MESOSCALE PRECIPITATION DISCUSSION 0052
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
725 PM EST WED FEB 28 2018

AREAS AFFECTED...NORTHEAST TX...SOUTHERN AR...NORTHERN
MS...SOUTHWEST TN...NORTHWEST AL

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 010030Z - 010500Z

SUMMARY...CONVECTION TRAINING OVER AREAS WITH SATURATED SOILS IS
EXPECTED TO PRODUCE FLASH FLOODING INTO THE LATE EVENING HOURS.

DISCUSSION...REGIONAL RADARS SHOWED AXES OF LOW TOPPED CONVECTION
FORMING AND MOVING IN A BROAD WARM AIR ADVECTION PATTERN AHEAD OF
A SHORT WAVE OVER THE SOUTHWEST STATES. THE BEST CONVECTION
EXTENDED FROM NORTHEAST TX INTO SOUTHERN AR AND NORTHERN
MS...CLOSEST TO THE BEST INSTABILITY (WHICH FORMED DURING
DIFFERENTIAL HEATING DURING THE AFTERNOON HOURS). THE MOST RECENT
GOES-16 CLEAN IR LOOP SHOWED A LONG FETCH OF MOISTURE STRETCHING
FROM THE EASTERN PACIFIC ACROSS WEST TX INTO THE LOWER MS AND
LOWER TN VALLEYS. WITHIN THIS DEEP MOISTURE PLUME...THE MOST
RECENT RAP ANALYSIS SHOWED 1.75 INCH PRECIPITABLE WATER
VALUES...WHICH ARE ABOUT THREE STANDARD DEVIATIONS ABOVE THE MEAN.

THE AXES OF LOW TOPPED CONVECTION WERE ALIGNED WITH THE 850-300 MB
MEAN FLOW...AND TRAINING WAS OCCURRING WITH THE CONVECTION ON THE
NORTHERN EDGE OF THE INSTABILITY. BASED ON REGIONAL
RADARS...HOURLY RAINFALL RATES RANGED BETWEEN 0.50 AND 1.00
INCHES. HOWEVER...THESE RAINFALL RATES WERE OCCURRING OVER AREAS
WITH SATURATED SOILS...WHERE 5 TO 10 INCHES OF RAINFALL HAS
OCCURRED OVER THE PAST WEEK. SO... DESPITE THE RELATIVELY MODEST
HOURLY RAINFALL RATES...FLASH FLOODING HAS OCCURRED WITH THIS
CONVECTION.

THE LOW LEVEL CONVERGENCE AHEAD OF THE MID LEVEL SHORT WAVE IS
EXPECTED TO REMAIN MORE OR LESS IN THE SAME PLACE THROUGH AT LEAST
01/05Z...EXTENDING FROM NORTHEAST TX INTO SOUTHWEST TN. THE DEEP
MOISTURE IN THE CONVERGENCE ZONE WILL CONTINUE TO FEED LOW TOPPED
CONVECTION...WHERE THE ALIGNMENT OF 850-300 MB MEAN WINDS AND
PROPAGATION VECTORS SUPPORT TRAINING. IT IS POSSIBLE THAT
INSTABILITY EVENTUALLY BECOMES A LIMITING FACTOR FOR THE EXPANSION
OF CONVECTION...ESPECIALLY AFTER 01/04Z OVER PORTIONS OF THE LOWER
MS VALLEY. HOWEVER...
INCREASING MID TO UPPER LEVEL DIVERGENCE AHEAD OF THE SHORT WAVE
PROVIDES SYNOPTIC SCALE ASCENT FOR MORE ORGANIZED CONVECTION TO
FORM.

THERE IS GOOD MULTI HIGH RESOLUTION MODEL SUPPORT FOR AN AXIS OF 2
TO 4 INCHES OF RAINFALL EXTENDING FROM NORTHEAST TX ACROSS CENTRAL
AND SOUTHERN AR INTO NORTHERN MS/NORTHWEST AL/WESTERN TN.

SOME HIGH RESOLUTION MODEL SPREAD EXISTS CONCERNING THE PLACEMENT
OF THE HIGHEST RAINFALL AXIS...WITH THE 12Z NSSL WRF AND 12Z WRF
ARW FURTHER SOUTH THAN THE MOST RECENT HRRR. GIVEN THE PLACEMENT
OF THE BEST INSTABILITY INTO THE LATE EVENING HOURS...A MORE
SOUTHERN PLACEMENT OF THE RAINFALL AXIS IS PREFERRED. THE
ADDITIONAL RAINFALL IS EXPECTED TO RESULT IN ADDITIONAL FLASH
FLOODING...OR AN EXACERBATION OF THE EXISTING FLOODING. GIVEN THE
SATURATED CONDITIONS IN PLACE... DAMAGING AND LIFE-THREATENING
FLASH FLOODING IS POSSIBLE INTO THE LATE EVENING HOURS.

HAYES

ATTN...WFO...BMX...FWD...HUN...JAN...LZK...MEG...OHX...OUN...
SHV...TSA...

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

LAT...LON   35628918 35098634 34398632 33978736 33309105
            31879734 31869735 32309765 34859449