MESOSCALE PRECIPITATION DISCUSSION 0392
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1246 PM EDT TUE JUL 03 2018

AREAS AFFECTED...LOUISIANA...EXTREME EAST TEXAS

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

VALID 031644Z - 032230Z

SUMMARY...NUMEROUS SHOWERS AND THUNDERSTORMS SHOULD DEVELOP THIS
AFTERNOON ACROSS LOUISIANA IN A VERY MOIST ENVIRONMENT. THE AREAS
OF HEAVY RAIN MAY BE SLOW-MOVING WHICH SHOULD CONTRIBUTE TO THE
POSSIBILITY OF FLASH FLOODING. RAIN RATES MAY REACH 2-3 IN/HR AND
6-HOUR RAINFALL TOTALS MAY EXCEED 5 INCHES ON A LOCALIZED BASIS IN
THE OUTLINED AREA.

DISCUSSION...A WEAK SURFACE LOW WAS EVIDENT IN SURFACE
OBSERVATIONS NEAR CHANDELEUR SOUND...OR JUST EAST OF NEW
ORLEANS...AND THIS LOW WAS SHOWING A GRADUAL DRIFT TO THE WNW WITH
TIME. MOST MODELS FORECAST THIS GENERAL MOTION TO CONTINUE THROUGH
THE DAY. RADAR VWPS AND GOES-16 DERIVED MOTION WINDS INDICATED
BROADLY CONFLUENT BOUNDARY LAYER FLOW OVER LOUISIANA...GENERALLY
TO THE NORTHWEST AND WEST OF THE SURFACE LOW. ADDITIONALLY...THE
DMW FROM GOES-16 SUGGESTED A WESTWARD TILT WITH HEIGHT TO THE LOW
PRESSURE SYSTEM...WITH THE 500MB TROUGH AXIS LIKELY CLOSER TO
CENTRAL LOUISIANA.

AS THE LOW RETROGRADES TO THE WEST THROUGH THE AFTERNOON...MODELS
ARE IN EXCELLENT AGREEMENT IN SHOWING WIDESPREAD CONVECTIVE
DEVELOPMENT IN THE BUBBLE OF HIGH PWS ACROSS THE REGION. CIRA
BLENDED TPWS ARE AROUND 2.0 TO 2.5 INCHES AND MANY OF THE GPS
OBSERVATIONS IN THE AREA ARE BETWEEN 2.2 AND 2.4 INCHES. THIS IS
CONSISTENT WITH DEEP TROPICAL MOISTURE...AND SHOULD SUPPORT VERY
HEAVY RAIN RATES. GIVEN THE WEAKNESS IN THE FLOW ALOFT COINCIDING
WITH THE LOW-LEVEL CONFLUENT FLOW...STORMS APPEAR MOST LIKELY TO
DEVELOP WHERE THEIR STORM MOTIONS MAY BE SLOWEST AND THIS IS
LIKELY TO COMPOUND THE RISK OF HEAVY RAINFALL.

TWO GENERAL AREAS OF CONCERN EXIST. THE FIRST AREA...NEAR THE
LOW-LEVEL CIRCULATION CENTER WHERE INSTABILITY WILL BE DIMINISHED
BY PERSISTENT DEEPER CLOUD COVER. HOWEVER...ADDITIONAL RAIN IN
SOME AREAS THAT HAVE ALREADY RECEIVED HEAVY RAIN IN THE PAST 12-24
HOURS COULD LEAD TO FURTHER FLOODING ISSUES. GIVEN THE WEAKER
INSTABILITY...WOULD EXPECT RAIN RATES TO BE LOWER IN THIS REGION
(SOUTHEAST LOUISIANA) AND THUS THE FLASH FLOOD RISK MAY ALSO BE
SLIGHTLY LOWER THROUGH 00Z. THE SECOND AREA OF CONCERN...FURTHER
WEST WHERE VISIBLE SATELLITE INDICATES SOME BREAKS IN THE CLOUD
COVER AND RAP ANALYSIS SHOWS GREATER INSTABILITY. RAIN RATES ARE
LIKELY TO BE HIGHER IN THIS REGION...AND RECENT HRRR FORECASTS
SHOW THEM REACHING 3 IN/HR IN LOCALIZED AREAS. THE 12Z HREF ALSO
SHOWS NEIGHBORHOOD PROBABILITIES OF OVER 3 INCHES OF RAIN IN 6
HOURS IN EXCESS OF 70 PERCENT OVER MOST OF SOUTHWEST LOUISIANA
(AND AS HIGH AS 90 PERCENT IN SOME AREAS). THESE PROBABILITIES ARE
SLIGHTLY HIGHER THAN THE HRRR-TLE...BUT THE GENERAL PLACEMENT OF
THE HIGHEST VALUES IS NEARLY IDENTICAL.

THEREFORE...CHANCES OF LOCALIZED HEAVY RAINFALL AND FLASH FLOODING
APPEAR HIGHEST IN SOUTHWEST LOUISIANA...BUT WILL BE POSSIBLE
ANYWHERE IN ADJACENT PORTIONS OF LOUISIANA OR EXTREME EAST TEXAS.
ANTECEDENT CONDITIONS ARE SOMEWHAT DRY OTHER THAN NEAR THE
BEAUMONT AND PORT ARTHUR AREAS OVER THE PAST COUPLE WEEKS...AND
THUS FLASH FLOODING SHOULD BE RELATED STRONGLY TO WHERE VERY HIGH
RAIN RATES CAN PERSIST FOR AT LEAST 1.5 TO 2.0 HOURS.

LAMERS

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

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

LAT...LON   32749371 32139257 31139181 30709011 30018921
            28958888 28959083 29469288 29579406 30169446
            31369481 32299452