Weather at the mid-latitudes are governed by cyclones and anticyclones mostly migrating eastward. These weather systems cause the jet streams to undulate, the meandering patterns are known as the Rossby waves. Occasionally, Rossby waves bring forth localized extreme weather phenomena. An example of finite-amplitude wave phenomenon is atmospheric blocking, which is often associated with heat waves and droughts. Recent development of a finite-amplitude local wave activity (FALWA) theory by Nakamura and collaborators enables comprehensive analysis of the dynamics of finite-amplitude Rossby waves observed in climate data, which helps understand the drivers of their life cycles. Despite the simplicity of interpretation it brings about, to apply the FALWA diagnostic to climate data requires more involved calculations than the traditional Eulerian framework. This article introduces the open-source Python package falwa which encapsulates the FALWA diagnostics implemented on gridded climate data presented in the authors' previous publications. It reviews the essence of the FALWA theory, the corresponding components in the package that implements the calculations, and where users can find sample notebooks to start with. It aims to serve as a road map for new users to easily navigate through this package. The latter half of this article documents the practices of the developers, which includes the documentation tools, continuous integration practice, and repository maintenance using automated GitHub functionalities. The authors also discuss existing numerical issues and future improvement plans. Hopefully, this open-source project encourages wider use of the FALWA diagnostics on climate data and model output by minimizing the impediment of the complex numerical computations.

A new procedure to obtain a longitudinally varying and slowly evolving atmospheric background state for the analysis of Rossby waveguides is described and discussed. The procedure is a rolling zonalization scheme, redistributing Ertel potential vorticity in a moving window to separate waves from the background. Waveguides are subsequently diagnosed from the gradient of the logarithm of potential vorticity. The effectiveness of the wave-background separation, even in large-amplitude conditions, is illustrated with reanalysis data. Established climatological mean waveguide structures are recovered from the rolling-zonalized state in the limit of long-term aggregation. Two contrasting episodes of Rossby wave packet propagation demonstrate how the evolution of waveguides derived from rolling zonalization can correspond to the development of superposed wave packets. The ability of the procedure to work with snapshots of the atmosphere provides new opportunities for waveguide research.