MESOSCALE PRECIPITATION DISCUSSION 0705
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
747 PM EDT SUN SEP 25 2016
 
AREAS AFFECTED...PORTIONS OF SOUTH TX 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 252350Z - 260350Z
 
SUMMARY...SLOW MOVING CONVECTION AHEAD OF A COLD FRONT WILL
TRAIN...PRODUCING HOURLY RAINFALL RATES NEAR 2.00 INCHES. THIS
MUCH RAINFALL COULD RESULT IN A FLASH FLOOD THREAT THROUGH THE
LATE EVENING HOURS. 

DISCUSSION...REGIONAL RADARS ACROSS SOUTH TX SHOWED A DEVELOPING
LINE OF CONVECTION AHEAD OF A COLD FRONT APPROACHING FROM THE
WEST. THE CONVECTION WAS FORMING AND STRENGTHENING IN AN AXIS OF
100O-1500 J/KG MLCAPE...WITHIN A RIBBON OF 2.00+ INCH PRECIPITABLE
WATER AIR. THE KCRP RADAR SHOWED HOURLY RAINFALL RATES BETWEEN
1.50 AND 2.50 INCHES STRETCHING FROM ATASCOSA AND MCMULLEN
COUNTIES. 

THE IR LOOP SHOWED COOLING IN THE COLUMN...WITH CLOUD TOP
TEMPERATURES NEAR -70 CELSIUS. A 15 TO 20 KNOT LOW LEVEL SOUTHEAST
INFLOW (PER REGIONAL VWP) SHOULD CONTINUE TO FEED 2.00+ INCH
PRECIPITABLE
WATER AIR OFF THE WESTERN GULF OF MEXICO AHEAD OF THE COLD FRONT.
THE MID LEVEL WINDS HAVE WEAKENED AHEAD OF THE FRONT... WHICH
COULD LEAD TO SLOW CELL MOVEMENT NORTHWARD ALONG THE LINE. CORFIDI
VECTORS SUGGEST THAT THE LINE ITSELF SHOULD MOVE SLOWLY EAST AHEAD
OF THE FRONT. 

THE LATEST RAP SHOWED SUFFICIENT INSTABILITY (MUCAPE VALUES
REMAINING BETWEEN 1000-2000 J/KG) INTO THE LATE EVENING HOURS TO
SUPPORT CONVECTION AHEAD OF THE FRONT. IN ADDITION...SYNOPTIC
SCALE ASCENT AHEAD OF A SHORT WAVE APPROACHING THE BIG BEND AREA
LATER THIS EVENING SHOULD ENHANCE CONVECTIVE DEVELOPMENT AS WELL.
IN ADDITION...IR TOPS WERE COOLING QUICKLY SOUTH OF THE RIO GRANDE
RIVER IN NORTHERN MEXICO...AND THERE IS SOME INDICATION IN THE
HRRR THAT THIS ACTIVITY COULD AFFECT PORTIONS OF SOUTH TX AFTER
02Z.

THE 22Z HRRR WAS JUST ABOUT THE ONLY PIECE OF HIGH RESOLUTION
GUIDANCE TO CAPTURE THE ONGOING CONVECTION (AS THE 12Z WRF ARW AND
12Z NSSL WRF WERE TOO FAR WEST AND A COUPLE OF HOURS BEHIND
REALITY). THE HRRR SHOWED THE POTENTIAL FOR LOCAL 3-5 INCH
RAINFALL AMOUNTS FROM SOUTH OF KBAZ TO WEST OF KBKS THROUGH ABOUT
04Z. GIVEN THE RAINFALL RATES SEEN ON THE CRP RADAR... THESE
AMOUNTS (AS LOCAL MAXIMA) SEEM PLAUSIBLE.

ONE AND THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE FAIRLY HIGH
ACROSS MUCH OF SOUTH TX (GENERALLY ABOVE 2.50 INCHES)... SO
TRAINING COULD BE NECESSARY FOR THE FLASH FLOOD THREAT THROUGH
LATE EVENING.

HAYES

ATTN...WFO...BRO...CRP...EWX...

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

LAT...LON   29429789 29129750 28439750 28009765 26719827 
            26249856 26189889 26399929 27169930 27949891 
            28689834 28989823 29029822 29369804