MESOSCALE PRECIPITATION DISCUSSION 0006
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
955 PM EDT FRI MAR 28 2014
 
AREAS AFFECTED...SOUTHEAST LA AND SOUTHERN MS 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 290154Z - 290754Z
 
SUMMARY...A SQUALL LINE IS APPROACHING AN AREA OF EXTREMELY LOW
FLASH FLOOD GUIDANCE VALUES - FFGS -- ACROSS SOUTHEAST LA AND
SOUTHERN MS.  REGIONAL FLASH FLOODING IS CONSIDERED LIKELY IN THIS
REGION.

DISCUSSION...A SHORTWAVE MOVING THROUGH CENTRAL TX HAS SPAWNED A
SQUALL LINE WHICH CROSSED THE SABINE RIVER INTO LA AROUND 0115
UTC.  THE 01Z HRRR APPEARS TO BE CAPTURING THIS FEATURE WELL,
THOUGH THE SIX HOURLY RADAR LOOP INDICATED THAT THE ACTIVITY COULD
OUTRACE THE PACE SEEN IN THE 01Z HRRR GUIDANCE.  DESPITE ITS
PROGRESSION, RADAR-DERIVED RAINFALL RATES OF 1-1.5 IN/HR ACCOMPANY
THE CONVECTIVE BAND.  PRECIPITABLE WATER VALUES OF 1.75" IN THE
REGION ARE NEAR THE 99TH PERCENTILE FOR LATE MARCH, OR TWO
STANDARD DEVIATIONS ABOVE NORMAL, WHICH IS AIDING PRECIPITATION
EFFICIENCY.  DIVERGENCE ALOFT IS BEING AIDED BY A JET COUPLET
CAUSED BY SPEED MAXIMA WITHIN THE POLAR AND SUBTROPICAL BRANCHES
OF THE JET STREAM.

AHEAD OF THE BAND, THERE IS A REGION OF FFGS OF 1.5"/3 HOURS -- OR
LESS -- WHICH WAS CAUSED BY THIS MORNING'S HEAVY RAINFALL -- THESE
RAINFALL RATES SHOULD BE EASILY EXCEEDED BY THE CONVECTIVE LINE.
THE HEAVIEST CONVECTION IS EXPECTED TO OCCUR ALONG A PRECEDING
SURFACE TROUGH/ OUTFLOW BOUNDARY LYING NORTHWEST- SOUTHEAST ACROSS
LA, WITH SOME DEVELOPING CIN OVERNIGHT POSSIBLY ALLOWING FOR
OVERRUNNING/ELEVATED CONVECTION ON THE COLD SIDE OF THE TROUGH. 
CORFIDI VECTORS ARE EXPECTED TO BE LIGHT IN THE REGION THROUGH
09Z, AS INCREASING 850 HPA INFLOW OF 30-40 KNOTS MOVES INTO THE
BOUNDARY.  THE CAPES/LOW-LEVEL SHEAR ARE SUFFICIENT FOR A WET
MULTI-CELL ENVIRONMENT AND BACKBUILDING ALONG THE OUTFLOW BOUNDARY
IS POSSIBLE, WHICH WOULD MAGNIFY THE FLASH FLOOD THREAT.  RAINFALL
RATES OF 1-1.5" PER HOUR SHOULD LEAD TO REGIONAL FLASH FLOODING. 
USED A SIX HOUR TIME HORIZON TO GIVE THE LONGEST POSSIBLE LEAD
TIME.

ROTH

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

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

LAT...LON   29048907 29969064 30929179 31579156 31609038 30388847
            29048907