MESOSCALE PRECIPITATION DISCUSSION 0837
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
600 PM EDT WED SEP 27 2017

AREAS AFFECTED...SOUTH-CENTRAL AND SOUTH TX

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 272200Z - 280400Z

SUMMARY...FLASH FLOODING WILL REMAIN POSSIBLE AS ADDITIONAL HEAVY
RAINS DEVELOP THROUGH THE EVENING HOURS OVER AREAS WITH NEARLY
SATURATED SOILS.

DISCUSSION...NON-OPERATIONAL GOES-16 10.3 MICRON/IR IMAGERY HAS
SHOWN SOME IMPRESSIVE COOLING OF CONVECTIVE TOPS OVER AREAS OF
SOUTH-CENTRAL TO SOUTH TX OVER THE LAST COUPLE OF HOURS IN
RESPONSE TO A MODERATELY UNSTABLE BOUNDARY LAYER DRIVEN BY DIURNAL
HEATING AND SURFACE DEWPOINTS IN THE MID 70S.

THE CONVECTION IS ALSO BEING ENHANCED BY BROADLY DIFFLUENT FLOW
ALOFT AND AN AXIS OF FOCUSED SURFACE CONVERGENCE NEAR THE
QUASI-STATIONARY FRONT DRAPED SOUTHWEST TO NORTHEAST FROM THE
MIDDLE RIO GRANDE VALLEY TO EAST TEXAS. THERE IS SOME EVIDENCE OF
A SURFACE WAVE ALONG THE FRONT WEST OF COTULLA WHICH IS HELPING TO
ENHANCE MOISTURE CONVERGENCE AND INSTABILITY TRANSPORT ACROSS THE
REGION. THIS LOW CENTER MAY WELL BE A SURFACE REFLECTION OF WHAT
REMAINS A RATHER WELL-DEFINED MCV A TAD FARTHER WEST BACK OVER
COAHUILA IN NORTHERN MEXICO.

PWATS REMAIN VERY HIGH WITH VALUES OF 2.2 TO 2.4 INCHES...AND WITH
SIGNIFICANT CONTRIBUTION FROM CONCENTRATED MID/HIGH LEVEL TROPICAL
MOISTURE STREAMING NORTH ACROSS MAINLAND MEXICO FROM THE FAR
EASTERN TROPICAL PACIFIC AS NOTED IN CIRA-LPW IMAGERY. THIS
CONTINUES TO SUPPORT HIGHLY EFFICIENT RAINFALL PROCESSES WITH
RAINFALL RATES CAPABLE OF EXCEEDING 3 INCHES/HR WITHIN THE
STRONGER CONVECTIVE CORES.

THE LATEST HIRES MODEL GUIDANCE INCLUDING THE 12Z ARW/NMMB AND
NSSL-WRF ARE ALL APPEARING TO BE UNDERDONE WITH THEIR CONVECTIVE
OUTPUT...AND THE LATEST HRRR APPEARS TO HAVE ITS HEAVIER QPF AXIS
NEAR THE FRONT DISPLACED A BIT TOO FAR BACK TO THE NORTHWEST. THE
LATEST SATELLITE AND RADAR TRENDS CERTAINLY WOULD TEND TO FAVOR
STRONGER CONVECTION AND HEAVIER RAINFALL A BIT FARTHER SOUTHEAST
AND GENERALLY INCLUSIVE OF SOUTH-CENTRAL AND SOUTH TX OVER THE
NEXT FEW HOURS. THIS INCLUDES THE SAN ANTONIO METROPOLITAN AREA.
EXPECT LOCALLY AN ADDITIONAL 3 TO 6 INCHES OF RAIN TO BE POSSIBLE
THROUGH LATE EVENING AS CONVECTIVE BANDS/CLUSTERS CONTINUE TO
DEVELOP SOUTHEAST OF THE FRONT AND LIFT NORTHWEST INTO THE AREA OF
MAXIMIZED SURFACE/FRONTAL CONVERGENCE.

GIVEN THE ALREADY SATURATED SOIL CONDITIONS ACROSS MUCH OF THE MPD
THREAT AREA...ANY ADDITIONAL HEAVY RAINS WILL RESULT IN RUNOFF
PROBLEMS AND CONCERNS FOR FLASH FLOODING.

ORRISON

ATTN...WFO...BRO...CRP...EWX...SJT...

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

LAT...LON   30359912 30159810 29179772 28139774 27349786
            26919819 26739872 26889933 27409962 27649988
            27890015 28250035 28760058 29150092 29530087
            29990028