The Rainfall In Texas Is Changing And Here Is Proof

It is tempting for people to make sweeping statements about the weather and climate based on a few isolated observations or experiences. As a meteorologist, I see this all of the time. People make broad commentaries about climate change because it snows or define the entire global climate based on what they observe in their backyard. This week Southeast Texas experienced another extreme weather event as the remnants of Tropical Storm Imelda dumped multiple feet of rainfall on the region. Numerous water rescues and hazards were reported from Beaumont to Houston and surrounding areas. Some residents say it was worse than Hurricane Harvey for them, which affirms that every storm is different, and they should be treated in that manner. However, there is compelling evidence that the rainfall in Texas is changing. Here’s the proof.

Last December, the National Oceanic and Atmosphere Administration (NOAA) quietly released a press release redefining how much rainfall is required to qualify as a 100-year or 1000-year event in Texas. Before I go further, a definition of what a 1000-year event means is required. Many people are confused by the concept. It is common to interpret a 1000-year event as one that happens once every 1000 years. I cannot blame them based on how the concept is identified. In actuality, a 1000-year event has a 1 in 1000 or .1% chance of happening in any year. Hydrologists or civil engineers will say that a 1000-year event has a 1000-year recurrence interval. The graphic below summarizes some of the more common intervals.

Brian Bledsoe is the Director of the Institute for Resilient Infrastructure Systems (IRIS) at the University of Georgia. He wrote in a 2017 Washington Post editorial,

most people are still surprised, if not astonished, to learn that the 100-year flood at a given location has more than a 1 in 4 chance of occurring within the term of a 30-year mortgage. For most of us, this 26 percent chance our home will be flooded before we have a chance to pay it off is troubling if not unacceptable

According to the NOAA press release, things have changed in Texas, and updates were required. It explicitly states:

The study, published as NOAA Atlas 14, Volume 11 Precipitation-Frequency Atlas of the United States, Texas, found increased values in parts of Texas, including larger cities such as Austin and Houston, that will result in changes to the rainfall amounts that define 100-year events, which are those that on average occur every 100 years or have a one percent chance of happening in any given year. In Austin, for example, 100-year rainfall amounts for 24 hours increased as much as three inches up to 13 inches. 100-year estimates around Houston increased from 13 inches to 18 inches and values previously classified as 100-year events are now much more frequent 25-year events.

These frequency atlases are not academic tools. They have real-world applications. They are used for planning activities at all levels and in infrastructure design. The information associated with these metrics also helps FEMA manage risks and development in floodplains under the auspices of its National Flood Insurance Program.

According to NOAA, the updated values will replace values dating back 50 years. The rainstorms of 2019 are very different than rainstorms of 1965. Peer review literature has established that the top 1-2% rainfall events are more intense than 50 years ago. Additionally, studies suggests that stalling tropical storms like Imelda and Harvey may be more common. Much of the stormwater management infrastructure in municipalities is designed for the storms of last century. According to a University of Georgia press release, Bledsoe argues for “investments in hybrid systems of traditional “gray” and natural “green” infrastructure that work together along with nonstructural measures such as insurance reform, zoning, buyout and relocation to improve outcomes across a wide range of future extreme weather scenarios.”