The floods that killed at least 20 people in Tennessee last weekend arrived with shocking speed and force — seemingly a case study of the difficulties of protecting people from explosive rainstorms as climate change gets worse.

A closer look at what happened in the days, years and even decades before the storm reveals that a series of government decisions — where and how to build, when to update flood maps, whether to join the federal flood insurance program and how to warn of dangerous floods — left residents more exposed to flooding than they had to be.

Record rainfall,

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