MESOSCALE PRECIPITATION DISCUSSION 0808
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1243 PM EST SAT DEC 31 2016
 
AREAS AFFECTED...UPPER TX COAST...SOUTHWEST LA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 311740Z - 312340Z
 
SUMMARY...VERY HEAVY RAINS DEVELOPING THIS AFTERNOON MAY RESULT IN
SOME RUNOFF CONCERNS AND POSSIBLE FLASH FLOODING. 

DISCUSSION...HEAVY RAINS WILL BE EXPANDING IN COVERAGE AND
INCREASING IN INTENSITY THIS AFTERNOON ACROSS SOUTHWEST LA AS A
WAVE OF LOW PRESSURE FOCUSES NEAR THE UPPER TX COAST AND HELPS
TRANSPORT DEEP MOISTURE NORTHWARD FROM THE GULF OF MEXICO INTO A
STALLED OUT FRONTAL ZONE. FACILITATING THE EXPANSION OF HEAVY
RAINS WILL BE FAVORABLE 250 MB RIGHT-ENTRANCE REGION JET
DYNAMICS/ASCENT AND A SOUTH-SOUTHWEST LOW LEVEL JET OF 40 TO 50
KTS OVERRUNNING THE FRONT.

ALREADY OVER THE LAST COUPLE OF HOURS THERE HAS BEEN A RATHER
RAPID DEVELOPMENT AND EXPANSION OF HEAVY CONVECTIVE RAINS ACROSS
AREAS OF THE UPPER TX COAST...AND THE COOLING CLOUD TOPS NOTED IN
GOES-EAST IR SATELLITE IMAGERY CONFIRMS DEEPER AND LARGER SCALE
ASCENT OCCURRING OVER THIS REGION. RAINFALL RATES ARE INCREASING
AND ARE EXPECTED TO INCREASE TO AS HIGH AS 1.5 INCHES/HR AS
STRONGER FORCING AND DEEPER MOISTURE ARRIVES THROUGH THE EARLY TO
MID AFTERNOON HOURS. PRECIPITABLE WATER VALUES AS PER BLENDED-TPW
DATA AND GPS MONITORS ARE ALREADY OVER 1.75 INCHES ALONG THE TX
COAST AND THIS WILL BE TRANSPORTED NORTHEAST INTO SOUTHWEST LA
THIS AFTERNOON AND THEN GRADUALLY FARTHER EAST ACROSS SOUTHERN LA
THIS EVENING.

THE 12Z HIRES GUIDANCE INCLUDING THE ARW/NMMB AND NSSL-WRF ALL
SUGGEST A FAIRLY STRONG SIGNAL FOR TRAINING CONVECTION NEAR AND
JUST NORTH OF THE FRONTAL ZONE...WITH HEAVY RAINFALL TOTALS OF AS
MUCH AS 3 TO 5 INCHES THROUGH 00Z. THIS IS SUPPORTED BY THE RECENT
HRRR GUIDANCE...ALTHOUGH THE MAIN FOCUS OF THE HEAVY RAINS SHOULD
BE FAIRLY CLOSE TO THE COAST WHERE THE FRONT IS. THE NMMB AND
NSSL-WRF BOTH APPEAR TO BE FOCUSING A LITTLE TOO MUCH RAIN TOO FAR
INLAND. GIVEN THE CONCERNS OVER TRAINING CONVECTION AND RESULTING
HEAVY RAINFALL TOTALS...THERE WILL BE SOME CONCERNS FOR RUNOFF
PROBLEMS AND POSSIBLE FLASH FLOODING.

ORRISON

ATTN...WFO...HGX...LCH...LIX...

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

LAT...LON   30819224 30609146 29999144 29579238 29639357 
            29369450 28979522 29079550 29519554 30009497 
            30539373