MESOSCALE PRECIPITATION DISCUSSION 0676
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
710 PM EST SAT DEC 12 2015
 
AREAS AFFECTED...EASTERN TX...FAR SOUTHEAST OK 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 130010Z - 130500Z
 
SUMMARY...HIGH MOISTURE CONTENT AIR IN A STRONG LOW LEVEL FLOW
TAPS INSTABILITY TO PRODUCE HEAVY TO EXCESSIVE RAINFALL ACROSS
PORTIONS OF EASTERN TX/FAR SOUTHEAST OK THROUGH THE LATE EVENING
HOURS. FLASH FLOOD IS LIKELY IN THIS SETUP.

DISCUSSION...A BAND OF HEAVY RAIN HAS ALREADY BECOME ESTABLISHED
IN THE STRENGTHENING LOW LEVEL FLOW ACROSS EASTERN TX INTO FAR
SOUTHEAST OK. THE TPW  PRODUCT SHOWED PRECIPITABLE WATER VALUES AS
HIGH AS 1.90 INCHES (WHICH IS BETWEEN THREE AND FOUR STANDARD
DEVIATIONS ABOVE THE MEAN) STREAMING NORTH FROM THE WESTERN GULF
AND NEARBY EASTERN PACIFIC ON A 40 TO 50 KNOT LOW LEVEL FLOW (PER
RECENT VWP). INSTABILITY HAS INCREASED IN THE FLOW IN THE PAST FEW
HOURS (AS SEEN ON THE KCRP 20Z RAOB)...WITH MLCAPE VALUES BETWEEN
1500 AND 2000 J/KG IN THE INFLOW ACROSS SOUTH TX.

LOCAL RADARS (SUCH AS KGRK AND KFWS) AND THE MOST RECENT MRMS
PASSES SUGGEST HOURLY RAINFALL RATES BETWEEN 0.75 AND 1.50
INCHES...THOUGH GROUND TRUTH HAS BEEN CLOSER TO AN INCH ACROSS
PORTIONS OF NORTHEAST TX. ONE AND THREE HOUR FLASH FLOOD GUIDANCE
VALUES ARE STILL RELATIVELY HIGH...BUT THE ONGOING HEAVY RAINFALL
SHOULD ALLOW THESE VALUES TO DROP INTO THE EVENING HOURS. 

THERE IS GOOD HIGH RESOLUTION MODEL AGREEMENT THAT BANDS OF HEAVY
RAINFALL ARE EXPECTED ACROSS EAST TX INTO FAR SOUTHEAST TX THROUGH
THE LATE EVENING HOURS. FOR THE MOST PART...THE HIGH RESOLUTION
MODELS HAVE BEEN UNDERESTIMATING THE INSTABILITY IN PLACE (BASED
ON THE KCRP SHOWING AND MLCAPE VALUES DEPICTED IN THE SPC
MESOANALYSIS)...AND THIS COULD BECOME IMPORTANT WHEN ASSESSING THE
FLASH FLOOD THREAT. 

THE MAJORITY OF HIGH RESOLUTION MODELS SUGGEST 2 TO 4 INCHES OF
RAINFALL IN NORTH/SOUTH ORIENTED CONVECTIVE BANDS THROUGH 13/05Z.
IT SHOULD BE NOTED THAT THE 22Z HRRR SHOWED THE POTENTIAL FOR 6+
INCHES OF RAINFALL ACROSS EASTERN TX IN A SINGLE CONVECTIVE BAND.
HOWEVER...THE 23Z HRRR (AS WELL AS THE MOST RECENT EXPERIMENTAL
HRRR) WERE NOT AS BULLISH WITH QPF AMOUNTS. 

AS THE HEAVY RAIN IS PLACE MOISTENS THE SOIL...THE MORE
CONVECTIVELY DRIVEN RAINFALL SHOULD BE ENOUGH TO PRODUCE FLASH
FLOODING. IN ADDITION...TRAINING OF THE CELLS IN THE CONVECTIVE
BANDS IS POSSIBLE AS THE ENTIRE AREA DRIFTS SLOWLY EASTWARD. FLASH
FLOODING IS CONSIDERED LIKELY INTO LATE EVENING...DUE TO THE
COMBINATION OF HIGH MOISTURE AIR AND THE POTENTIAL FOR TRAINING.

HAYES

ATTN...WFO...EWX...FWD...HGX...LZK...OUN...SHV...TSA...

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

LAT...LON   35029497 34789457 33959427 32969444 31479484 
            30579525 29559583 29639650 30359690 31249682 
            32159657 32759643 33249624 33209624 34559577 
            34949543 
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