In June 2025, the North Carolina Department of Environmental Quality (NCDEQ) released the state’s first statewide, site-specific Probable Maximum Precipitation (PMP) Study, along with a companion PMP Evaluation Tool to support the NCDEQ Dam Safety Program.

This marks the first time North Carolina has developed PMP values tailored specifically to its geography, climate, and storm history. The study provides a modern, science-based foundation for understanding extreme rainfall and storm risk, giving dam owners, engineers, and regulators access to more accurate, locally relevant data for dam safety evaluations, flood risk management, and storm preparedness planning.

RiskHydro was honored to serve on NCDEQ’s PMP Technical Advisory Committee, contributing to the development of this statewide effort to modernize rainfall estimation and strengthen North Carolina’s dam safety program.

What’s New in NCDEQ’s PMP Tool

The new PMP Study and Evaluation Tool replaces outdated guidance from the 1970s and provides a much higher level of accuracy and usability. Key improvements include:

• Three distinct storm types: local, general, and tropical, each representing different rainfall patterns and storm behaviors across the state.
• Shorter-duration storms: rainfall durations as brief as one hour, improving modeling accuracy for small watersheds and high-intensity events.
• Gridded rainfall data: higher spatial resolution that supports more precise and regionally specific rainfall estimates.
• Modern storm datasets: over 150 years of storm data, including recent tropical systems, to reflect contemporary rainfall behavior.

The full study and tool can be downloaded directly from the NCDEQ Dam Safety Program PMP Study and Evaluation Tool webpage. The site includes the PMP data files, evaluation tool, user guide, and training videos for public use.

Why It Matters

The new PMP Tool changes how consultants and dam owners in North Carolina evaluate rainfall risk and plan for extreme events.

• Smarter decision-making: Updated PMP inputs give dam owners a stronger basis to evaluate potential retrofits, breach alternatives, and risk mitigation strategies using current science.
• More accurate hydrologic modeling: Updated rainfall data improves the precision of inflow design floods (IDFs), helping ensure dams perform safely during the most severe storms.
• Improved regulatory alignment: Projects based on older HMR-51 rainfall data may require review or recalibration. Early adoption of the new PMP data streamlines approvals and reduces uncertainty.

How RiskHydro is Helping Clients Transition

RiskHydro is already applying the new PMP data and methodology across ongoing dam safety and hydrologic studies throughout the state. Our team combines deep technical expertise with direct insight from our participation on the NCDEQ Technical Advisory Committee to help clients adapt efficiently and confidently. We help clients with:

• Training and implementation support for integrating the new PMP methodology into active and future projects.
• Hydrologic model updates using NCDEQ’s new PMP datasets and rainfall distributions.
• Scenario testing for local, general, and tropical storm events.
• Regulatory coordination to ensure documentation aligns with NCDEQ expectations.

In Summary

North Carolina’s new PMP Tool represents a major advancement in how the state approaches extreme rainfall, dam safety, and storm preparedness. It reflects modern data, rigorous analysis, and collaboration among engineers, scientists, and regulators.

At RiskHydro, we remain committed to helping clients apply the new data with clarity, precision, and confidence to protect both infrastructure and communities across North Carolina.