Flash Flood Guidance
Issued by NWS

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826
AWUS01 KWNH 210241
FFGMPD
TXZ000-210839-

MESOSCALE PRECIPITATION DISCUSSION 0108
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1040 PM EDT WED MAY 20 2015

AREAS AFFECTED...SOUTHWEST TX

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

VALID 210239Z - 210839Z

SUMMARY...ORGANIZED CONVECTION IS EXPECTED TO MAINTAIN RAINFALL
EFFICIENCY OVER THE NEXT SEVERAL HOURS, WITH HOURLY RAIN RATES OF
2-3" REMAINING POSSIBLE.  FLASH FLOODING IS A CONCERN HERE.

DISCUSSION...RADAR TRENDS SINCE 20Z INDICATE THAT CONVECTION
ACROSS WEST-CENTRAL TX HAS BEEN EXPANDING WHILE DROPPING SOUTHEAST
AT AROUND 12 KTS, ROUGHLY IN LINE WITH THE 1000-500 HPA THICKNESS
PATTERN AND MAGNITUDE OF THE FORWARD PROPAGATION VECTORS.  CLOUD
TOPS CONTINUE TO COOL OVER THIS CLUSTER.  CAPPING ACROSS SOUTHERN
TX (SHOWING UP AS 9-12C IN THE 700 HPA TEMPERATURE FIELD) APPEARS
TO BE CAUSING CONVECTION IN NORTHEAST MX TO WEAKEN/BECOME MORE
ISOLATED TO THE WEST OF THE RIO GRANDE, HINTING IT SHOULD NOT ROB
INFLOW OR INSTABILITY FROM THE MORE NORTHERN CONVECTION.  INFLOW
AT 850 HPA IS 20-25 KTS PER RECENT VAD WIND PROFILES, 5-10 KTS
GREATER THAN RECENT RAP FORECASTS AND CLOSE TO DOUBLE THE MEAN
850-400 HPA WIND FLOW IN THE REGION.  PRECIPITABLE WATER VALUES OF
CLOSE TO 1.75" EXIST, ABOVE THE 90TH PERCENTILE FOR MID TO LATE
MAY.  CAPE VALUES (MIXED LAYER AND MOST UNSTABLE) OF 2000-3500
J/KG LIE NEAR AND AHEAD OF THE SLOWLY PROGRESSIVE COLD FRONT.
HOURLY RAIN RATES AS OF LATE HAVE MAXIMIZED IN THE 2-3" AN HOUR
RANGE, ABOVE FLASH FLOOD GUIDANCE VALUES.

WITH RECENT RAP FORECASTS SHOWING AN INCREASE IN THE LOW-LEVEL
FLOW TO 30+ KTS AND THE 850-400 HPA FLOW COLLAPSING TO LIGHT AND
VARIABLE, THINK HOURLY RAIN RATES OF 2-3" SHOULD REMAIN POSSIBLE
FOR THE NEXT SEVERAL HOURS WHILE THE CONVECTION CONTINUES TO
ORGANIZE AND MOVE SOUTHEAST CLOSE TO 15 KTS.  THE 18Z SSEO
NEIGHBORHOOD PROBABILITIES SUGGEST THE HEAVY RAIN THREAT SHOULD
PERSIST UNTIL AT LEAST 09Z.  THE 12Z RUNS OF THE ARW, NMM,
WRF4NSSL, NAM CONEST AND 00Z HRRR APPEAR TO HAVE THE BEST HANDLE
ON THE CURRENT/EXPECTED CONVECTIVE ACTIVITY.  THE WORRY IS THAT
THIS CONVECTIVE AREA COULD DISPLAY LOCALLY HIGHER HOURLY RAIN
RATES WHILE IT MERGES WITH SCATTERED CONVECTION TO THE SOUTH.
SHORT PERIODS OF CELL TRAINING ALONG WITH CELL MERGERS SHOULD
ALLOW FLASH FLOODING TO BE POSSIBLE OVER THE NEXT SEVERAL HOURS.

ROTH

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...EWX...FWD...SJT...

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

LAT...LON   31059841 31049842 30819804 29659727 29409756
            29149828 28749954 28470024 28570041 29240076
            29940147 30290112 30640012



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