MESOSCALE PRECIPITATION DISCUSSION 0078
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1104 AM EDT WED MAR 28 2018

AREAS AFFECTED...SOUTH-CENTRAL TO SOUTHEAST TX

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

VALID 281500Z - 282100Z

SUMMARY...SHOWERS AND THUNDERSTORMS WITH INTENSE RAINFALL RATES OF
UP TO 2 INCHES/HR ARE EXPECTED TO BECOME MORE FOCUSED OVER THE
NEXT FEW HOURS. FLASH FLOODING IS LIKELY.

DISCUSSION...GOES-16 1-MINUTE WV IMAGERY SUITE SHOWS A FAVORABLY
DIVERGENT FLOW PATTERN ALOFT OVER MUCH OF SOUTH-CENTRAL TX OUT
AHEAD OF AN UPPER TROUGH EJECTING EAST OUT ACROSS THE SOUTHERN
HIGH PLAINS. THIS IS ALREADY FACILITATING UPSCALE GROWTH OF
CONVECTION IN AND AROUND THE GREATER SAN ANTONIO METROPOLITAN
AREA. CLOUD TOPS IN GENERAL WITH THIS CONVECTION AND ALSO
CONVECTION OVER SOUTHEAST TX TO THE NORTHWEST OF HOUSTON ARE
TENDING TO COOL AND GIVEN THE APPROACHING UPSTREAM DYNAMICS AND
PROXIMITY OF A GRADUALLY DESTABILIZING BOUNDARY LAYER...ADDITIONAL
CONVECTIVE ORGANIZATION IS LIKELY TO OCCUR.

THE STRONGER INSTABILITY PARAMETERS ARE CURRENTLY OVER
SOUTH-CENTRAL TX WITH MUCAPE VALUES OF 1000 TO 2000 J/KG. DIURNAL
HEATING SHOULD GRADUALLY ALLOW THIS INSTABILITY TO BECOME MORE
ROOTED NEAR THE SURFACE OVER THE NEXT SEVERAL HOURS...AND THIS
INSTABILITY IS EXPECTED TO POOL NORTHEASTWARD ALONG AN OUTFLOW
BOUNDARY SITUATED AHEAD OF A COLD FRONT. IN TIME...THE COLD FRONT
SHOULD SETTLE SOUTHEAST AND LIKELY WILL MERGE WITH THE OUTFLOW
BOUNDARY AND WILL FOSTER AN ENHANCED AXIS OF SURFACE CONVERGENCE.
THIS COUPLED WITH THE IMPROVING THERMODYNAMIC ENVIRONMENT AND
LARGER SCALE ASCENT IS EXPECTED TO RESULT IN A SW/NE AXIS OF
FOCUSED CONVECTION WITH POTENTIAL FOR TRAINING CELL ACTIVITY.

PWATS ARE RATHER HIGH WITH VALUES OF 1.5 TO 1.7 INCHES...BUT THE
CIRA-LPW DATA SETS CONFIRM A RATHER DEEP POOLING OF THE MOISTURE
WITH A NOTABLE EASTERN TROPICAL PACIFIC CONNECTION SEEN IN THE
500/300 MB LAYER. THIS SUPPORTS GREATER RAINFALL EFFICIENCY FOR
INTENSE RAINFALL RATES WHEN CONSIDERING THE VERY COLD CLOUD
TOPS/STRONG UPDRAFT SIGNALS. ADDITIONALLY...THE LATEST RAP
FORECASTS THE PERSISTENCE OF A MOIST 40 TO 45 KT S/SW LOW LEVEL
JET AHEAD OF THE MAIN COLD FRONT.

EXPECT RAINFALL RATES OF UP TO 2 INCHES/HR AS CONVECTION
ORGANIZES...AND WITH THE CONVECTION BECOMING MORE ALIGNED WITH THE
MEAN LAYER FLOW...TRAINING CONVECTION WILL FOSTER THE POTENTIAL
FOR AS MUCH AS 3 TO 6 INCHES OF RAIN AT LEAST LOCALLY THROUGH 21Z.
THIS IS CONSISTENT WITH THE LATEST HRRR OUTPUT. FLASH FLOODING
WILL BECOME LIKELY...AND ESPECIALLY OVER ANY AREAS THAT SAW HEAVY
RAINFALL EARLIER THIS MORNING AND LAST NIGHT.

ORRISON

ATTN...WFO...CRP...EWX...FWD...HGX...LCH...SHV...

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

LAT...LON   31879460 31679402 31409381 30989396 30639444
            30099539 29619626 29139703 28529827 28339912
            28639943 29209926 29759874 30069819 30349761
            30869665 31519548