Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 251802
FFGMPD
TXZ000-252200-

MESOSCALE PRECIPITATION DISCUSSION 0825
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
201 PM EDT MON SEP 25 2017

AREAS AFFECTED...MIDDLE RIO GRANDE VALLEY

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

VALID 251800Z - 252200Z

SUMMARY...SMALL CLUSTER OF VERY SLOW-MOVING THUNDERSTORMS WILL
LIKELY PERSIST OVER THE NEXT FEW HOURS WITH EXTREMELY HEAVY
RAINFALL. FLASH FLOODING WILL BE LIKELY AS THESE RAINS ARRIVE
ACROSS PARTS OF THE MIDDLE RIO GRANDE VALLEY.

DISCUSSION...NON-OPERATIONAL GOES-16 10.3 MICRON/IR IMAGERY SHOWS
A HIGHLY PERSISTENT AND FOCUSED CLUSTER OF THUNDERSTORMS MOVING
VERY SLOWLY EAST ACROSS THE NORTHERN MEXICAN STATE OF COAHUILA.
THE CONVECTION WHICH HAS BEEN ONGOING FOR SEVERAL HOURS NOW IS
EXPECTED TO BEGIN CROSSING THE RIO GRANDE RIVER AND INTO ADJACENT
TX COUNTIES ON THE U.S. SIDE OF THE BORDER...AND SPECIFICALLY
KINNEY AND MAVERICK COUNTIES IN THE SHORT TERM.

THE CONVECTION IS BEING AIDED BY DIFFLUENT FLOW ALOFT ASSOCIATED
WITH STREAMING VORT ENERGY LIFTING NORTH-NORTHEAST ACROSS CENTRAL
AREAS OF MAINLAND MEXICO AND WITH A VERY MOIST COLUMN IN PLACE
THAT IS BEING AIDED IN PART BY WEAKENING T.D. PILAR NEAR THE
SOUTHWEST PACIFIC COAST OF MEXICO. MEANWHILE...THERE IS PLENTY OF
LOW LEVEL SOUTHEAST FLOW AIMING UP THE RIO GRANDE VALLEY WHICH IS
HELPING TO TRANSPORT GREATER DIURNALLY ENHANCED INSTABILITY INTO
THE AREA OF ACTIVE CONVECTION. GIVEN DOWNSTREAM MUCAPE VALUES OF
1000 TO 1500 J/KG...AND EFFECTIVE BULK SHEAR VALUES OF 30 TO 35
KTS...THE CURRENT CONVECTIVE CLUSTER WILL LIKELY MAINTAIN A FAIRLY
HIGH DEGREE OF ORGANIZATION OVER THE NEXT FEW HOURS AS IT MOVES
VERY SLOWLY EASTWARD.

EXTREMELY HEAVY RAINFALL RATES OF UP TO 3 INCHES/HR ARE EXPECTED
BASED ON KDFX DUAL-POL RADAR ESTIMATES AND TRENDS ALSO SEEN WITH
GOES-16 SATELLITE IMAGERY WHICH SHOW SOME ADDITIONAL COOLING OF
CONVECTIVE TOPS INCLUDING A COUPLE OF VERY WELL-DEFINED
OVERSHOOTING TOPS WHICH ARE INDICATIVE OF HIGHLY ORGANIZED/FOCUSED
AND STRONG UPDRAFTS. EXPECT AS MUCH AS 4 TO 6+ INCHES OF RAIN TO
BE POSSIBLE AS THIS SMALL MCS MOVES EAST. THESE RAINFALL AMOUNTS
ARE LIKELY TO CAUSE SOME FLASH FLOODING.

ORRISON

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

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

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

LAT...LON   29660021 29359927 28489868 27779868 27629926
            27689969 28050012 28290039 28730071 29380078




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