GOES water vapor images frequently show exceptionally dark filaments, ranging in size from short "dry slots" curling into cyclonic centers to equally narrow but very long regions extending longitudinally for many hundreds of kilometers. The implied mid-level dryness within these filaments suggests air with a history of subsidence. Furthermore, the largest of these longitudinal filaments indicate the presence of a jet stream immediately to the south, and recent studies show them to be closely collocated with similarly-shaped filaments of enhanced potential vorticity ("PV streamers") also found on the cyclonic side of the jet. Occasionally there is dramatic mesoscale convective development over the central United States within and toward the nose of the streamers/filaments. Among the possible mechanisms that play a role in these cases of MCS development are dynamical processes involving jet interactions and potential vorticity (eg., circulations driven by superposed upper-level and low-level jetstreams,gravity waves generated by unbalanced flow, or midlevel spinup produced by vortex translation across slanting isentropes, the "Raymond mechanism"). Another possibility is that the critical factor promoting MCS development is simply the destabilization caused by the advection of dry air aloft and/or the creation of a dry, cool air mass essential for development of a cool pool and descending jet as it interacts with precipitation.

A first step toward determining which of these processes are relevant is a careful description of the meteorological fields during streamer occurrences. First, to assess the ubiquity of features like these and to select good cases for analysis, we follow water vapor imagery during several months of 2001 and present representative images. We also describe a sequence of 6 days in June and July of 1999 during which a large and persistent streamer extended from the east Pacific to the central U.S. This streamer appeared to play a role in the development of several mesoscale convective complexes (MCCs) on successive days, each associated with heavy rain and severe weather over the Central and Southern Plains. To put these streamers in context, we overlay model initialization fields of upper-level winds, potential vorticity, and other relevant meteorological quantities. Diagnostic analysis of flow imbalance and gravity waves, isentropic conveyor belts, and cool pools are also made. Where possible, we show radiosonde ascents made through the filaments to help detect the vertical structure of moisture and winds. Since the return in the water vapor channels vary with temperature, filaments at different latitudes may appear artificially darker. To assess the impact of this variation, we compare streamer cases with corresponding images corrected at the University of Virginia.