SUN OCT 22 2017

AREAS AFFECTED...ERN LA INTO SRN MS

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

VALID 221227Z - 221700Z

SUMMARY...CLUSTERS OF SLOW MOVING CONVECTION WILL SLOWLY MOVE EAST
ACROSS ERN LA INTO SRN MS THROUGH 17Z. ADDITIONAL RAINFALL OF 3-5
INCHES WILL BE POSSIBLE ALONG WITH AREAS OF FLASH FLOODING.

DISCUSSION...12Z RADAR SHOWED SCATTERED AREAS OF CONVECTION OVER
SRN MS INTO ERN LA WITH RAINFALL RATES OF 2-3 IN/HR. AN MCV NEAR
THE MARION/JEFFERSON COUNTY BORDER IN SRN MS APPEARS TO BE FORWARD
PROPAGATING AND ASSOCIATED WITH WEAKENING RAINFALL RATES AS THE
SYSTEM MOVES INTO REDUCED INSTABILITY. FARTHER SOUTH
HOWEVER...SLOWER MOVING RAINFALL CORES WERE NOTED INTO PORTIONS OF
ERN LA WHERE A SLOWER OVERALL MOVEMENT TO THE EAST WAS OBSERVED.
FLOW ALOFT REMAINS DIFFLUENT TO THE SOUTHEAST OF A SHARPLY
ORIENTED UPPER LEVEL TROUGH AXIS CURRENTLY OVER THE W-CNTRL TX/OK
BORDER. LOW LEVEL FLOW WAS CONFLUENT INTO THE ONGOING NORTH-SOUTH
ORIENTED AXIS OF HEAVY RAINFALL WITH GPS OBSERVED PRECIPITABLE
WATER VALUES OF 1.9 TO 2.0 INCHES NEAR THE NEW ORLEANS METRO AREA.

SHORT TERM FORECASTS FROM THE RAP INDICATE CONTINUED LOW LEVEL
CONFLUENCE OVER ERN LA INTO SRN MS THROUGH THE REMAINDER OF THE
MORNING BUT GRADUALLY WEAKENING UPPER LEVEL DIFFLUENCE AS THE
UPPER TROUGH AXIS APPROACHES AND THE SPLIT IN THE FLOW BECOMES
LESS PRONOUNCED. CAPE IS CURRENTLY LIMITED AS ONE MOVES
NORTHEASTWARD FROM THE SRN LA/MS BORDER WITH SOME INCREASE
EXPECTED OVER THE NEXT FEW HOURS...BUT THE GREATER VALUES SHOULD
REMAIN NEAR AND SOUTH OF I-10 WHICH IS EXPECTED TO CONTINUE
SUPPORTING RAINFALL RATES OF 2-3 IN/HR.

AN ELONGATED OUTFLOW BOUNDARY OVER NRN LA/SERN TX WAS OBSERVED TO
BE MOVING SOUTHEAST AT 30-35 KT...WITH PRESENT MOVEMENT REACHING
THE ONGOING ERN LA AXIS BETWEEN 16-17Z. WHILE IT IS UNCLEAR
EXACTLY HOW LONG CONVECTION WILL PERSIST OVER ERN LA/SRN
MS...OBSERVATIONS AND SHORT TERM TRENDS SUGGEST A FLASH FLOOD
THREAT WILL LINGER FOR THE NEXT 3-4 HOURS. HRRR AND OTHER HI-RES
MODELS ARE NOT HANDLING ONGOING TRENDS IN RADAR VERY WELL WHICH
REDUCES CONFIDENCE IN THE FORECAST.

OTTO

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

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

LAT...LON   32268980 32268914 31698817 30758772 30088782
            29468869 29028945 29199135 30309119 31629063