What a '1 in 1,000 year' rainfall event actually means

Extreme rain in parts of New York, Connecticut, and Vermont led to life-threatening flash flooding this week. This rainfall has been referred to as a 'once-in-a-thousand-year' event, but what does that mean?

When you hear the term 1,000-year rainfall event, it sounds like the type of event that only happens once every 1,000 years, but that's not actually the case.

It's a statistical way of describing the probability of an extreme weather event at any time in any given year. A 1,000-year event has a 1 in 1,000 or 0.1 percent chance of happening in any given year.

The amount of rainfall that would be considered a 1 in 1,000-year event varies depending on the location and the time of year. In some areas, it might be a few inches of rain, while in others it might be much more. For example, a 1 in 1,000-year rainfall event in Los Angeles might be 2 inches of rain, while in Seattle, it might be 10 inches of rain.

Why does it seem like these events are happening more often? It's because they are. Within a span of 5 weeks last summer, the U.S. experienced five 1,000-year rain events in the lower 48. In April of this year, Fort Lauderdale experienced the rainiest day in the city's history, with more than two feet falling in a 24-hour period. That was another 1,000-year rain event.

So why are these events becoming more frequent? Scientists have linked these intense rain events to a warming climate. A warmer atmosphere can hold more moisture. For every degree Fahrenheit the Earth warms, the atmosphere can hold nearly 4 percent more moisture. Since 1880, the global average temperature has increased by 1.9 degrees Fahrenheit. So now, when it rains, it really pours.