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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California FLASH FLOOD FORECASTING OVER COMPLEX TERRAIN With an Assessment of the Sulphur Mountain NEXRAD in Southern California Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California Board on Atmospheric Sciences and Climate Division on Earth and Life Studies NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. Support for this project was provided by the National Oceanic and Atmospheric Administration under Contract No. 50-DGNA-1-90024. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project. International Standard Book Number 0-309-09316-3 (Book) International Standard Book Number 0-309-54580-3 (PDF) Library of Congress Control Number 2004116335 Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Cover: Base (0.5°) reflectivity images as seen from the Sulphur Mountain NEXRAD (front) and the adjoining Santa Ana Mountains NEXRAD (back) on February 18, 2004 approximately two minutes apart. Images courtesy of WeatherTAP Inc. Copyright 2005 by the National Academy of Sciences. All rights reserved. Printed in the United States of America
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council. www.national-academies.org
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California COMMITTEE TO ASSESS NEXRAD FLASH FLOOD FORECASTING CAPABILITIES AT SULPHUR MOUNTAIN, CALIFORNIA PAUL L. SMITH, (Chair), South Dakota School of Mines and Technology, Rapid City ANA P. BARROS, Duke University, Durham, North Carolina V. CHANDRASEKAR, Colorado State University, Fort Collins GREGORY S. FORBES, The Weather Channel, Inc., Atlanta, Georgia EVE GRUNTFEST, University of Colorado, Colorado Springs WITOLD F. KRAJEWSKI, University of Iowa, Iowa City THOMAS D. POTTER, University of Utah, Salt Lake City RITA ROBERTS, National Center for Atmospheric Research, Boulder, Colorado MATTHIAS STEINER, Princeton University, New Jersey ROGER M. WAKIMOTO, University of California, Los Angeles NRC Staff JULIE DEMUTH, Study Director ELIZABETH A. GALINIS, Senior Program Assistant
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ROBERT J. SERAFIN (Chair), National Center for Atmospheric Research, Boulder, Colorado FREDERICK R. ANDERSON, McKenna Long and Aldridge LLP, Washington, D.C. ROBERT C. BEARDSLEY, Woods Hole Oceanographic Institution, Massachusetts ROSINA M. BIERBAUM, University of Michigan, Ann Arbor RAFAEL L. BRAS, Massachusetts Institute of Technology, Cambridge MARY ANNE CARROLL, University of Michigan, Ann Arbor WALTER F. DABBERDT, Vaisala Inc., Boulder, Colorado KERRY A. EMANUEL, Massachusetts Institute of Technology, Cambridge CASSANDRA G. FESEN, Dartmouth College, Hanover, New Hampshire JENNIFER A. LOGAN, Harvard University, Cambridge, Massachusetts WILLIAM J. RANDEL, National Center for Atmospheric Research, Boulder, Colorado ROGER M. WAKIMOTO, University of California, Los Angeles JOHN C. WYNGAARD, Pennsylvania State University, University Park Ex Officio Members ANTONIO J. BUSALACCHI, JR., University of Maryland, College Park ERIC F. WOOD, Princeton University, New Jersey NRC Staff CHRIS ELFRING, Director AMANDA STAUDT, Senior Program Officer JULIE DEMUTH, Program Officer SHELDON DROBOT, Program Officer ELIZABETH A. GALINIS, Senior Program Assistant ROB GREENWAY, Senior Program Assistant DIANE GUSTAFSON, Administrative Coordinator PARIKHIT SINHA, Postdoctoral Fellow ANDREAS SOHRE, Financial Associate
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California Preface The request from the National Oceanic and Atmospheric Administration (NOAA) to the National Academy of Sciences (NAS) to conduct a study to examine the availability, performance, and capability of the Sulphur Mountain NEXRAD located in Ventura County, California, originated from U.S. Senator Barbara Boxer (California). In a letter to NAS President Bruce Alberts she explained that her motivations were based on the potential threat that flash floods pose to the residents of Ventura and Los Angeles Counties and concerns that the Sulphur Mountain NEXRAD is not effective in providing sufficient warnings (Appendix A). In addition, Senator Boxer specifically requested an examination of the warning failure rate and the radar data gap. Upon receipt of this request, the National Academies Board on Atmospheric Sciences and Climate (BASC) developed a charge that was agreed to by Senator Boxer, NOAA, and the National Academies Governing Board. In January 2004, the Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California, was formed to undertake this study. Over the next six months, we proceeded to hold four meetings, three of which included information-gathering sessions. At the first meeting, we learned first-hand about the motivation for this study as well as the NOAA National Weather Service (NWS) modernization effort and its flash flood warning mission. The committee held its second meeting in Ventura, California, where it conducted two site visits—one to the Los Angeles-Oxnard NWS Weather Forecast Office (WFO) and one to the site of the Sulphur Mountain NEXRAD. In addition, a full day of that meeting was devoted to hearing nine invited presentations on topics ranging from flash flood hydrology to forecasting flash floods to technical issues associated with radars sited in complex terrain and near coastal regions. At the end of that same day, the committee opened the floor to the public, allowing anyone to make comments for consideration by the committee. During this
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California open forum, some of the concerns expressed were with regard to potential health effects posed by the radar. However, the committee’s interpretation is that the issue of health effects is not within the purview of the Statement of Task, which was agreed upon by both Senator Boxer and NOAA, and therefore it is not discussed in this report. Our public information-gathering efforts were concluded at our third meeting by two more experts who provided information about operational flash flood forecasting and the siting of the Sulphur Mountain radar. The committee considered carefully all of the input and materials provided at these meetings, as well as statistics on nationwide flash flood warnings and events, the Sulphur Mountain radar availability, and atmospheric inversion data, which the committee specifically requested from NWS officials and others. After reviewing all of this information, the committee found little substantive basis for the concerns that had been expressed about the flash flood forecasting and warning capabilities of the Los Angeles-Oxnard WFO. Because there always is room for improvement, however, the committee developed its own analyses of the coverage provided by the Sulphur Mountain radar and determined ways in which that coverage could be improved. This report summarizes the information obtained throughout the study and presents the resulting findings and recommendations. On behalf of the entire committee, I want to thank the many people who provided data, information, and opinions throughout the course of this study; the committee is solely responsible for its interpretations of all the input received. I especially want to thank Daniel Melendez at NWS Headquarters for fielding the numerous data requests of the committee; Dave Danielson and Dan Keeton at the Los Angeles-Oxnard WFO for their assistance with the Los Angeles-Oxnard data; and Michal Kraszewski, Radoslaw Goska, Anton Kruger, and Alexandros Ntelekos for providing us with detailed radar coverage and power loss calculations. I also want to express appreciation to Milton Kramer for his attention to this issue; not far into this study, the committee learned to appreciate the passion of the Ventura County people for this issue, and I think the committee would agree that this made for an interesting and informative study. Thank you also to Bob Serafin, our liaison to BASC, who attended a couple of our meetings and contributed many thoughtful ideas, and to Connie Crandall for her assistance in preparing some of our drafts of this report. And a special thank you to the BASC staff who assisted with this project, including our Study Director, Julie Demuth, and our Senior Program Assistant, Liz Galinis. Finally, I want to commend the committee—a tremendous group of thoughtful, hard-working, talented people who volunteered generously their
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California time in order to complete this study in a condensed time frame. Via our many contemplative and provocative conversations, we provide this assessment of using NEXRAD radars in complex terrain for flash flood forecasting with a special assessment of the Sulphur Mountain NEXRAD. We hope it proves useful to operational forecasters, researchers, policymakers, and stakeholders. Paul L. Smith, Chair Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California Acknowledgments This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: Howard B. Bluestein, University of Oklahoma, Norman Leon E. Borgman, University of Wyoming, Laramie Elbert (Joe) W. Friday, Jr., University of Oklahoma, Norman Stanley Q. Kidder, Colorado State University, Fort Collins Clifford E. Mass, University of Washington, Seattle Iraj Nasseri, Los Angeles County Department of Public Works, California Dallas Raines, KABC-TV, Los Angeles, California James A. Smith, Princeton University, New Jersey Soroosh Sorooshian, University of California, Irvine James Wilson, National Center for Atmospheric Research, Boulder, Colorado Although the reviewers listed above have provided constructive comments and suggestions, they were not asked to endorse the report’s conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Kuo-Nan Liou, University of California, Los Angeles, and George Frederick, Vaisala, Inc. Appointed by the National Research Council, they were responsible for
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California Contents EXECUTIVE SUMMARY 1 1 INTRODUCTION 9 2 FLASH FLOODS 12 The Physical Processes of Flash Floods, 13 3 THE NATIONAL WEATHER SERVICE AND FLASH FLOODS 26 The Modernization and Associated Restructuring of the National Weather Service, 27 The Process of Issuing Flash Flood Forecasts, Watches, and Warnings, 29 4 NEXRAD 35 The NEXRAD Network, 35 NEXRAD Scan Strategies, 40 Precipitation Estimation, 41 The Evolving NEXRAD System, 49 Siting of the NEXRADs, 51 5 OBSERVATIONAL CHALLENGES OF LOW-LEVEL RADAR COVERAGE IN COMPLEX TERRAIN AND COASTAL AREAS 59 Beam Visibility, 60 Propagation of Electromagnetic Radar Signals, 61 6 FLASH FLOOD WARNING PROCESS IN LOS ANGELES AND VENTURA COUNTIES 74 Rainfall Records for Los Angeles and Ventura Counties, 77 The NWS Los Angeles-Oxnard Flash Flood Warning Process, 82
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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California Private Sector Role in Forecasts and Dissemination, 85 Stakeholders, 86 Need for Multiagency and Public Collaboration, 88 7 EVALUATION OF THE SULPHUR MOUNTAIN RADAR AND FLASH FLOOD WARNINGS IN LOS ANGELES AND VENTURA COUNTIES 91 Radar Coverage, 91 Availability of the Sulphur Mountain Radar, 100 Flash Flood Warning Statistics, 101 8 POTENTIAL IMPROVEMENTS IN FLASH FLOOD WARNINGS 117 NEXRAD and Other Radar Sources, 118 Modeling, Data Assimilation, and Decision Support Systems, 135 9 CONCLUDING THOUGHTS 146 REFERENCES 149 APPENDIXES A Origin of the Study 169 B Characteristics of the NEXRAD Radar 172 C Chronology of the Siting of the Sulphur Mountain NEXRAD 175 D National Weather Service Flash Flood Verification Procedures 176 E Acronyms 180 F Community Participation 184 G Committee and Staff Biographies 186