MESOSCALE DISCUSSION 0498
   NWS STORM PREDICTION CENTER NORMAN OK
   1210 PM CDT FRI APR 17 2009
   
   AREAS AFFECTED...PORTIONS SE AND E-CENTRAL TX.
   
   CONCERNING...HEAVY RAINFALL 
   
   VALID 171710Z - 171945Z
   
   SLOW EWD MOTION OF INITIAL MCS...AND ESPECIALLY ITS MERGER WITH
   SCATTERED-NUMEROUS TSTMS THAT HAVE DEVELOPED JUST TO ITS E BETWEEN
   VCT-CLL...WILL CONTRIBUTE TO SUSTAINED PERIODS OF 1.5-2.5 INCH/HOUR
   RAIN RATES.  THIS THREAT WILL SHIFT EWD ACROSS I-10 CORRIDOR BETWEEN
   SAT-HOU AND OVER HOU METRO...AS WELL AS OVER AREA BETWEEN
   HOU-TYR...THROUGH MID-AFTERNOON.  REF WWS 159-160 AND RELATED
   MESOSCALE DISCUSSIONS FOR SVR THREAT.
   
   SURFACE MESOANALYSIS CONTINUES TO INDICATE MARINE FRONTAL ZONE FROM
   OFFSHORE SABINE PASS WNWWD ACROSS GLS BAY AND INTERSECTING
   CONVECTION OVER AUSTIN COUNTY.  AIR MASS ALONG AND S OF THIS
   BOUNDARY WILL REMAIN CHARACTERIZED BY RICH MOISTURE...WITH SFC DEW
   POINTS UPPER 60S TO LOW 70S...AND GPS PW DATA SHOWING VALUES
   CONSISTENTLY AROUND 1.5 INCH. MODIFIED RAOBS AND RUC SOUNDINGS
   INDICATE NEARLY-SATURATED RH -- FROM SFC TO NEARLY 700 MB.  LOWER
   PORTION OF THIS DEEP SLAB OF MOIST AIR WILL BE LIFTED ATOP FRONTAL
   ZONE AMIDST 45-55 KT LLJ...WHICH ALSO WILL HELP TO ENHANCE
   STORM-RELATIVE INFLOW..AS WELL AS PROXIMITY TO AXIS OF MAX 850 MB
   MOISTURE TRANSPORT...AND ZONE OF DEEP LOW LEVEL CONVERGENCE/ASCENT. 
   MOISTURE...DEEP LIFT AND INSTABILITY...AND THEREFORE POTENTIAL FOR
   HEAVY RAIN...ALL WILL DIMINISH ROUGHLY N OF I-20.  FROM ABOUT 20 NM
   S OF I-10 TOWARD COAST...THREAT BECOMES MORE CONDITIONAL AND SPOTTY
   BECAUSE OF LACK OF SUSTAINED LOW LEVEL LIFT...ALTHOUGH SIMILAR RATES
   ARE LIKELY FROM INDIVIDUAL/LOCALIZED CORES.
   
   ..EDWARDS.. 04/17/2009
   
   
   ATTN...WFO...LCH...SHV...HGX...FWD...EWX...
   
   LAT...LON   29409768 29549732 30159684 31289612 32139608 32499576
               32479502 32119445 31229427 30359475 29719541 29309685
               29409768