Even before the second megastorm in as many weeks brings to the Southeast US, it's already clear that federal flood-risk maps underpinning decisions by millions of American homeowners and businesses are severely out of sync with a new era of climate-intensified disasters.

The best-known guide to flood risk in the US is the Federal Emergency Management Agency's set of flood maps, which designate high-hazard areas where homeowners with mortgages must buy flood policies. Those maps -- which are now widely and incorrectly used to understand flood risk more broadly -- are often out of date and don't focus on the danger of rain-caused ...