MESOSCALE PRECIPITATION DISCUSSION 0630
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
806 AM EDT SAT OCT 31 2015
 
AREAS AFFECTED...SOUTHEAST TX, LA, & SOUTHWEST MS 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 311205Z - 311805Z
 
SUMMARY...A LARGE AREA OF THUNDERSTORMS IS EXPECTED TO PRODUCE
HOURLY RAIN RATES UP TO 3".  FLASH FLOODING SHOULD OCCUR,
PARTICULARLY WITHIN URBAN AREAS.

DISCUSSION...A CONVECTIVELY-REINFORCED FRONTAL SYSTEM LIES ACROSS
SOUTHEAST TX THIS MORNING, TO THE SOUTHEAST OF A SHORTWAVE LIFTING
PAST THE BORDER OF NORTH-CENTRAL TX/SOUTHWEST OK. DIVERGENCE ALOFT
IS SIGNIFICANT AND BROAD AS THE POLAR AND SUBTROPICAL JETS
SLIGHTLY DIVERGE IN THIS AREA.  PRECIPITABLE WATER VALUES OF
1.7-2.4" LIE ACROSS THE REGION PER GPS INFORMATION.  MUCAPES OF
1000-3000 J/KG LIE TO THE SOUTH OF THE WARM FRONT, WHICH ARE BEING
ADVECTED IN BY 40-60 KTS OF 850 HPA INFLOW (PER VAD WIND
PROFILES), WHICH ACROSS LA IS ABOUT 30% ABOVE THE MEAN 850-400 HPA
WIND.  THIS HAS LED TO PRECIPITATION EFFICIENCY, WITH DUAL POL
ESTIMATES AS HIGH AS 3" AN HOUR OVER THE PAST COUPLE HOURS. 
SATELLITE TRENDS AS OF LATE SHOW SLIGHTLY WARMING CLOUD TOPS,
IMPLYING THE EVENT IS PASSING ITS PEAK.

CONVECTION HAS BEEN MOVING ALONG THE LINES OF FORWARD PROPAGATING
CONVECTIVE VECTORS, BETWEEN NORTHEAST AND EAST-NORTHEAST AT 30-35
KTS.  A CONVECTIVE LINE ASSOCIATED WITH THE COLD FRONT COULD LEAD
TO CELL MERGERS WITH PRECEDING ACTIVITY OVER THE PINEY WOODS/BIG
THICKET OF SOUTHEAST TX AND SOUTHWEST/CENTRAL LA, WHICH COULD LEAD
TO RENEWED COOLING OF CLOUD TOPS AND HIGHER RAIN RATES.  CELL
MERGERS AND TRAINING ARE EXPECTED TO BE THE PRIMARY FLASH
FLOOD/EXCESSIVE RAINFALL THREAT.  WHILE LOW-LEVEL INFLOW IS
EXPECTED TO SLOWLY DECREASE TODAY, THE MAXIMUM SHOULD FALL FROM 60
TO 50 KTS, MORE THAN SUFFICIENT TO CONTINUE ORGANIZED CONVECTIVE
DEVELOPMENT.  INSTABILITY IS EXPECTED TO CONTINUE CREEPING
NORTHWARD, WITH THE WARM FRONT MOVING NORTHWARD IN THE GULF
POTENTIALLY MERGING WITH THE COASTAL FRONT ACROSS SOUTHERN LA OVER
THE NEXT SEVERAL HOURS.  

THE CAM GUIDANCE WHICH HAS BEEN BEST REPRESENTING THE CONVECTIVE
PATTERN/ PLACEMENT OF THE HEAVY RAINFALL IS THE 00Z ARW, WITH THE
OTHER GUIDANCE DISPLACED MORE TO THE NORTH (THEIR USUAL BIAS). 
THE GENERAL SIGNAL IN THE GUIDANCE IS FOR LOCAL AMOUNTS IN THE
4-7" RANGE, WHICH COULD FALL QUICKLY.  THIS IS NEAR THE 50 YEAR
RECURRENCE INTERVAL FOR INDIVIDUAL LOCATIONS IN THIS AREA OVER A
SIX HOUR PERIOD.  THE AVAILABLE MOISTURE AND NOTABLE PRECIPITATION
EFFICIENCY SHOULD ALLOW FOR 3" AN HOUR RATES.  THE 06Z SSEO
NEIGHBORHOOD PROBABILITIES OF EXCEEDING THE THREE HOUR FLASH FLOOD
GUIDANCE VALUES SUGGEST THE APEX OF THE RAIN EVENT WOULD BE IN THE
15-18Z TIME FRAME.  FLASH FLOODING SHOULD OCCUR, PARTICULARLY
WITHIN URBAN AREAS.

ROTH

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

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

LAT...LON   32209175 31799093 30679071 29929084 29409162 
            29549265 29699321 29509423 29109486 28899609 
            29029748 30139720 31009587 31929377