On the Role of Storm Motion in Events When Both Tornadoes and Flash Floods Occur at the Same Place and Date

In some situations, an episode of severe local storms begins with tornadoes and later evolves into mostly heavy rain.  The urgency of tornado warnings can cause forecasters to fail to recognize a developing flash flood, and so fail to issue timely warnings.  It is argued that the ability to estimate likely cell and storm system motions is an important factor in being able to anticipate this dual threat.  Pattern recognition of archetypical situations can be helpful, but the combination of tornado and flash flood events can arise in many different ways.  Case studies are presented to illustrate some of the variety of pre-storm environments in which tornado/flash flood events can develop.  Several methods for estimating cell and storm motions are discussed.  Specifically, severe storm forecasters would benefit by looking at hodographs and plan-view displays of predicted supercell motion to help increase situational awareness of these dual-threat environments.  Attention especially should be given to those situations when forecast cell motions are <8 m s^–1 (15 kt).   Further, forecasters should be alert to situations where multicell convective rain systems have movements very different from component cell motions, potentially creating a “training” situation.