How is it that catastrophes always seem predictable ex post but never ex ante? The paradox is recurrent. Our study of the warnings issued prior to one of the last major natural disasters in France – the tempest of December 27, 1999 – focuses on the organizational factors accounting for the surprise effect of the catastrophe, to try and understand why, though the phenomenon had been predicted and announced by the French weather bureau, the protagonists of Civil Defense said the magnitude of the event took them by surprise. A multi-level, qualitative analysis of the inter-organizational meteorological alert system shows that a combination of factors pertaining to structure, context and individuals transformed the warnings into routine messages. The case of the 1999 storm allows us to introduce a particular, ideal-type of risk, the treacherous risk, which, though clearly announced, was disregarded because seemingly so familiar.