Ellicott City flood: Here's what a '1,000-year storm' actually means

The downpour that ravaged Ellicott City last weekend was supposed to be the rarest of storms — so unlikely to happen that meteorologists call it a 1,000-year rain storm. That doesn’t mean it happens every thousand years. It is not quite that unusual.

The number is a statistical measure that means there is 1/10 of 1 percent of a chance in any year that such a storm will hit.

“That is a hard statistic to grasp,” said Jason Elliott, a meteorologist with the National Weather Service. “To put it in laymen terms, it is just exceptionally rare.”

It was the second time in two years a massive rainstorm devastated Ellicott City, causing flash flooding that ripped the historic city’s Main Street. The storm toppled cars, submerged buildings and gutted businesses.

It was the same story that played out in 2016. Many seemed in disbelief that it could happen again so soon. But Elliott said that though the risk is small, the storm can happen in any given year.

You could even experience such a storm twice in a year. But there also might not be a storm like this for another 50 years.

Meteorologists use other similar statistical measures to rate rare storms. A 100-year flood means that there is one chance in 100 of a flood or storm in a year. A 500-year flood means there is a 0.2 percent chance.

Elliott said it is not clear why the Howard County town was hit again so soon. The Mid-Atlantic and Northeast corridors are in general experiencing more frequent and heavier rainfall, he said.

Others have argued that over development and slow implementation of storm water control plans are contributing to the flooding.

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