MESOSCALE PRECIPITATION DISCUSSION 0621
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
953 AM EDT FRI OCT 30 2015
 
AREAS AFFECTED...SOUTHERN/SOUTH-CENTRAL TX 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 301352Z - 301952Z
 
SUMMARY...TWO AREAS OF CONCERN ACROSS SOUTHERNMOST TX AND
SOUTH-CENTRAL TX COULD SEE HOURLY RAIN RATES UP TO 4" AND LOCAL
AMOUNTS IN THE 4-8" RANGE.  FLASH FLOODING IS LIKELY ACROSS A
SATURATING REGION.

DISCUSSION...THE SYNOPTIC SETUP IS COMPLEX.  AT THE SURFACE, THE
APPARENT POLAR FRONT IS BEING REINFORCED BY A CONVECTIVE COMPLEX
ACROSS SOUTH-CENTRAL TX, WITH A COASTAL FRONT NOTED CLOSER TO THE
UPPER TX COAST/OFFSHORE THE LA COAST.  IN SOUTHERNMOST TX, A WEAK
DEW POINT GRADIENT/CONVERGENCE ZONE IS STARTING TO DRIVE NEW
THUNDERSTORM ACTIVITY TO THE WEST AND NORTH OF BROWNSVILLE. 
ALOFT, THE POLAR AND SUBTROPICAL JETS MERGE ACROSS NORTHERN
MEXICO, DOWNSTREAM OF AN ORGANIZED CONVECTIVE COMPLEX IN SOUTHWEST
MEXICO WHICH IS CONTRIBUTING MOISTURE ALOFT.  INFLOW AT 850 HPA IS
CONVERGENT ALONG A NORTH-SOUTH AXIS IN EAST-CENTRAL TX, WITH A
MAGNITUDE OF 30-60 KTS PER VAD WIND PROFILES.  MUCAPE IN THE
REGION IS 1000-3000 J/KG, WHICH WAS VERIFIED BY THE 12Z
BROWNSVILLE TX SOUNDING.  THE LOW-LEVEL INFLOW IS UP TO 30% HIGHER
THAN THE MEAN 850-400 HPA WIND, WHICH HAS LED TO PRECIPITATION
EFFICIENCY.  LOCAL AMOUNTS UP TO 10" HAVE BEEN NOTED ON DUAL POL
IMAGERY TO THE WEST AND NORTHWEST OF NEW BRAUNFELS TX. 
PRECIPITABLE WATER VALUES OF 1.7-2.4" INHABIT THE REGION, WHICH IS
NEAR THREE SIGMAS ABOVE THE MEAN FOR THE END OF OCTOBER.

ACROSS SOUTH-CENTRAL TX, THE NON-SPCWRF GUIDANCE HAS HAD A GOOD
HANDLE ON LOCATION AND MAGNITUDE OF RAINFALL.  THESE PIECES OF
GUIDANCE ADVERTISE LOCAL AMOUNTS IN THE 4-8" RANGE OVER THE NEXT
SEVERAL HOURS, WHICH COULD FALL QUICKLY.  BACKBUILDING AND CELL
TRAINING ARE BOTH CONSIDERED THE MAIN RISKS FOR EXCESSIVE
RAINFALL.  HOURLY RAIN RATES UP TO 4" HAS BEEN OBSERVED VIA DUAL
POL ESTIMATES NEAR SAN ANTONIO/AUSTIN TX.  THE 06Z SSEO
NEIGHBORHOOD PROBABILITIES INDICATE A LULL IN HEAVY RAINFALL IS
POSSIBLE IN THE 15-18Z PERIOD, PERHAPS DISRUPTED BY THUNDERSTORM
ACTIVITY IN SOUTHERNMOST TX AND THE DEPARTING OF THE SHORTWAVE
MOVING ACROSS NORTHERN TX, BEFORE RAMPING UP AGAIN AS INSTABILITY
INCREASES DUE TO ADVECTION AND INSOLATION AND A NEW SHORTWAVE
APPROACHING FROM NORTH-CENTRAL MEXICO.  IN SOUTHERNMOST TX, A
COMPROMISE OF THE 12Z HRRR AND 00Z NAM CONEST APPEARS IDEAL, WHICH
INDICATE LOCAL AMOUNTS OF 3-7" ARE POSSIBLE NEAR THE WEAK
CONVERGENCE ZONE/DEW POINT GRADIENT.  THE MAGNITUDE OF RAIN
EXPECTED SHOULD LEAD TO FLASH FLOODING, PARTICULARLY WITHIN URBAN
CENTERS.

ROTH

ATTN...WFO...BRO...CRP...EWX...FWD...HGX...

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

LAT...LON   30979621 30169506 29399486 28899530 28189658 
            27329726 25999708 25799743 25869825 26839795 
            27959788 28879816 29309900 30319852 30969742