re: Flood threat on northern Gulf coast

MESOSCALE PRECIPITATION DISCUSSION 0544
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
954 AM EDT THU AUG 11 2016

AREAS AFFECTED...SOUTHERN LA...EXTREME SOUTHERN MS...EXTREME
SOUTHERN AL...WESTERN FL PANHANDLE

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

VALID 111350Z - 111930Z

SUMMARY...VERY SLOW-MOVING AND INTENSE SHOWERS AND THUNDERSTORMS
WILL BE FOCUSING OVER THE CENTRAL GULF COAST AND AREAS JUST INLAND
THROUGH EARLY AFTERNOON. EXTREMELY HEAVY RAINFALL WILL BE POSSIBLE
AND MAY RESULT IN LOCALLY SIGNIFICANT RUNOFF PROBLEMS.

DISCUSSION...A VERY BROAD AND ELONGATED AREA OF LOW PRESSURE OVER
THE CENTRAL GULF COAST THIS MORNING IS GENERATING VERY HEAVY AND
SLOW-MOVING SHOWERS AND THUNDERSTORMS FROM MUCH OF SOUTHERN LA
EAST INTO FAR SOUTHERN MS/AL AND THE WESTERN FL PANHANDLE.

THE MAIN AXIS OF CONVECTION IS FOCUSING WITHIN AN AXIS OF
CONFLUENT LOW LEVEL FLOW AND ALONG A RATHER WELL-DEFINED
INSTABILITY GRADIENT. OVER THE LAST COUPLE OF HOURS THERE HAS BEEN
SOME EXPANSION AND CONCENTRATION OF THE BROKEN BANDS OF
CONVECTION...BUT THERE CONTINUES TO BE VERY SLOW CELL MOTION AND
THIS IS BEGINNING TO RAISE SIGNIFICANT CONCERNS FOR VERY HEAVY
RAINFALL TOTALS.

THE 12Z LIX RAOB INDICATES A PWAT OF 2.69 INCHES AND A 18C
DEWPOINT AT 850 MB. THIS AIRMASS ALONG THE CENTRAL GULF COAST IS
DEEPLY TROPICAL AND EXTREMELY MOIST. IN FACT...THE CIRA-LPW
PRODUCT IS SHOWING VERY CONCENTRATED LEVELS OF MOISTURE IN THE
500/300 MB LAYER OVER SOUTHERN LA. WET-BULB ZERO LEVELS ACROSS THE
REGION ARE OVER 16000 FEET AT BOTH LCH AND LIX THIS MORNING WHICH
CONFIRMS THE DEEP WARM/MOIST COLUMN.

THE ENVIRONMENT AS A RESULT IS CONDUCIVE FOR EXTREMELY EFFICIENT
RAINFALL PRODUCTION WITHIN THE AXIS OF DEEPER CONVECTION...AND
THUS EXTREMELY HIGH RAINFALL RATES WHICH WILL LIKELY BE ON THE
ORDER OF 3 TO 4 INCHES/HR IN SOME CASES.

THE 12Z HRRR/HRRRP GUIDANCE INDICATES AN AXIS OF 4 TO 8 INCH
RAINFALL POTENTIAL THROUGH 18Z ACROSS PORTIONS OF
SOUTHERN/SOUTHEAST LA AND INTO POSSIBLY FAR SOUTHERN MS...WITH
LESSER AMOUNTS FARTHER EAST INTO THE WESTERN FL PANHANDLE.

SIGNIFICANT RUNOFF PROBLEMS AND/OR FLASH FLOODING WILL BE POSSIBLE
GIVEN THESE RAINFALL AMOUNTS...AND ESPECIALLY NEAR ANY URBANIZED
AREAS. WILL CONTINUE TO MONITOR.

ORRISON