MESOSCALE PRECIPITATION DISCUSSION 0614
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
719 AM EDT WED AUG 02 2017

AREAS AFFECTED...SOUTHEAST TX...SOUTHWEST LA

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

VALID 021118Z - 021645Z

SUMMARY...EXPECTED INCREASE IN ONSHORE BANDED CONVECTIVE CELLS
POSING RISK FOR FLASH FLOODING PARTICULARLY IN URBAN SETTINGS.

DISCUSSION...EXPERIMENTAL GOES-16 10.3UM IR DEPICTS A FEW NW-SE
SHALLOW CU BANDS WITH PERCOLATING TOPS THAT PRESENT THE SUGGESTION
THAT THE CAP HAS WEAKENED SUFFICIENTLY FOR FURTHER GROWTH AND
EXPANSION IN COVERAGE IN THE NEXT FEW HOURS.
RECENT SURFACE ANALYSIS ALSO SHOWS MODEST 1013 LOW OFF THE CENTRAL
TX COAST WITH SUBTLE BUT IMPORTANT SURFACE RIDGING ACROSS S
CENTRAL LA.  THIS PROVIDES A EXCELLENT CORRIDOR FOR MOISTURE
ADVECTION AND LOCALIZED INCREASE IN ONSHORE FLOW FOCUSED ON THE
UPPER TX COAST. 

CIRA LAYERED PW DOES SUGGEST THAT THE NARROW RELATIVELY DRY LAYER
SEEN IN 00Z SOUNDING FROM LCH HAS BEGUN TO MOISTEN WITH THE
APPROACH OF RETURN MID-UPPER LEVEL FLOW WITH 7-3H PWS SINKING
SOUTHWARD PINCHING OUT THE DRY ZONE NEAR/NORTH OF THE COAST.  
ADDITIONALLY...SUB-8H FLOW IS QUITE MOIST AS WELL WITH LOW 60S
850MB TDS AND LOW 70S AT THE SURFACE.  AS SUCH TPWS ARE JUST ABOVE
2.0" SUPPORTING WARM CLOUD/RAINFALL EFFICIENCY FOR THESE
DEVELOPING CELLS.

GOES-WV DOES SUGGEST SLOW SOUTHWARD DRIFT OF SHORTWAVE IN N TX
WILL AT LEAST PROVIDE GLANCING MID TO UPPER LEVEL
SUPPORT/EVACUATION FOR DEEPER ASCENT...WHILE ALSO PROVIDING SLOW
BUT EASTWARD CELL MOTIONS.  WITH STRONGER LOW LEVEL SUPPORT
PROPAGATION VECTORS ARE LIKELY TO SUPPORT SOME BACKBUILDING WHILE
AT THE SAME TIME ALLOWING FOR CONCENTRATION OF THE ONSHORE
CORRIDOR TO FOCUS IN THE UPPER TX COAST.  THIS MAY SUPPORT
ADDITIONAL ASCENT ON THE EASTERN SIDE WHERE FLOW/CELL MOTIONS
SUPPORT SLIGHT ANTICYCLONIC CONVERGENCE.   RAP/HI-RES GUIDANCE ARE
FAIRLY AGGRESSIVE IN STRENGTH/RAINFALL EFFICIENCY WITH RATES UP TO
2-2.5"/HR POSSIBLE.

SOME LIMITING FACTORS...MAINLY THE OVERALL DRY PATTERN OVER THE
LAST FEW WEEKS HAS LEFT GROUND CONDITIONS MORE SUPPORTIVE OF
ABSORBING/WITHSTANDING THE MAGNITUDES OF UP TO 2.5"/HR AND TOTALS
IN THE 2-4" RANGE.  OVERALL COVERAGE IS ALSO SUGGESTED BY GUIDANCE
TO BE LIMITED...EITHER WITH ONE CONSOLIDATING BAND OR MODERATELY
SCATTERED CONVECTIVE CORES...THINKING THE PRIOR IS MORE LIKELY AT
THIS TIME BUT THERE IS SOME LOWERED CONFIDENCE IN PRECISE
EVOLUTION.  

ALL CONSIDERED THERE REMAINS SUFFICIENT ENVIRONMENTAL AND GUIDANCE
SUPPORT TO SUGGEST FLASH FLOODING IS POSSIBLE THROUGH THE EARLY
MORNING HOURS...PARTICULARLY IN URBAN/TRADITIONAL SLOW DRAINING
AREAS.

GALLINA

ATTN...WFO...HGX...LCH...

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

LAT...LON   30639391 30299295 29549229 29689319 29519410
            29239471 28909508 29079539 29999539 30409474