MESOSCALE PRECIPITATION DISCUSSION 0262
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
400 PM EDT FRI SEP 20 2013
 
AREAS AFFECTED...TX HILL COUNTRY TO THE ARKLATEX 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 202000Z - 210200Z
 
SUMMARY...A BAND OF HEAVY RAINFALL IS EXPECTED TO SLOWLY SHIFT
SOUTHEAST OVER THE NEXT SEVERAL HOURS.  AS THE BAND IS MOVING INTO
INCREASING FLASH FLOOD GUIDANCE VALUES, REGIONAL FLASH FLOODING IS
CONSIDERED POSSIBLE.

DISCUSSION...A LONG BAND OF HEAVY RAIN EXTENDS FROM THE TX HILL
COUNTRY NORTHEAST THROUGH EASTERN TX TOWARDS THE LA/AR BORDER,
WITH SCATTERED THUNDERSTORMS COVERING PORTIONS OF THE UPPER TX
COASTAL PLAIN AND WESTERN LA.  THE 15Z HRRR APPEARS TO BE HANDLING
THE CONVECTIVE PATTERN THE BEST WITHIN THIS REGION AT THIS TIME. 
THE MAIN FOCUS OF FOR THE RAIN BAND IS A FRONTAL ZONE EXTENDING
FROM UVALDE TX NORTHEAST THROUGH THE PINEY WOODS OF EASTERN TX
TOWARDS THE CENTRAL LA/AR BORDER.  PRECIPITABLE WATER VALUES OF
2-2.5 INCHES, IN THE 95TH PERCENTILE FOR LATE SEPTEMBER, ARE
HELPING INCREASE PRECIPITATION EFFICIENCY.  THE 12Z ARW/18Z RAP
INDICATE CAPE VALUES OF 1000-2000 J/KG IN THE WARM SECTOR ARE
BEING ADVECTED TOWARDS THE FRONTAL ZONE.  LOW-LEVEL INFLOW/0-3 KM
SHEAR OF AROUND 35 KTS EXISTS NEAR THE TX/LA BORDER.  THIS
COMBINATION OF CAPE AND LOW-LEVEL SHEAR FAVORS THE DEVELOPMENT OF
WET MACROBURST MULTICELLS.  AN UPPER-LEVEL SHORTWAVE MOVING
THROUGH THE SOUTHERN HIGH PLAINS IS ACTING AS THE MAIN SOURCE OF
DIVERGENCE ALOFT.  THE 850-400 HPA FLOW IS PARALLEL TO THE FRONTAL
BOUNDARY, WHICH IS A PLUS FOR HEAVY RAINFALL.

AS THE FRONTAL ZONE SLOWLY SHIFTS EASTWARD, THE BAND SHOULD MOVE
IN SUIT.  THE MAIN AXIS OF LOW-LEVEL INFLOW IS EXPECTED TO SHIFT
TOWARDS THE LA/MS BORDER, WHICH WAS USED TO DEFINE THE EASTERN
BOUND OF THE AREA.  AS LOW-LEVEL INFLOW AND INSTABILITY IS
EXPECTED TO PERSIST, THE THREAT OF FLASH FLOODING SHOULD CONTINUE
INTO THE EVENING HOURS. RAINFALL RATES OF 1-2" PER HOUR WERE
RECENTLY NOTED IN RADAR IMAGERY.  COMBINED WITH RELATIVELY HIGH
FLASH FLOOD GUIDANCE VALUES IN THE AREA, REGIONAL FLASH FLOODING
IS CONSIDERED POSSIBLE.


ROTH/TERRY

ATTN...WFO...JAN...LZK...LCH...SHV...HGX...FWD...EWX...

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

LAT...LON   29659584 29119783 29229889 30519771 32179577 33029405
            33949188 33749099 32439122 30909309 29659584 
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