MESOSCALE DYNAMICS IN THE PALM SUNDAY TORNADO OUTBREAK

Radar and satellite imagery suggest that strong mesoscale forcing occu rred in the Palm Sunday tornado outbreak on 27 March 1994. Parallel li nes of severe thunderstorms within each of three mesoscale convective systems developed just ahead of a cold front in Mississippi and Alabam a on this date. Analyses of routine meteorological observations, barog raph data, and forecasts from the Era and NGM models and a mesoscale r esearch model (MASS) are used to examine the relative roles of large-s cale dynamics and mesoscale processes in triggering and organizing the mesoscale convection. Quasigeostrophic forcing was absent in the outb reak region. Likewise, a thermally direct circulation system transvers e to the upper-level jet that was present to the northwest of the outb reak region was decoupled from the strong low-level ascent occurring i n northern Alabama and Mississippi at the lime of the outbreak. Strong ageostrophic frontogenesis in the presence of conditional symmetric i nstability (CSI) was the chief cause for the intense low-level ascent along and behind the front, consistent with the line of severe storms that developed explosively along the front and an observed postfrontal precipitation band. However, the strongest supercells developed in se gmented lines 100-200 km ahead of and parallel to the frontal boundary in an atmosphere that the MASS model indicates was inertially unstabl e due to a mesoscale midlevel jetlet. Analysis suggests that these sto rms developed in a manner consistent with the predictions of asymmetri c inertial instability theory in the presence of convective instabilit y. Several mesolows were observed to have traveled along the frontal b oundary and to have played a key role in focusing the frontogenesis. S imilar frontal mesolows were simulated by the MASS model. Strong low-l evel ascent in the presence of conditional instability helped to deepe n the mesolows, but they were strongly modulated by a train of gravity waves propagating on the cold side of the front. A combination of due ling and wave-CISK (conditional instability of the second kind) proces ses maintained the waves, which remained coupled to the jetlets as the y propagated from intense convection in northeastern Texas. A time-to- space conversion objective analysis of bandpass-filtered barograph dat a reveals that similar waves emanated from this same region. The lifti ng patterns produced by the complex interactions between the gravity w aves, CSI, asymmetric inertial instability, and frontogenesis satisfac torily explains the development, configuration, spacing, and relative movement of the severe mesoconvective systems on Palm Sunday. All of t hese mesoscale phenomena were coupled to or strongly influenced by the jetlets, which were produced by strong convection at an earlier time within the region of quasigeostrophic forcing far removed from the tor nado outbreak.