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They Just Keep Coming Back

The night of August 20, 1998, a massive thunderstorm dumped buckets of rain over eastern Nebraska. As morning arrived, the storm had gone but a similar storm appeared the following night, ravaging the region once again. This situation is not atypical. Throughout the Midwest large storm clusters called Mesoscale Convective Systems (MCS) are quite common during the warm weather months and may reoccur for several nights.

Sometimes an MCS will reoccur in the same region night after night due to stationary weather features. At other times, a series of MCSs can traverse the central and eastern United States from west to east over several nights. Scientists using advanced computer models have found the culprit that is able to trigger these sequels. NCAR researchers Chris Davis and Stanley Trier found that, after some MCSs dissipate, a vortex is left behind which can prompt storm development the following night. They call the phenomenon a Mesoscale Convective Vortex (MCV).

An MCV is generated from the cyclonic vortex within an MCS. These giant storms follow the same general morphology and evolution as bow echoes, except on a larger scale. They too have cyclonic and anticyclonic vortices at the ends of their massive squall lines and, like a bow echo, the cyclonic vortex is favored in the northern hemisphere, becoming larger over time. It is this dominant vortex what will become an MCV if it lasts beyond the lifespan of the MCS.

From mid-May to July 2003 Davis and Trier are stationed in the Midwest United States as part of the BAMEX field program to better our understanding of the processes that form MCVs and how the vortices are able to prompt storm development. Hopefully their research will enable forecasters to track vortices after their parent storms have dissipated allowing them to predict where and when the next MCS will spring up.