Tag Archive for: precipitation-frequency estimates

Here Come New Precipitation-Frequency Estimates…Again

NOAA’s new Atlas 15 precipitation-frequency estimates will soon replace recently introduced Atlas 14 estimates – even before the Atlas 14 estimates have been fully adopted and integrated into local regulations.

About Precipitation-Frequency Estimates

Engineers use precipitation-frequency estimates to design, plan and manage infrastructure under Federal, State and local regulations. For instance, to ensure homes are built X feet above the 100-year floodplain, engineers must “know” how much rain will fall in a 100-year storm. Predicting that is one of NOAA’s jobs.

However, haphazard adoption of the new estimates has created a patchwork quilt of regulations across Texas and the U.S. One of the dirty, little secrets in the flood mitigation business is that many jurisdictions fail to adopt the new estimates and update their regulations accordingly. It’s costly, time-consuming, and raises the bar for developers.

So, many jurisdictions continue to use lower estimates to help attract development.

But designing infrastructure around artificially low rainfall estimates can lead to insufficient mitigation that increases flood risk for everyone.

Some Estimates Now in Effect Go Back 60 Years

In 2018, NOAA introduced Atlas 14 precipitation-frequency estimates for Texas. They replaced earlier estimates published by NOAA as early as the 1960s. Some parts of the Houston region still use those earlier estimates today.

Atlas 14 estimates superseded those published in:

  1. Weather Bureau Technical Paper No. 40, Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years (Hershfield, 1961)
  2. Weather Bureau Technical Paper No. 49, Two- to Ten-Day Precipitation for Return Periods of 2 to 100 Years in the Contiguous United States (Miller, 1964).

Newest Estimates Will Incorporate Climate Change

Compared to those, Atlas 14 estimates are more accurate. They incorporate data from newer technologies and more data collected over longer periods. Atlas 14 totals increased 30-40% for the Lake Houston Area.

The Atlas 15 estimates are just getting underway and have not yet been developed. NOAA expects to release them in 2027.

Atlas-15 timeline by NOAA

NOAA claims its Atlas-15 update will improve precipitation-frequency estimates by leveraging non-stationary climate estimates. Previous estimates, such as Atlas 14, have assumed a stationary climate.

In statistics, “non-stationary” means the underlying environment changes, say due to some strong trend or seasonality. Many people believe climate is changing and hence the desire to build that into the new precipitation-frequency estimates.

National Funding, New Updates Every 10 Years

Historically, NOAA precipitation-frequency estimates have been funded by states and other users, on a cost-reimbursable basis. However, that is changing.

Moving forward, the Federal government will fund precipitation-frequency updates. Under the Floods Act, signed into law in December 2022, NOAA will update precipitation-frequency estimates every 10 years.

Goals include:

  • Updating standards
  • Incorporating climate change
  • For the entire country.

Voluntary, Local Participation

But there’s a dirty little secret that not many people know about. Nothing forces individual cities, counties or states to adopt the estimates and work them into their regulations.

That’s a big job. And an expensive one. So, not all jurisdictions do it. Many areas surrounding Houston still plan infrastructure using data developed 60 years ago.

If you plan on less rain, channels can be narrower and stormwater-detention basins smaller. But residents are not protected as much as they should be.

Posted by Bob Rehak on 9/5/2023 based on information from NOAA.

2198 Days since Hurricane Harvey