MESOSCALE PRECIPITATION DISCUSSION 0442
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
604 AM EDT TUE JUL 04 2017

AREAS AFFECTED...NORTHERN AL AND NEARBY SOUTHERN TN...FAR
NORTHWEST GA

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

VALID 041000Z - 041400Z

SUMMARY...CONVECTION TRAINING IN A UNIDIRECTIONAL FLOW AHEAD OF A
LONG LIVED MCV COULD RESULT IN FLASH FLOODING THIS MORNING.

DISCUSSION...CONVECTION STRETCHING FROM NORTHERN MS ACROSS
NORTHERN AL INTO NEARBY SOUTHERN TN IS BEGINNING TO TRAINING IN A
UNIDIRECTIONAL FLOW AHEAD OF A LONG LIVED MCV OVER WESTERN TN.
AHEAD OF THE MCV...AN AXIS OF 500-1000 J/KG MUCAPE EXTENDS FROM
NORTHERN AL INTO CENTRAL AND EASTERN TN...AND THE CONVECTION IS
ORGANIZING IN THE AXIS OF INSTABILITY...IN THE SYNOPTIC SCALE
ASCENT AHEAD OF THE MCV.

THE KHUN RADAR SHOWED HOURLY RAINFALL RATES NEAR 1.50 INCHES WITH
THE CURRENT CONVECTION. RAP MODEL SOUNDINGS AHEAD OF THE MCV
SHOWED UNIDIRECTIONAL FLOW FROM THE SURFACE THROUGH AT LEAST 700
MB...WITH WET BULB ZERO VALUES CLOSE TO THE FREEZING
LEVEL...INFERRING WARM CLOUD PROCESSES ARE BECOME A FACTOR IN
RAINFALL EFFICIENCY.

MUCH OF THE HIGH RESOLUTION GUIDANCE (INCLUDING THE MOST RECENT
HRRR AND EXPERIMENTAL HRRR) WAS AN HOUR OR TWO SLOWER THAN
REALITY...FIRING THE CONVECTION FURTHER WEST ACROSS WESTERN AND
CENTRAL TN. THERE IS A SIGNAL IN THE HIGH RESOLUTION GUIDANCE FOR
LOCAL 2.00 TO 3.00 INCH RAINFALL AMOUNTS IN THE UNIDIRECTIONAL
FLOW...AND BASED ON THE RAINFALL RATES ON THE KHUN RADAR...THESE
AMOUNTS APPEAR REASONABLE. ONE AND THREE HOUR FLASH FLOOD GUIDANCE
VALUES ARE AS LOW AS 1.50 INCHES...SO FLASH FLOODING IS POSSIBLE.

HAYES

ATTN...WFO...BMX...FFC...HUN...MRX...OHX...

ATTN...RFC...LMRFC...OHRFC...SERFC...

LAT...LON   36098479 35748423 34918487 34498523 34288561
            34148631 34348697 35448594