Flash Flood Guidance
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133
AWUS01 KWNH 260208
FFGMPD
TXZ000-260805-

MESOSCALE PRECIPITATION DISCUSSION 0827
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1007 PM EDT MON SEP 25 2017

AREAS AFFECTED...MID-RIO GRANDE VALLEY INTO SOUTH TEXAS PLAINS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 260205Z - 260805Z

SUMMARY...LOCALIZED HEAVY RAINFALL...ABOVE 6 INCHES THROUGH
08Z...APPEARS LIKELY WHICH WOULD CAUSE FLASH FLOODING OVER
PORTIONS OF THE MID-RIO GRANDE VALLEY INTO THE SOUTH TEXAS PLAINS.

DISCUSSION...OVER THE PAST 2 HOURS...KDFX RADAR IMAGERY SHOWED
TRANSIENT MESO-VORTICES EMBEDDED WITHIN A LARGER CLUSTER OF
CONVECTION SLOWLY MOVING ACROSS MAVERICK COUNTY TOWARD I-35.
RECENT COOLING ON EXPERIMENTAL GOES 16 10.3 MICRON IMAGERY OF -75
TO -80 C OVER EASTERN ZAVALA AND DIMMIT COUNTIES CORRESPONDED TO
DUAL-POL RADAR ESTIMATED RATES OF 1.5 TO 2.5 IN/HR. SLOW MOVEMENT
OF THESE HEAVY RAIN CORES IS AND WILL CONTINUE TO BE A CONCERN FOR
FLASH FLOODING. 00Z SOUNDINGS FROM BRO AND DRT SHOWED SELY FLOW OF
ABOUT 20 KT NEAR AND BELOW 850 MB WITHIN A PRECIPITABLE WATER
ENVIRONMENT OF 1.8 TO 1.9 INCHES AS WELL AS 500-1000 J/KG MLCAPE.

ALSO WORTH NOTING WAS A MID-UPPER LEVEL VORTICITY MAX...LIKELY
TIED TO THE REMNANTS OF PILAR IN THE EAST PACIFIC...LOCATED OVER
SRN COAHUILA AT 0130Z...TRACKING TOWARD THE NORTHEAST. AN AREA OF
COLDER CLOUD TOPS WAS NOTED AHEAD OF THIS FEATURE APPROACHING THE
RIO GRANDE NEAR THE MAVERICK/WEBB COUNTY BORDER.

THE RAP HAS BEEN STEADFAST WITH FORECASTS OF 30-40 KT 850 MB FLOW
DEVELOPING BY 06Z...FLOWING UP THE RIO GRANDE VALLEY AND THE SOUTH
TEXAS PLAINS. INCREASING MOISTURE TRANSPORT INTO THE ONGOING
CLUSTER OF CONVECTION WILL COMBINE WITH MODESTLY DIFFLUENT UPPER
LEVEL FLOW WHICH WILL LIKELY CONTINUE TO SUPPORT SLOW MOVING
BURSTS OF HEAVY RAIN. HOWEVER...FORECASTING THE LONG TERM MOVEMENT
AND DISSIPATION OF THESE MESO-GAMMA SCALE VORTICES TIED TO THE
HEAVIEST RAINFALL RATES IS DIFFICULT TO DO. STILL...THE REGION
FROM THE RIO GRANDE...ACROSS I-35 TO NEAR ROUTE 16 APPEARS LIKELY
TO REMAIN IN A FAVORABLE ENVIRONMENT FOR CONTINUED HEAVY RAIN OVER
THE NEXT SEVERAL HOURS WITH LOCALIZED ADDITIONAL RAINFALL TOTALS
OVER 6 INCHES EXPECTED BY 08Z.

OTTO

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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

LAT...LON   29319999 29319925 28769865 28029847 26909851
            26389879 26339928 27249980 28470062 29120048




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