Flash Flood Guidance
Issued by NWS

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026
AWUS01 KWNH 281604
FFGMPD
MSZ000-LAZ000-281900-

MESOSCALE PRECIPITATION DISCUSSION 0144
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1202 PM EDT THU APR 28 2016

AREAS AFFECTED...EXTREME EASTERN LA...SOUTHERN MS

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

VALID 281600Z - 281900Z

SUMMARY...INTENSE BACKBUILDING SHOWERS AND THUNDERSTORMS CONTINUE
TO RESULT IN EXTREME FLASH FLOODING. CONDITIONS TO IMPROVE TOWARD
MID-AFTERNOON.

DISCUSSION...IR SATELLITE IMAGERY CONTINUES TO SHOW VERY
COLD-TOPPED CONVECTION WITH SIGNIFICANT BACKBUILDING OF CELL
OCCURRING OVER SOUTHEAST MS AND MORE RECENTLY JUST NORTH AND EAST
OF LAKE PONCHARTRAIN. THE ACTIVITY IS EMBEDDED WITHIN A MODEST LOW
LEVEL WARM ADVECTION REGIME BUT IS BEING ENHANCED BY A COMBINATION
OF MODERATE INSTABILITY FUNNELING IN OFF THE NORTHERN GULF OF
MEXICO AND A SPLIT UPPER JET PATTERN ACROSS THE GULF COAST WHICH
IR SATELLITE IMAGERY SHOWS.

THE LATEST GOES-14 SRSO IMAGERY SHOWS A CLASSIC CONVECTIVE WEDGE
SIGNATURE ACROSS FAR EASTERN LA AND SOUTHEAST MS WHICH COUPLED
WITH REPEATING OVERSHOOTING TOPS AS PER THE GOES-OTD ALGORITHM IS
INDICATIVE OF STRONG FORCING. CONSEQUENTLY...RAINFALL RATES HAVE
BEEN VERY HEAVY AND HAVE BEEN REPORTEDLY AS HIGH AS 3 INCHES/HR.
HELPING TO FACILITATE THE HEAVIER RAIN RATES IS A PWAT FIELD OF
1.5 TO 1.7 INCHES AS PER GPS-DERIVED INFORMATION.

MULTIPLE RUNS OF THE HRRR SUGGEST THAT THERE SHOULD BE ANOTHER 3
TO 4 HOURS OF LOCALLY VERY INTENSE RAINFALL BEFORE THEN SEEING
SOME INTRUSION OF WARMER AND DRIER MID LEVEL DRY AIR INTRUSION FOR
CONVECTION TO BEGIN WEAKENING AND POSSIBLY ALSO BOWING SOUTHEAST
OUT OVER THE NORTHERN GULF OF MEXICO. THE HRRR THOUGH SUGGESTS
ADDITIONAL AMOUNTS OVER 3 TO 5 INCHES FOR AREAS JUST EAST OF LAKE
PONCHARTRAIN AND INTO FAR SOUTHERN MS BEFORE THIS WEAKENING OCCURS.

ALREADY OBSERVED RAINFALL TOTALS OF OVER 10 INCHES HAVE BEEN NOTED
OVER SOUTHERN LA...AND ESPECIALLY IN VICINITY OF HARRISON COUNTY
MS. DUAL-POL RADAR ESTIMATES ARE LOCALLY OVER 1 FOOT WHICH IS
LIKELY A LITTLE HIGH GIVEN SOME HAIL CONTAMINATION.

NEVERTHELESS...THE ADDITIONAL RAINS WILL FOSTER ADDITIONAL FLASH
FLOODING WHICH MAY LOCALLY BE EXTREME GIVEN THE HEAVY TOTALS SEEN
THIS MORNING.

ORRISON

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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

LAT...LON   31068920 30798851 30238851 29808929 29648994
            30019029 30519028 30898977



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