A potential vorticity streamer and its role in the development of a week-long series of mesoscale convective systems. Part I: Severe weather and precipitation

From 27 June to 5 July 1999 a narrow region of positive upper tropospheric potential vorticity (a "PV streamer") formed over the tropical eastern Pacific and persisted over the southwest and south-central United States. During this period, apparently related to the streamer, a succession of mesoscale convective complexes (MCCs) and other severe-weather-producing phenomena developed in the central and southern Plains states, primarily Kansas, Oklahoma, Missouri, Texas, and Arkansas. One of these events, a large supercell and MCC that occurred on 27-28 June, produced high winds in Nebraska and Kansas and rainfall and flooding in Kansas and Missouri, inflicting several million dollars of property damage.

Because they formed over a data-rich region, these MCCs provide an excellent opportunity to investigate the role that PV streamers of this kind play in the development of mesoscale convection and the severe weather that in several cases preceded the MCCs. In Part I of this paper, we describe the location, extent, and type of severe weather that occurred during this period. We also describe the persistent large-scale pattern, particularly the potential vorticity field. A detailed study of the first of these serial MCCs on 28 June and its interaction with the streamer is presented in Part II by Caracena et al.

Satellite water vapor imagery clearly delineate the dry slot related to the streamer, and demonstrate that the MCCs interacted with the streamer during their formation and as they moved eastward. Using first the streamer location, and then the MCC anvil, as anchor points, we create a composite of severe weather reports and heavy rainfall (up to 7 inches in regions of eastern Kansas) during the period to gain a sense of where dangerous rainfall and severe weather occurred relative to the streamer and the MCCs.