National Oceanic and Atmospheric Administration’s (NOAA’s) National Weather Service (NWS) proposes updating precipitation frequency estimates for the states not included or in process for publication in the following Volumes of NOAA Atlas 14 “Precipitation-Frequency Atlas of the United States”: Volume 1 AZ, NM, NV, UT Volume 2 DE, DC, IL, IN, KY, MD, NC, NJ, OH, PA, SC, TN, VA, WV Volume 3 PR, VI Volume 4 HI Volume 5 Pacific Islands (selected) Volume 6 CA Volume 7 AK Volume 8 CO, IA, KS, MI, MN, MO, ND, NE, OK, SD, WI Volume 9 AL, AR, FL, GA, LA, MS Volume 10 CT, MA, ME, VT, NH, NY, RI, VT. Volume 11 TX Precipitation frequency estimates for the northwest states will be published as Volume 12 on the web at www.nws.noaa.gov/ohd/hdsc. The new NOAA Atlas 14 estimates are greatly improved in terms of accuracy, reliability and resolution. They are derived using improved data in terms of period of record and station density, state of the art statistical techniques, and spatial interpolation method that accounts for variation in terrain.

The purpose of this project is to develop precipitation frequency estimates for the five Northwestern states (ID, MT, OR, WA, WY) for durations of 5-minute through 60-day at average recurrence intervals (ARIs) of 1-year through 1,000-year. The estimates and associated bounds of 90% confidence intervals will be provided at 30 arc-sec resolution (approximately 800 x 800 m; varies with latitude). The study results will be published as NOAA Atlas 14 Volume 12, a wholly web based publication available at Precipitation Frequency Data Server (PFDS). The publication will include the artifacts provided in previous NOAA Atlas 14 Volumes, including access through the PFDS, base grids in standard formats together with error estimates, electronic copies of maps, charts of seasonal distributions and probabilistic temporal distributions of heavy precipitation, and detailed documentation. Updated areal reduction factors, which are needed to calculate analogous areal precipitation frequency estimates, will not be developed as a part of this project.

Task 1 - Data collection and quality control Task 2 - Regionalization and calculation of regional statistics Task 3 - Frequency distribution selection and frequency calculations Task 4 - Estimates for 5-minute and 10-minute durations Task 5 - Internal consistency at observing locations Task 6 - Error estimates for DDF/IDF curves Task 7 - Rainfall frequency estimates with confidence limits Task 8 - Spatial interpolation and consistency Task 9 - Temporal distributions Task 10 - Peer review Task 11 - Documentation Task 12 - Final deliverables Task 13 - Status Reporting