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11 Utah has published two updates to HMR 49:  “2002 Update for Probable Maximum Precipitation, Utah 72 Hour Estimates to 5,000 sq. mi. - March 2003” (USUL)  “Probable Maximum Precipitation Estimates for Short Duration, Small Area Storms in Utah - October 1995” (USUS) These augment, not supersede, HMR 49 and are intended only for use in the state of Utah. HMRs 55a, 49, and 57 provide precipitation values for a local storm (thunderstorm) with 6 hours of duration and a general storm with 72 hours of duration. The results from both the general and local storm should be used in hydrologic trials to determine the critical design values. The HMR methods require obtaining index precipitation from maps, and then adjusting precipitation depths for drainage area size, elevation, and orographic effects specific to the watershed being studied. An alternative to the HMR documents for PMP estimates is a site-specific analysis. Colorado has developed an Extreme Precipitation Analysis Tool (EPAT) and is currently conducting a formal 3rd party meteorological peer review set for completion in April 2014. In general, a site-specific analysis is not readily achievable for small dam owners and because it typically requires a custom analysis by a consultant engineer/meteorologist. Conclusions To determine design precipitation depths for precipitation frequency events, NOAA Atlas 2 and/or Atlas 14 are available (Montana also uses USGS WRI Report 97-4004). NOAA Atlas 14 is an update and supersedes NOAA Atlas 2 in Utah and Colorado, while no update is currently planned for Montana or Wyoming. One advantage of NOAA Atlas 14, the precipitation depths can be easily obtained online using NOAA’s Precipitation Frequency Data Server. PMP estimates can be estimated using HMR methods. Precipitation for both the local and general storms is derived for hydrological evaluation. An alternative to the HMR methods is a site-specific extreme precipitation analysis. Site-specific analysis is not easily achieved and typically requires a custom analysis by a consultant engineer/meteorologist. Selecting the design precipitation is the first step in the hydrologic analysis used to size the dam and spillway. If the dam and spillway are not sized appropriately, an extreme precipitation event can lead to dam overtopping and failure. As the recent precipitation event in Colorado shows, large or rare precipitation events can occur and when they do, the importance of appropriately selecting the design precipitation for a given dam is reinforced. References (with Links where available) Colorado Rules and Regulations for Dam Safety and Dam Construction, January 01, 2007. Colorado Launching Massive Emergency Dam Inspection Program, The Denver Post, by David Olinger, September 23, 2013. Montana Dam Safety Rules and Regulations, Rule 36.14.502 - Hydrologic Standard for Emergency and Principal Spillways, November 5, 1999. Regional Analysis of Annual Precipitation Maxima in Montana, USGS Water- Resources Investigations Report: 97-4004, by Charles Parent, 1997. Utah Requirements for the Design, Construction and Abandonment of Dams, R655-11-4 Hydrologic Design, August 01, 2013. Wyoming Surface Water Regulations and Instructions, Chapter 5 - Reservoirs, 1977. NOAA Data Links Current NWS Precipitation Frequency Documents (Including NOAA Atlas 2 and NOAA Atlas 14): http://www.nws.noaa.gov/oh/hdsc/currentpf.htm Current NWS Probable Maximum Precipitation Documents (Including HMR 55A, HMR 49, and HMR 47): http://www.nws.noaa.gov/oh/hdsc/studies/pmp.html HDSC Exceedance Probability Analysis for Select Storm Events: http://www.nws.noaa.gov/oh/hdsc/aep_storm_analysis/index.html Areal Reduction (Technical Paper No. 29): http://www.nws.noaa.gov/oh/hdsc/Technical_papers/TP29P4.pdf