Background
Flooding from heavy rainfall events is a dangerous phenomenon that has become increasingly probable and severe in the United States due to climate change. As discussed in the report, “The 8th National Risk Assessment: The Precipitation Problem,” as air temperatures increase, more water vapor may be held in the atmosphere and discharged during rainfall events. For every 1°C increase, 7% more water vapor is carried by the same air volume. As a result, increasing temperatures have created changes in the expectations of the Intensity, Duration, and Frequency (IDF) of rainfall events. Rainfall events that were thought to occur only once every hundred years are now occurring with far greater frequency. In some places, these formerly rare events are now occurring as often as every 5 or 10 years, based on the First Street Foundation Precipitation Model (FSF-PM).
Methodological overview
The National Oceanic and Atmospheric Administration (NOAA) created the Atlas 14 and Atlas 2 publications from 1973 to 2018. These are used to provide a national understanding of rainfall estimates and are utilized by engineers to design infrastructure projects that can protect communities. However, the analyses do not reflect current rainfall conditions or the changing climate and were generated for different parts of the country at different times, leading to inconsistencies across regions. NOAA received appropriations in 2023 from the US Congress to update these issues through the creation of Atlas 15, but it won’t be completed until 2027 at the earliest.
To correct this, First Street Foundation (FSF) analyzed 795 high-quality NOAA Automated Surface Observing Station (ASOS) data points across the US to estimate the likely rainfall IDF characteristics in the current year. The FSF-PM offers a methodology that corrects the known issues in NOAA Atlas 14, providing more accurate and reliable estimates of heavy precipitation occurrence and flood risk. The analysis reveals that a significant number of highly populated areas are experiencing higher flood risk than what the local communities currently consider a 1-in-100-year event. This highlights the urgent need for accurate rainfall data to inform infrastructure design and investment decisions. Furthermore, the analysis shows that cities like Baltimore, Dallas, Washington D.C., and New York City face substantial increases in risk compared to the NOAA Atlas 14 estimates, with the magnitude of correction expected to amplify in the future due to climate change. The FSF-PM findings provide a more comprehensive and representative understanding of heavy precipitation occurrence and flood risk, assisting in better flood risk assessment, infrastructure preparedness, and mitigation efforts.
National overview
The First Street Foundation Precipitation Model (FSF-PM) results reveal that 167.2 million people in the US (over 51% of its population) reside in a county where stormwater system failure is likely to occur today, as those areas are now at least twice as likely to experience severe levels of flooding (associated with the previously thought of 1-in-100-year events) from rainfall each year. Of that group, 43.6 million Americans (13.3% of the population) are over five times more likely to experience that same level of severe flooding. The depths of water associated with severe flooding that was previously considered a rare 1-in-100-year storm in Atlas 14 will now be experienced every 20 years on average by those Americans.
Over the past 20 years, NOAA’s ASOS rain gauges have recorded 30 locations that have experienced multiple 1-in-100-year events and 13 locations that have reported 1-in-500-year events, based on the Atlas 14 classification. The traditional method of estimating flood risk not only emphasizes out-of-date data but also is more inaccurate because it ignores precipitation during extreme weather events. Using FEMA’s Special Flood Hazard Area (SFHA) designation as the primary measure of flood risk in the United States further exacerbates this issue, as SFHA does not factor in precipitation when calculating flood risk. Over half the risk in the country is unaccounted for due to the lack of precipitation in the SFHA modeling process. Across the nation, the FSF-FM, which has been updated with the newly developed FSF-PM, reveals that approximately 17.7 million properties across the country are at substantial risk or greater (1-in-100-year risk or greater), which is 2.2 times more than the amount included in the FEMA 1-in-100 SFHA designation.
Policy implications
The United States plans to use $1.2 Trillion in Federal appropriations from the Infrastructure Investment and Jobs Act (IIJA) of 2022 to protect against future flooding. However, inaccurate rainfall information poses a significant threat to the allocation and design standards of these projects. The First Street Foundation’s analysis provides data to understand the risk and design effective protections more accurately. The current flood risk standard in the US, FEMA’s SFHA designation, does not account for precipitation, leaving over half of the country’s risk unaddressed. The FSF-FM reveals that 17.7 million properties across the country are at substantial risk or greater, which is 2.2 times more than FEMA’s designation. Without accurate data, infrastructure projects may be based on inaccurate data, leading to wasted taxpayer dollars and inadequate protection against flooding. Policymakers must ensure accurate data is available and used in infrastructure planning and design by requiring funding for research and data collection and updating FEMA’s SFHA designation to account for precipitation. Taking these steps protects communities from flooding and ensures infrastructure investments are based on accurate data and analysis.
Conclusion
The First Street Foundation Flood Model highlights the urgent need for accurate data collection and analysis to protect communities from the impacts of climate change. Policymakers must prioritize the collection and analysis of accurate data to ensure that infrastructure investments provide effective protection against flooding. This includes allocating funding for research, ensuring that climate change and extreme events are consistently taken into account in precipitation models, and updating FEMA’s SFHA designation to consider precipitation. By taking these steps, policymakers can ensure that communities are adequately prepared for the increasingly severe impacts of climate change.