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Abstract Results from a large eddy simulation of a tornadic supercell developing in a horizontally homogeneous environment are presented which clearly illustrate a connection between low‐level mesoyclone development and the development of a streamwise vorticity current (SVC). Although the environment supports tornadic supercells, a strong low‐level mesocyclone (LLM) does not develop until a well‐defined SVC forms in the storm's forward flank. As the streamwise vorticity in the SVC flows southward and is tilted into the storm updraft creating updraft helicity, the LLM strengthens and lowers toward the surface. The SVC also focuses LLM development in a confined storm‐relative position favorable for converging/stretching preexisting vertical vorticity. Tornadogenesis occurs within ∼5 min of the establishment of a strong LLM. These results illustrate a possible mode of internal storm variability that may be an important factor in explaining why some supercells produce tornadoes while others do not in similar favorable environments. 

Abstract Tornado motion changes occurring with major internal rear-flank momentum surges are examined in three significant tornado-producing supercells. The analysis primarily uses fixed-site Doppler radar data, but also utilizes in situ and videographic observations when available. In the cases examined, the peak lowest-level remotely sensed or in situ rear-flank surge wind speeds ranged from 48 to at least 63 m s −1 . Contemporaneous with major surges impacting the tornadoes and their parent low-level mesocyclones, longer-duration tornado heading changes were leftward and ranged from 30° to 55°. In all cases, the tornado speed increased substantially upon surge impact, with tornado speeds approximately doubling in two of the events. A storm-relative change in the hook echo orientation accompanied the major surges and provided a signal that a marked leftward heading change for an ongoing tornado was under way. Concurrent with the surge interaction, the hook echo tip and associated low-level mesocyclone turned leftward while also moving in a storm-relative downshear direction. The major rear-flank internal surges influenced tornado motion such that a generally favorable storm updraft-relative position was maintained. In all cases, the tornado lasted well beyond (≥21 min) the time of the surge-associated left turn with no evident marked loss of intensity until well down-track of the turn. The local momentum balance between outflow and inflow that bounds the tornado or its parent circulation, especially the directionality evolution of the bounding momentum, is the most apparent explanation for tornado down-track or off-track accelerations in the featured events.