How Often Do 100-year Rain Events Actually Happen?
DURATION 3 minute read
- It’s been 57 years since Texas has updated rainfall precipitation estimates
- According to Atlas-14 estimates, a 100-year rain event in Harris County is now thought to be closer to a 25-year rain event
- FEMA National Flood Insurance Program has been updated accordingly
What is Atlas 14?
Developed by the National Oceanic and Atmospheric Administration (NOAA) to serve as the United States Government’s source for precipitation frequency estimates, Atlas 14 quantifies the expected frequency of extreme precipitation events. The estimates quantify how much precipitation at a location for a given duration qualifies as a certain year event, such as a 100-year rainfall.
In comparison to previous studies, the Atlas 14 study shows significant increases in rainfall frequency values throughout Texas. Larger cities, like Houston and Austin, show values increasing from 13 inches to 18 inches and 10 inches to 13 inches respectively, in a 24-hour period. What was previously considered a 100-year event in the Houston area is now classified as a 25-year event.
How are Atlas-14 Estimates Determined?
All known sources of historical information were used for this study, including newspaper records from more than 100 years ago. The records are the most accurate for precipitation data, rain gauge networks, and improved methods in statistical hydrology. NOAA developed a Comparison Map showing Atlas 14 estimates alongside previous estimates. You can also access Atlas 14 maps.
How are Atlas-14 Estimates Used?
Precipitation frequency information is essential when it comes to informing infrastructure design, hydrometeorological analysis and forecasting, and flood risk. Engineers and planners utilized the precipitation frequency estimates to understand expected extreme rain events and bring knowledge of areas susceptible to flooding into land use and development decisions, including managing and designing stormwater infrastructure.
The estimates are also used as input to hydrological models that delineate flood zones and identify flood risks for FEMA’s National Flood Insurance Program. The updated values replace previous federal studies completed to this point.