Study Finds That 100-Year Rain Events May Happen More Frequently

Oct. 3, 2018

Decades of data have led officials to rainfall totals that define 100-year weather events

The National Oceanic and Atmospheric Administration (NOAA) released a study finding that in the Houston area, 100-year estimates increased from 13 inches to 18 inches in 24 hours. Rainfall previously classified as 100-year events are now more frequent 25-year events.

According to The Associated Press, decades of this data have led federal officials to rainfall totals in Texas that define 100-year weather events and caution that extreme rainstorms will strike the state more frequently. A 100-year storm is a storm that, on average, occurs every 100 years. That is a 1% chance of happening any given year.

In the Austin, Texas region, 100-year rainfall amount for 24 hours have increased as much as 3 inches up to 13 inches. The increase was the same for the area north of Corpus Christi, a portion of West Texas that includes the border county of Val Verde, and others.

“Current standards used for infrastructure design and floodplain regulations will possibly be revised based on the new values,” said Mark Glaudemans with NOAA’s Office of Water Prediction. “Officials in locations that have seen significant increases are already assessing the potential impacts of adopting the new estimates.”

The National Hurricane Center determined that the Houston metro area last year experienced a flood brought by Hurricane Harvey that was a greater than a one-in-1,000 year event.

Until Harvey, the record for rainfall from a hurricane or tropical storm in the Lower 48 states was 48 inches in 1978, and 52 inches in Hawaii in 1950.

The NOAA findings can have wide-ranging implications. NOAA rainfall values guide state and federal regulations the dictate how infrastructure design and development is done. They also determine flood risks and are used in the development of floodplains.

The Harris County commissioners earmarked $14.5 million for updated floodplain maps in the wake of Harvey. The maps rely on rainfall data to help Houston-area regulators determine where homes can be built, insurance costs and locations for flood-control projects. It was not clear how the NOAA report may affect the revision of those maps. A spokeswoman for the Harris County Flood Control District did not return a message seeking comment from AP.