MESOSCALE PRECIPITATION DISCUSSION 0900
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
913 PM EDT SAT OCT 21 2017

AREAS AFFECTED...CNTRL LA...SRN LA...SW MS

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

VALID 220112Z - 220542Z

SUMMARY...VERY HEAVY RAIN RATES...LOCALLY PERSISTENT...COULD LEAD
TO FLASH FLOODING.

DISCUSSION...OBSERVATIONAL TRENDS AND A SMALL INTENSE QPF SIGNAL
IN RECENT HRRR RUNS ARE CAUSE FOR SOME CONCERN. LOW LEVEL INFLOW
WAS VERY MOIST WITH PW 1.70 INCH OR GREATER ON 00Z SOUNDINGS FROM
LAKE CHARLES AND SLIDELL.

ONGOING STORMS AT 01Z HAD SHOWN CONTINUED STRENGTHENING...COOLING
CLOUD TOPS ALONG WITH INCREASING DUAL POL RAIN RATE
ESTIMATES...AND A GRADUAL ABILITY TO BECOME ORGANIZED. THE STORMS
WERE SUPPORTED BY RICH GULF MOISTURE STREAMING INTO A REGION OF
MODEST BUT PERSISTENT UPPER LEVEL DIFLUENCE AHEAD OF THE LARGE
SCALE AMPLIFYING TROUGH OVER THE PLAINS STATES. STORMS WEST OF
ALEXANDRIA...ALONG THE NORTHWEST EDGE OF THE RICHEST MOISTURE PER
TPW SATELLITE IMAGERY AND THE SPC 
MESOANALYSIS...HAD BECOME ANCHORED WHERE SOUTHEASTERLY INFLOW WAS
BALANCED AGAINST SOUTHWESTERLY UPPER LEVEL FLOW. AT 01Z DUAL POL
RADAR AT FORT POLK... KPOE...ESTIMATED SMALL SCALE RAIN RATES NEAR
4 INCHES IN AN HOUR. CONTINUATION OF THIS ACTIVITY IS SOMEWHAT IN
DOUBT...AS THE UPSTREAM INFLOW WAS RAIN-COOLED...BUT INCREASING
SYNOPTIC ASCENT MAY SUSTAIN A NARROW CONVECTIVE WEDGE.

FARTHER SOUTH A COMBINED COLD POOL HAD GRADUALLY DEVELOPED FROM
BATON ROUGE TO LAFAYETTE...ACTING TO ENHANCE THE LOW LEVEL
CONVERGENCE AND SLOW THE INDIVIDUAL CELL MOTIONS. CONTINUED
MERGERS WITHIN THIS CLUSTER OF DEEP THUNDERSTORMS IS LIKELY
THROUGH AT LEAST 05Z...AND PERHAPS LONGER...PER RAP MODEL
FORECASTS OF INCREASINGLY DIFLUENT UPPER FLOW. RECENT HRRR RUNS
PRODUCE A SMALL BUT POTENTIALLY ACCURATE AREA OF EXCEPTIONAL
RAINFALL TOTALS THROUGH THE NIGHTTIME HOURS OVER SOUTHERN
LOUISIANA. ALTHOUGH THE HRRR SOLUTION OF GREATER THAN 10 INCHES
REPRESENTS A WORSE CASE SCENARIO...ANYTHING CLOSE TO THIS WOULD
EASILY EXCEED FFG...AND WPC UPGRADED THIS SMALL AREA TO A SLIGHT
RISK OF EXCESSIVE RAINFALL IN THE 01Z UPDATE.

BURKE

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

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

LAT...LON   32389218 31939124 30979092 30259093 29729152
            30029222 30869291 32029306