Two 100-year floods in a year? Here’s how it’s possible

Weather Specials

PITTSBURGH (WKBN) – People across our region are still cleaning up after the last round of high water and flooding. We’ve heard the terms “50-year” and “100-year” flood before but what does that actually mean — especially when you get two of them in a year?

“We call it the ring of fire, where everything around the high-pressure system gets very active,” said Fred McMullen, a meteorologist with the National Weather Service in Pittsburgh.

He said that’s why our area has been dealing with repeated rounds of thunderstorms and heavy rain.

“Around that high-pressure system has been the storm track across the Ohio Valley and that has led to a very active pattern — not only in the Ohio Valley, but in the plains.”

The severe weather and extreme flooding have stretched from the plains over the past few days to places like Dayton and eventually into the Valley.

In Boardman, Poland and Canfield, high water quickly caused widespread damage.

“Therefore, the same location got hit with multiple rounds of precipitation. We had rainfall estimates, over six inches of rainfall in the heaviest band,” McMullen said.

With that much rain, it doesn’t matter where you are — the ground just can’t handle it in a short period of time.

“When we see the aging infrastructure of storm systems and then when you have a lot of pavement, a lot of asphalt, new development and areas that weren’t developed before, you have more runoff,” McMullen said.

But what does it mean to have a “50-year” flood?

“That means there is a 2% chance of that occurring. Of that much rainfall occurring either in a one-hour, three-hour, six-hour or 12-hour period. Two percent chance,” McMullen said.

The bottom line is it has to do with chance, not the actual number of years.

In other words, if weather probabilities worked in the lottery, you could win the jackpot twice.

“You could have a 50-year flood back-to-back years because, again, it is a very low probability to have a 2% chance one year, hit it and the next year, have a 2% chance and get it again,” McMullen said. “We can tell people a ‘100-year flood.’ Once you have one, you’re not done for a hundred years. You could have one later that year or you can have one next year.”

He wants to remind people that it only takes six inches of moving water to sweep you off your feet and 12 inches to sweep away an SUV. So be safe — turn around, don’t drown.

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