Modelling rainfall trends in England and Wales

The monthly England and Wales precipitation (EWP) series (once power transformed to induce symmetry and to stabilise variance) may be characterised as having linear seasonal trends with a white noise error process superimposed. However, these trends are not stable, for they are interrupted by four regime shifts occurring in 1828, 1871, 1917 and 1976. If these shifts are ignored then the series is consistent with a trend pattern in which winters are becoming increasingly wet and summers drier. If only the last regime from 1976 is considered, then summers are still becoming drier but winters have no trend, with spring becoming wetter. The unusually wet winter of 2014 is seen to have been a consequence of very high January and February rainfall relative to that predicted, the conjunction of which is unprecedented during the two and a half centuries over which the EWP series has been available, during which time such pairs of values have been essentially uncorrelated.