What It Really Means When a 1-in-1,000-Year Rainstorm Hits

Officials called Hurricane Helene’s deadly rainfall and floods “biblical” and “generational.” But weather forecasters used another term: “once in 1,000 years.”

Helene was actually the second once-in-a-millennium storm to strike North Carolina in a matter of days. Less than two weeks before Helene made landfall, an unnamed tropical storm brought 1,000-year rains to communities on the opposite side of the state, inundating homes along the coast.

The idea of two such rainfall events occurring back-to-back might seem confusing. After all, it sounds like they should only occur every 1,000 years. But in reality, it’s all about probability. Understanding the odds ...