### Abstract

Prediction of long-lived anomalous behaviour in the atmosphere is fundamental to extended-range and seasonal forecasting. Dynamical theory provides insight into the properties of quasi-steady solutions of the governing equations. Since barotropic dynamics cannot describe the development and decay of anomalous patterns on scales larger than the deformation radius, the dynamics of large-scale anomalies are essentially baroclinic and controlled by changes in the deformation radius. In particular the vertical structure of the atmosphere should play a key role.

These predictions have been examined using best estimates of the state of the atmosphere produced by a numerical weather prediction centre (so-called 'operational analyses'). It is shown that changes in the effective depth of the atmosphere are linked to changes in the horizontal scale of the developing perturbations while larger-scale perturbations persist for long periods. Implications for atmospheric modelling are discussed.