MESOSCALE PRECIPITATION DISCUSSION 0834
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
741 AM EDT WED SEP 27 2017

AREAS AFFECTED...SOUTH-CENTRAL AND SOUTH TEXAS

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

VALID 271140Z - 271730Z

SUMMARY...WIDESPREAD RAIN WILL CONTINUE ACROSS SOUTH-CENTRAL AND
SOUTH TEXAS TO THE EAST OF A MID-LEVEL CIRCULATION OVER NORTHERN
MEXICO. SMALL-SCALE VERY HEAVY RAIN BANDS MAY PRODUCE RAIN RATES
UP TO 2-3 IN/HR AND LOCALIZED FLASH FLOODING.

DISCUSSION...A PROMINENT MESOSCALE CONVECTIVE VORTEX (MCV)
CONTINUES TO BE CENTERED OVER FAR NORTHERN MEXICO WITH A PLUME OF
VERY MOIST AIR JUST TO THE EAST OVER SOUTHERN TEXAS. WITHIN THIS
MOIST ENVIRONMENT...PRECIPITABLE WATER VALUES ARE ON THE ORDER OF
2.0 TO 2.5 INCHES (CLOSE TO 200% OF NORMAL) WHICH IS SUSTAINING A
BROAD AREA OF MODERATE TO OCCASIONALLY HEAVY RAIN. GIVEN THE SLOW
DRIFT OF THE MCV...PORTIONS OF THE OUTLINED DISCUSSION AREA HAVE
ALREADY RECEIVED SIGNIFICANT RAINFALL OVER THE PAST COUPLE
DAYS...PARTICULARLY IN THE VICINITY OF LAREDO AND AREAS JUST TO
THE NORTH AND NORTHEAST...WITH LOCALIZED TOTALS UPWARDS OF 10
INCHES PER SEVERAL PRECIPITATION ANALYSES.

THE RAIN IS EXPECTED TO PERSIST THROUGH THE MORNING HOURS IN THE
PLUME OF DEEP TROPICAL MOISTURE...AND MAY CONTRIBUTE TO A MORE
GRADUAL AREAL FLOOD THREAT OVER A BROADER AREA. THIS MAY BE
PARTICULARLY TRUE IN THE AXIS OF STRONGEST LOW-LEVEL MOISTURE
TRANSPORT CLOSER TO THE RIO GRANDE RIVER. HOWEVER...SMALLER SCALE
RAIN BANDS ARE LIKELY TO DEVELOP IN FAVORED AREAS OF LOW-LEVEL
CONVERGENCE THAT ARE LIKELY TO BE MORE LOCALIZED AND TRANSIENT
(TIME SCALES OF 1-3 HOURS). THESE FOCUSED VERY HEAVY RAIN BANDS
COULD PRODUCE RAIN RATES IN THE 2-3 IN/HR RANGE...CERTAINLY
SUFFICIENT TO PRODUCE FLASH FLOODING GIVEN THE BROAD AREA OF
SATURATED SOILS.

THE LOWEST FLASH FLOOD GUIDANCE VALUES ROUGHLY COINCIDE WITH THE
HIGHEST RAINFALL TOTALS OVER THE PAST 24-48 HOURS AND ARE
GENERALLY SOUTH AND SOUTHEAST OF THE AREAS THAT ARE CURRENTLY
SEEING THE MOST PERSISTENT RAIN AND LOCALIZED HEAVIER RAIN BANDS.
HOWEVER...GIVEN GREATER INSTABILITY TO THE SOUTH...CONVECTIVE RAIN
BANDS MAY HAVE A PROPENSITY TO REDEVELOP GRADUALLY FURTHER SOUTH
AND SOUTHEAST WITH TIME...AND INTO AREAS THAT WOULD BE MORE
HYDROLOGICALLY PRIMED FOR FLASH FLOODING. AN AXIS OF LOW-LEVEL
CONVERGENCE STRETCHING IN THE SAN ANTONIO TO VICTORIA CORRIDOR MAY
ALSO PROVIDE ANOTHER FOCUSING MECHANISM FOR CONVECTIVE RAIN BANDS
AND LOCALIZED FLASH FLOODING.

LAMERS

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

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

LAT...LON   30829996 30729940 30469885 30189823 29789781
            29399740 28789689 28369675 28089679 27729718
            27219746 26899790 26629849 26459892 26539922
            26769930 26949946 27219957 27469961 27649979
            27750003 27930002 28100018 28260032 28420039
            28670052 28950071 29090079 29260094 29410117
            29540133 29700148 29710172 29740200 29840204
            30280180 30590128 30700058