Colorado - New Mexico Regional Extreme Precipitation Study

The states of Colorado and New Mexico require that the owners of state-regulated dams construct and operate facilities that will safely accommodate runoff from extreme rainfall events. Because dams have the potential to impact life and property, there is a clear public safety need to develop safe hydraulic features at these dams.The construction of features to safely convey these events often require owners to bear significant costs. There has been a growing need to evaluate and update the methodology to estimate both the magnitude and frequency of rare storm events in general, and to specifically update the estimate of Probable Maximum Precipitation (PMP). In response to this need, the Colorado and New Mexico state dam safety programs carried out the Colorado-New Mexico Regional Extreme Precipitation Study.

The sponsors of this study developed three project goals: (1) To create updated tools that are broadly accepted as representing the state-of-the-practice in estimating the magnitude of PMP and calculating its equivalent precipitation frequency annual exceedance probability (AEP). (2) To use the results of the study to develop a recommendation for a draft standard of practice for use as a model nationally, and (3) To identify areas for future research toward improving estimates of extreme precipitation. To estimate the magnitude of PMP, storm-based deterministic methods
are used. The existing tools to do this are dated and use only storms that occurred before about 1980. This study utilized the list of historic and also added analysis of more recent extreme storm events, such as the September 2013 event that occurred in both Colorado and New Mexico. Additional advances in the deterministic methodologies were also utilized. Probabilistic Precipitation Frequency analysis has not been previously done as part of state-wide PMP studies. This analysis was added to this study to provide an independent check of the rarity of deterministic PMP estimates, and to also provide the states with tools for probabilistic risk analyses. Although the NOAA Hydrometeorological Design Studies Center (HDSC) has not done research in deterministic PMP analysis since 1999, the NOAA Earth Systems Research Laboratory (ESRL) Physical Sciences Division (PSD) has been conducting research in physically-based dynamical weather modeling. This study contracted with the NOAA ESRL PSD to test application of their High Resolution Rapid Refresh (HRRR) and Weather Research and Forecasting (WRF) models in PMP magnitude and frequency estimating.

The CO-NM REPS was performed from June 2016 to August 2018, and encompassed three critical technical components:
Task 1: Estimation of Deterministic Probable Maximum Precipitation
Task 2: Preparation of Probabilistic Precipitation-Frequency Estimates
Task 3: Applications of Physically-based Dynamical Weather Modeling for PMP and Precipitation Frequency analyses