Low-Level Boundary Intensification and Convective Regeneration in the Lower Mississippi River Valley Region Severe Weather and Flash Flood Event of April 6-7 2003

An unusual mix of severe weather and excessive rainfall ravaged much of the Lower Mississippi River Valley Region during 6-7 April 2003, causing more than $250 million in damage. The potent, early spring storm system responsible for instigating the severe convection featured strong dynamical forcing, a very moist, moderately unstable atmosphere (K-Index in the low 40s, precipitable water values approaching 2.00 in, lifted index between -5 and -8 and MUCAPE greater than 3000 J kg-1) and strong low-level wind shear (0-1 km storm-relative helicity greater than 300 m2 s-2). Several high precipitation supercell thunderstorms developed over northeast Louisiana and central and southern Mississippi during the late morning and early afternoon hours of 6 April 2003, resulting in numerous reports of large hail, wind damage, tornadoes and flash flooding. In all, 11 confirmed tornadoes touched down across the region: 8 F0s, 2 F1s and 1 F2.

As the event unfolded through the evening hours of 6 April 2003, focus shifted from severe weather to flash flooding. Rainfall from earlier in the day measured between one and four inches over central Mississippi, and soil conditions had become increasingly moist. A lingering, nearly stationary boundary that stretched across northeast Louisiana and central Mississippi was reinforced by the earlier convection and became a strong focus for continued convective generation. Strong, difluent flow aloft was characterized by a split upper-level jet formation. A strong southwesterly low-level jet combined with this difluent upper-level pattern to provide a continuous feed of rich low-level moisture to the region.

During the evening of 6 April 2003 and early morning hours of 7 April 2003, convection continually regenerated over northeast Louisiana and trained over central Mississippi. In excess of four inches of rainfall was observed along and just north of Interstate 20 from Tallulah, LA to Jackson and Meridian, MS. By the end of the event, Jackson International Airport reported 8.5 inches of rainfall, setting the all-time 24-hour rainfall record for the station. In addition, several NWS cooperative stations reported 24-hour rainfall accumulations greater than ten inches, with northeastern suburbs of Jackson, MS reporting up to 12.2 inches of rainfall. Such intense rainfall resulted in a top ten crest of 35.3 feet at the Pearl River at Jackson, MS, and record flooding on the Chickasawhay and Chunky Rivers in east Mississippi. Widespread flash-flooding was reported across much of central Mississippi with several creeks and tributaries inundating populated urban and suburban regions of the Jackson metropolitan area.

While this spring storm exhibited classic severe thunderstorm characteristics, it is likely to be most remembered for its anomalous rainfall production and subsequent record flooding. It is of great operational significance to determine the meteorological factors that led to the intensification of an apparent low-level meso-beta scale boundary. This boundary was largely responsible for focusing intense convective rainfall in a relatively narrow corridor (< 45 km wide) from Tallulah, LA to Jackson and Meridian, MS.