MESOSCALE PRECIPITATION DISCUSSION 0095
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
233 PM EDT FRI APR 13 2018

AREAS AFFECTED...ARKANSAS...SE OK...NE TX...FAR NW LA

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

VALID 131832Z - 140030Z

SUMMARY...AN INCREASING THREAT OF FLASH FLOODING IS EXPECTED THIS
AFTERNOON ACROSS THE ARKLATEX REGION AS THUNDERSTORMS BECOME MORE
NUMEROUS. HOURLY RAIN RATES OF UP TO 2 IN/HR WILL BE POSSIBLE WITH
LOCALIZED RAIN TOTALS THROUGH 00Z (7 PM CDT) POSSIBLY REACHING 4-5
INCHES. THE THREAT OF FLASH FLOODING IS EXPECTED TO CONTINUE
BEYOND THE EXPIRATION TIME OF THIS PRODUCT.

DISCUSSION...ACROSS THE ARKLATEX REGION THE LOW-LEVEL FLOW WAS
QUITE STRONG FOR THE 18Z TIMEFRAME...TYPICALLY A DIURNAL LULL IN
THE STRENGTH OF THE LOW-LEVEL JET. HOWEVER...RADAR VWPS FROM KSHV
AND KLZK SHOWED 850MB FLOW AROUND 50 KNOTS WHICH WAS BOTH STEADY
FROM THE 12Z REGIONAL SOUNDINGS AND NEAR THE DAILY RECORD VALUES
IN SOUNDING CLIMATOLOGY FOR MID-APRIL. THE ANOMALOUSLY STRONG LLJ
WAS CONTRIBUTING TO A SUBSTANTIAL NORTHWARD FLUX OF BOUNDARY LAYER
MOISTURE...WHICH HAS LED TO A PERIOD OF DESTABILIZATION OVER THE
PAST SEVERAL HOURS ACROSS THE REGION. PRECIPITABLE WATER VALUES
ARE ALSO INCREASING...AND BY LATER THIS AFTERNOON SHOULD BE (PER
RAP FORECASTS) IN THE RANGE OF 1.7 TO 1.8 INCHES. THESE VALUES
WOULD BE NEAR...BUT JUST SHY OF...APRIL RECORD VALUES AT SHV AND
LZK IN THE SOUNDING CLIMATOLOGY. IN OTHER WORDS...THE AVAILABLE
MOISTURE AND THE NORTHWARD MOISTURE FLUX (DRIVEN BY THE LLJ) ARE
ANOMALOUSLY HIGH EVEN FOR MID-SPRING...AND THIS SHOULD DRIVE THE
THREAT OF HEAVY RAINFALL THIS AFTERNOON AND INTO THE EVENING HOURS.

THE CONFLUENT LOW-LEVEL FLOW IS EXPECTED TO BE FOCUSED ALONG THE
AXIS OF HIGHEST PWATS AND PARALLEL TO THE OVERALL DEEP LAYER FLOW.
FURTHERMORE...THE CORE OF THE LLJ WILL BE FOCUSING INTO THE SSW/SW
PERIPHERY OF THE RISK AREA (AND LIKELY INITIATING
CONVECTION)...WITH STRONGER INSTABILITY ALSO SITUATED TO THE SOUTH
AND SOUTHWEST. THIS MAY FAVOR PERIODS OF TRAINING CONVECTION WITH
SOME BACKBUILDING ON THE UPSHEAR FLANKS OF ORGANIZED MULTICELL
CLUSTERS INTO THE MOIST AND UNSTABLE INFLOW. THIS APPEARS MOST
LIKELY TO OCCUR FROM NORTHEAST TEXAS INTO CENTRAL ARKANSAS WHERE
THE HREF HAS THE HIGHEST PROBABILITIES OF 3+ INCHES OF RAINFALL BY
00Z (AS HIGH AS 50-60 PERCENT). STORMS WERE ALREADY INITIATING
ALONG THE DRYLINE OVER CENTRAL OKLAHOMA...AS WELL AS CLOSER TO THE
LOW-LEVEL THETAE AXIS IN NORTHEAST TEXAS. THE OUTLINED AREA OF
THIS DISCUSSION DOES NOT EXTEND AS FAR W/NW AS CENTRAL OKLAHOMA
AND THE DRYLINE...AS PWATS DROP OFF THAT FAR NORTHWEST. GOES-16
AIR MASS RGB SHOWS A CHANNEL OF DRIER AIR IN THIS REGION AND THAT
SHOULD MAKE THUNDERSTORMS IN THAT AREA LESS EFFICIENT AT RAINFALL
PRODUCTION. HOWEVER...THE STORMS THAT INITIATE CLOSER TO THE
MID-LEVEL MOISTURE GRADIENT DEPICTED ON THE AIR MASS RGB WOULD BE
MORE LIKELY TO CONGEAL INTO MULTICELL CLUSTERS... TRAIN... AND
PROPAGATE THROUGH A MORE FAVORABLE ENVIRONMENT FOR HEAVY RAINFALL.

LAMERS

ATTN...WFO...FWD...LZK...MEG...OUN...SHV...TSA...

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

LAT...LON   36249285 36249151 35579110 34659152 33339259
            32369360 31639504 31889599 32599666 33549633
            34429528 35589417