MESOSCALE PRECIPITATION DISCUSSION 0079
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
420 PM EDT WED MAR 28 2018

AREAS AFFECTED...SOUTH-CENTRAL/SOUTHEAST TX...WESTERN/NORTHERN
LA...FAR SOUTHEAST AR...WEST-CENTRAL MS

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

VALID 282020Z - 290220Z

SUMMARY...VERY HEAVY RAINFALL IS EXPECTED FROM STRONG
THUNDERSTORMS WITH 2.5 INCH/HR RAINFALL RATES AND CONCERNS OF
TRAINING CELL ACTIVITY. FLASH FLOODING IS LIKELY.

DISCUSSION...THE LATEST GOES-16 10.3 MICRON/IR IMAGERY SHOWS AN
EXPANSIVE AREA OF STRONG AND WELL-ORGANIZED COLD-TOPPED CONVECTION
WITH SEVERAL EMBEDDED OVERSHOOTING TOPS THAT EXTENDS ALONG AND
NORTHWEST OF A LINE FROM NEAR VICTORIA TX...TO JASPER TX...TO
NATCHITOCHES LA.

THE CONVECTION IS BEING FACILITATED BY NOTABLY DIVERGENT FLOW
ALOFT COINCIDING WITH A PRONOUNCED POOLING OF DEEP LAYER
MOISTURE...SHEAR AND INSTABILITY ALONG AN OUTFLOW BOUNDARY AND
WITH PROXIMITY TO AN APPROACHING COLD FRONT. BLENDED-TPW AND
GPS-DERIVED PWATS ARE ON THE ORDER OF 1.7 TO 1.9 INCHES ALONG THIS
BOUNDARY...WITH MLCAPE VALUES APPROACHING OR EXCEEDING 1500 J/KG
AND EFFECTIVE BULK SHEAR VALUES OF 50 TO 55 KTS.

THIS WILL SUSTAIN A STRONG AND ORGANIZED CONVECTIVE THREAT WELL
INTO THE EVENING HOURS WITH A GRADUAL EAST AND SOUTHEAST SHIFT TO
THE CONVECTIVE AXIS THAT WILL THREATEN THE NORTHWEST SUBURBS OF
THE HOUSTON AND BEAUMONT/PORT-ARTHUR METROPOLITAN AREAS OF
SOUTHEAST TX...AND ALSO EXTENDING NORTHEAST INTO CENTRAL LA WITH A
FOCUS ON THE NATCHITOCHES...FORT POLK AND ALEXANDRIA AREAS.

THIS IS ALL STRONGLY SUPPORTED BY THE 12Z CAM GUIDANCE...INCLUDING
THE ARW...ARW2...NMMB AND THE NAM-CONEST. RECENT HRRR RUNS SUPPORT
THIS AS WELL...BUT THE MAIN CONCERN MOVING FORWARD WILL BE AN
ENVIRONMENT BECOMING HIGHLY CONDUCIVE FOR TRAINING CONVECTION IN
THE HOURS AHEAD AS CONVECTION BECOMES STRONGLY ALIGNED SW/NE WITH
THE MEAN LAYER FLOW DOWNSTREAM OF AN UPPER TROUGH EJECTING OUT
ACROSS THE SOUTHERN PLAINS. THIS WILL BE COINCIDENT WITH A HIGHLY
CONFLUENT LOW LEVEL JET THAT WILL BE PEAKING NEAR 50 KTS AND
MAXIMIZING LOW LEVEL CLOUD-BEARING MOISTURE TRANSPORT.

GIVEN THE DEGREE OF DEEP MOISTURE AND FOCUSED
INSTABILITY...RAINFALL RATES WILL BE QUITE HIGH AND MAY EXCEED 2.5
INCHES/HR GOING INTO THE EVENING HOURS. THIS COUPLED WITH TRAINING
CELL ACTIVITY WILL SUPPORT RAINFALL TOTALS OF AS MUCH 5+ INCHES
GOING THROUGH THE MID-EVENING HOURS. THIS WILL LIKELY RESULT IN
ENHANCED SHORT-TERM RUNOFF AND FLASH FLOODING CONCERNS. WILL
CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...CRP...EWX...HGX...JAN...LCH...LZK...MEG...SHV...

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

LAT...LON   33939007 33458942 32718966 31969052 31069197
            29579467 28659689 28409794 28799814 29619754
            30869604 32319386 33499163