Return intervals for rainfall amounts from Hurricane Harvey. Graphic: Dr. Shane Hubbard / Cooperative Institute for Meteorological Satellite Studies

By Jason Samenow
31 August 2017

(The Washington Post) – As Harvey’s rains unfolded, the intensity and scope of the disaster were so enormous that weather forecasters, first responders, the victims, everyone really, couldn’t believe their eyes. Now the data are bearing out what everyone suspected: This flood event is on an entirely different scale than what we’ve seen before in the United States.

A new analysis from the University of Wisconsin’s Space Science and Engineering Center has determined that Harvey is a 1-in-1,000-year flood event that has overwhelmed an enormous section of  Southeast Texas equivalent in size to New Jersey.

There is nothing in the historical record that rivals this, according to Shane Hubbard, the Wisconsin researcher who made and mapped this calculation. “In looking at many of these events [in the United States], I’ve never seen anything of this magnitude or size,” he said. “This is something that hasn’t happened in our modern era of observations.”

Hubbard made additional calculations that accentuate the massive scale of the disaster:

  • At least 20 inches of rain fell over an area (nearly 29,000 square miles) larger than 10 states, including West Virginia and Maryland (by a factor of more than two).
  • At least 30 inches of rain fell over an area (more than 11,000 square miles) equivalent to Maryland’s size.

A 1,000-year flood event, as its names implies, is exceptionally rare. It signifies just a 0.1 percent chance of such an event happening in any given year. “Or, a better way to think about it is that 99.9 percent of the time, such an event will never happen,” Hubbard said.

Apart from Harvey, there’s simply no record of a 1,000-year event occupying so much real estate. […]

Precipitation areas in Southeast Texas, 23 August 2017 - 30 August 2017. Graphic: Dr. Shane Hubbard / Cooperative Institute for Meteorological Satellite Studies

Climate change studies have found that what’s considered a 500-year flood today may become much more frequent in coming decades. […]

“Expect #HarveyFlood record will be broken in 5, 15, 25 years from now — sooner rather than later,” tweeted David Titley, professor of meteorology at Penn State. [more]

Harvey is a 1,000-year flood event unprecedented in scale

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