MESOSCALE PRECIPITATION DISCUSSION 0149
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
416 PM EDT THU JUL 04 2013
AREAS AFFECTED...NORTHWEST AL...CENTRAL TN
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY
VALID 042016Z - 050016Z
SUMMARY...HEAVY RAINFALL OVER THE PAST SEVERAL HOURS HAS LOWERED
FLASH FLOOD GUIDANCE VALUES SIGNIFICANTLY ACROSS THE REGION. WITH
ANOTHER 2-4 INCHES OF RAIN POSSIBLE IN THE NEXT FEW HOURS, FLASH
FLOODING IS LIKELY IN NORTHWEST AL AND CENTRAL TN.
DISCUSSION...A WEAK SURFACE LOW EXISTS SOUTH OF SOUTHERN AL, AND A
RELATED SURFACE TROUGH EXTENDS NORTH-NORTHWEST NEAR THE AL/MS
BORDER, WHILE AN OUTFLOW BOUNDARY ACROSS NORTHWEST AL FROM EARLIER
CONVECTIVE ACTIVITY AND A NORTHWARD-MOVING BOUNDARY AT THE 850 HPA
LEVEL ARE ACTING AS THE DRIVING FORCE FOR NEW CONVECTION. INFLOW
INTO THE REGION AT 850 HPA IS AROUND 35 KNOTS PER VAD WIND
PROFILES IN THE REGION, AND CAPE VALUES ARE IN THE 1000-1500 J/KG
RANGE PRECIPITABLE WATER VALUES IN THE AREA ARE AROUND TWO
INCHES. BROAD DIVERGENCE EXISTS ALOFT TO THE EAST OF A SLOWLY
RETROGRADING UPPER-LEVEL TROUGH.
SINCE 1915Z, CONVECTION ACROSS NORTHWEST ALABAMA HAS INTENSIFIED
ALONG ITS SOUTHWEST PERIPHERY, LIKELY DUE TO INCREASING CAPES NEAR
THE AL/MS BORDER. WITH TIME, A WEAK SURFACE LOW IS EXPECTED TO
MOVE UP THE SURFACE TROUGH, UP TO THE CENTRAL MS/AL BORDER BY 00Z.
THIS WILL LEAD TO A STRENGTHENING OF THE LOW-LEVEL INFLOW INTO
THE OUTFLOW BOUNDARY WHILE ITS ORIENTATION BECOMES MORE
SOUTH-NORTH ORIENTED, POSSIBLY UP TO 50 KNOTS, AND EXTEND THE
HEAVIEST RAINFALL RATES NORTHEAST ACROSS CENTRAL TN. CORFIDI
VECTORS ARE LIGHT, AND ALONG THE AXIS OF THIS CONVECTIVE LINE.
FLASH FLOOD GUIDANCE VALUES ARE REGIONALLY DOWN TO NEAR OR BELOW
1.5 INCHES IN THREE HOURS, AND 2-4 INCHES OF RAINFALL IS EXPECTED
IN THE NEXT FEW HOURS. AS SUCH, FLASH FLOODING IS CONSIDERED
LIKELY ACROSS THE REGION INTO THIS EVENING.
ROTH
ATTN...WFO...OHX...BMX...HUN...MEG...JAN...
ATTN...RFC...LMRFC...OHRFC...SERFC...
LAT...LON 35818575 34408664 33548784 33708832 34798785 35908691
36628579 35818575
Last Updated: 416 PM EDT THU JUL 04 2013