Multiscale overview of a violent tornado outbreak with attendant flash flooding

On 1 March 1997 violent tornadoes caused numerous fatalities and widespread damage across portions of central and eastern Arkansas and western Tenness ee. In addition, the associated thunderstorms produced very heavy rainfall and Rash flooding, with a few locations receiving up to 150 mm (6 in.) of r ainfall in 3 h. The initial environment appeared favorable for strong torna does with unseasonably warm moist air at lower levels resulting in signific ant instability (convective available potential energy values between 1400 and 1800 J kg(-1)) where 0-2-km storm-relative helicities exceeded 300 m(2) s(-2) and the middle-tropospheric storm-relative flow was conducive for to rnadic supercells. The most destructive tornadoes developed along a preexis ting surface boundary where lower-tropospheric moisture convergence and fro ntogenesis were enhanced. Tornadoes and heaviest rainfall only ensue after upward motion associated with the direct circulation of an upper-tropospher ic jet streak became collocated with lower-tropospheric upward forcing alon g the surface boundaries. From a flash flood perspective the event occurred in a hybrid mesohigh-synoptic heavy rain pattern as thunderstorms develope d and moved along surface boundaries aligned nearly parallel to the mean wi nd. In addition, strong flow and associated moisture Bur convergence in the lower troposphere favored the formation of cells to the southwest or upstr eam of the initial convection with thunderstorms, including a a tornadic su percell, traversing over the same area. The available moisture and ambient instability also supported both vigorous updrafts and high precipitation ra tes.