MESOSCALE PRECIPITATION DISCUSSION 0830
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
311 PM EDT TUE SEP 26 2017

AREAS AFFECTED...LOWER RIO GRANDE VALLEY....SOUTH-CENTRAL TX

CONCERNING...HEAVY RAINFALL

VALID 261905Z - 270005Z

SUMMARY...HEAVY RAINS ARE EXPECTED TO EXPAND IN COVERAGE OVER THE
NEXT FEW HOURS. SOME FLASH FLOODING WILL BE POSSIBLE GIVEN VERY
INTENSE RAINFALL RATES OF NEAR 3 INCHES/HR.

DISCUSSION...THE LATEST NON-OPERATIONAL GOES-16 IR IMAGERY SHOWS
EXPANDING COLD TOPPED CONVECTION OVER THE LOWER RIO GRANDE VALLEY
AS AN MCV EXITS VERY SLOWLY EAST OUT OF THE FAR NORTHEAST PORTION
OF THE MEXICAN STATE OF COAHUILA. THE MCV IS INTERACTING WITH A
VERY MOIST AND INCREASINGLY UNSTABLE BOUNDARY LAYER OVER THE LOWER
RIO GRANDE VALLEY AND ADJACENT AREAS OF SOUTH-CENTRAL TX...WHERE
THERE IS PLENTY OF LOW LEVEL SOUTHEAST FLOW TRANSPORTING HIGHER
THETA-E AIR WELL INLAND OFF THE GULF OF MEXICO. ACROSS ADJACENT
AREAS OF SOUTH-CENTRAL TX...THERE ARE SOME LOOSELY ORGANIZED BANDS
OF CONVECTION ATTEMPTING TO DEVELOP WITHIN THE WELL-DEFINED
SOUTHEAST LOW LEVEL FETCH WHICH IS BEING AIDED BY DIURNALLY
ENHANCED INSTABILITY.

PWATS ACROSS THE REGION ARE APPROACHING 2 INCHES...AND THE LATEST
CIRA-LPW DATA SHOWS A WELL-DEFINED 500/300 MB CONCENTRATION OF
MOISTURE WHICH WILL FACILITATE HIGHLY EFFICIENT RAINFALL PROCESSES
FOR ENHANCED RATES. MEANWHILE...MLCAPE VALUES ARE 1500 J/KG DOWN
ACROSS MUCH OF THE LOWER RIO GRANDE VALLEY. A COMBINATION OF THESE
WILL TEND TO SUPPORT RAINFALL TOTALS OF UP TO 3 INCHES/HR.

THE LATEST HIRES MODEL CONSENSUS TENDS TO SUPPORT THE MCV NEAR THE
RIO GRANDE RIVER MOVING ONLY VERY SLOWLY EAST THROUGH THE EVENING
HOURS...AND SO THE OVERALL CONVECTIVE ENVIRONMENT WILL BE
CONDUCIVE FOR HIGHLY FOCUSED/CONCENTRATED AREAS OF CONVECTION.
EXPECT STORM TOTALS OF AS MUCH AS 3 TO 6+ INCHES OF RAIN...AT
LEAST LOCALLY...AND THUS SOME FLASH FLOODING WILL BE POSSIBLE
THROUGH 00Z.

ORRISON

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

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

LAT...LON   31849878 31839839 31009787 30099721 29179648
            28359635 27669702 27239721 26699745 26559809
            26779925 27129945 27579970 27629983 28280019
            28810023 29849963 30599925 31419927 31729907