Winter NAO tracked the Wales-to-England rainfall ratio in 1951-2025

We tested whether the same-winter North Atlantic Oscillation (NAO) tracks the rainfall ratio between Wales and England. We used 75 complete winters from 1951 to 2025, Met Office country rainfall and NOAA CPC monthly NAO. A linear model included time and winter NAO. A one-unit increase in NAO was linked to a 10.94% increase in the fitted rainfall ratio. The conditional 95% five-winter block interval was 7.04% to 14.27%. Period log slopes were 0.10692 in 1951-1987 and 0.10514 in 1988-2025. The change interval, -0.07881 to 0.06496, did not resolve a change and does not prove stability. A model fitted in the early period gave -0.054% later-period reconstruction skill against a trend-only model, with limits of -92.81% to +39.52%. Thus a clear full-record association did not give clear transfer gain for the fixed early model. This extends known regional rainfall links; it does not identify a cause, measure water storage or provide a forecast. Country averages, one index, revised data and block assumptions limit inference. The package contains all used monthly values, exact source hashes, code, draws, a figure and an audit.

NAO · winter rainfall · Wales · England · regional contrast · rainfall ratio · block bootstrap · model transfer