MESOSCALE PRECIPITATION DISCUSSION 0424
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1140 AM EDT MON JUL 09 2018

AREAS AFFECTED...EASTERN TEXAS HILL COUNTRY AND I-35 CORRIDOR

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

VALID 091540Z - 091930Z

SUMMARY...NEARLY STATIONARY RAIN AND THUNDERSTORMS HAVE PERSISTED
THIS MORNING JUST NORTHWEST OF SAN ANTONIO...AND AREA CONTRIBUTING
TO A CONCENTRATED AREA OF HEAVY RAINFALL. CONTINUATION OF THIS
ACTIVITY...OR ADDITIONAL REDEVELOPMENT OF SLOW-MOVING
THUNDERSTORMS THIS AFTERNOON MAY LEAD TO SOME FLASH FLOODING IN
THE EASTERN PORTION OF TEXAS HILL COUNTRY... NEAR THE BALCONES
ESCARPMENT... AND INTO THE AUSTIN-SAN ANTONIO PORTION OF THE I-35
CORRIDOR. RAIN RATES COULD REACH 1-2 IN/HR.

DISCUSSION...KEWX RADAR SHOWS A PERSISTENT BAND OF CONVECTION THAT
IS SITUATED JUST NORTHWEST OF SAN ANTONIO...ROUGHLY NEAR THE
TERRAIN GRADIENT OF THE BALCONES ESCARPMENT...MARKING THE EASTERN
EXTENT OF TEXAS HILL COUNTRY. IT WAS ALSO ON THE NOSE OF SOME
BROAD SOUTHERLY LOW-LEVEL INFLOW WHICH MAY BE COMBINING WITH
EXISTING THUNDERSTORM OUTFLOW BOUNDARY AND COLD POOL...AS WELL AS
THE TERRAIN GRADIENT TO PROVIDE FOR REGENERATION OF LOW-TOPPED
CONVECTION THAT THEN QUICKLY LIFTS INTO THE EXISTING CONVECTIVE
BAND. IT IS UNCERTAIN HOW LONG THIS WILL CONTINUE...AS WE ARE
APPROACHING A TIME OF DAY (15-17Z) WHEN STRONGER INSTABILITY TENDS
TO BUILD AND CONVECTION CAN BECOME DEEPER. THIS MAY ALLOW FOR
DIFFERENT PROPAGATION AND EVOLUTION IN A COUPLE HOURS TIME.
NEVERTHELESS...LOW-TOPPED CONVECTION CONTINUES TO PERSIST OVER
AREAS THAT KEWX DUAL POL ESTIMATES PLACE 2-4 INCHES OF RAINFALL
ALREADY IN THE PAST SIX HOURS.

THERE WILL ALSO BE THE POSSIBILITY OF REDEVELOPMENT IN THE
AFTERNOON. HI-RES MODELS FAIL TO SHOW MUCH FOCUSED CONVECTIVE
DEVELOPMENT DURING THE AFTERNOON...BUT THEY MAY BE UNDERESTIMATING
THE STRENGTH OF THE COLD POOL AND CONVECTIVELY INFLUENCED
WEST-EAST OUTFLOW BOUNDARY THAT SHOULD BE NEARLY STATIONARY OVER
THE NEXT FEW HOURS. THE RAP MODEL FORECASTS LOW-LEVEL SOUTHERLY
FLOW TO INCREASE BY 18Z...PARTICULARLY THE PORTION THAT WILL BE
FOCUSED INTO THE SAN ANTONIO TO UVALDE AREA. THIS MAY ALLOW FOR
REDEVELOPMENT OF DEEPER CONVECTION NEAR AREAS THAT HAVE ALREADY
RECEIVED HEAVY RAINFALL.

CONFIDENCE IS SLIGHTLY BELOW AVERAGE GIVEN A LACK OF HI-RES MODEL
SUPPORT...BUT GIVEN THE VERY MOIST ENVIRONMENT (PWATS AROUND 2.0
TO 2.1 INCHES)...ALREADY HEAVY RAINFALL...AND ONGOING HYDROLOGIC
RESPONSE INDICATED BY THE CREST MODEL IN PORTIONS OF
BANDERA...KENDALL...AND NORTHWEST BEXAR COUNTIES...IT DOES APPEAR
THAT ANY ADDITIONAL RAINFALL COULD EXACERBATE THE FLOOD POTENTIAL
IN A CLIMATOLOGICALLY FAVORABLE REGION FOR FLASH FLOODING.

LAMERS

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

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

LAT...LON   30499762 30109748 29329812 28989906 29239981
            29859972 30459844