Significance Forecasts of summer weather patterns months in advance would be of great value for a wide range of applications, from tourism to agriculture, water resource management, energy production, construction, and retail. However, dynamical model seasonal forecasts for European summers have very little skill. Here, we analyze atmosphere and ocean observations and identify evidence that a specific pattern of summertime atmospheric circulation over Western Europe is predictable from the previous spring North Atlantic sea-surface temperatures, and identify the mechanism involved. The atmospheric pattern has a particularly strong influence on rainfall in the British Isles and in the north coast of France, which we find can also be predicted months ahead with significant skill.