By compiling rainfall data over the past several decades, NOAA has redefined some instances of the 100-year rainfall total to be a 25-year event for Houston, and it increased the rainfall amount attributed to a 100-year event.
The data is reflected in an update released Thursday to the agency's Atlas 14 study, a comprehensive analysis of probabilities of rainfall events for various durations, from 5 minutes to 60 days, across the state of Texas.
"The 100-year estimates around Houston increased from 13 inches to 18 inches—almost 35 percent in value. Previously classified as 100 year events are now classified as much more frequent 25-year events," said Mark Glaudemans, an official with the agency's water prediction office. Those inch totals are for a 24-hour rain event.
What is called a 100-year event is a reference to the statistical likelihood of a rain event occurring in any given year in any given location. For example, a 100-year event has a 1 percent chance of happening every year. A 25-year event has a 4 percent chance.
Previous estimates that the agency had been using for the state were almost 60 years old in some cases, Glaudemans said. The new data is far more accurate and draws on many more sources of rainfall information, in some cases going back as far as 100 years, he said.
Hurricane Harvey dropped 6.8 inches over Harris County in the first hour and almost four feet of water over four days, according to data compiled by the Harris County Flood Control District. Using the new Atlas 14 standard, that would make Harvey's first hour somewhere between a 500 and 1,000-year event, and its four-day total beyond a 1,000 year event.
The new NOAA standards override all other existing rainfall data, he said, and have implications for drainage systems, development guidelines, flood maps and flood insurance.
"I think with everything the Houston-Harris County area has been through, three major events in four years, I think that has a lot to do with these new numbers," said Mark Hanna, a spokesman for the Insurance Council of Texas. "Anyone living anywhere with a even a hint that their home could flood, they should purchase flood insurance."