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Probable Maximum Precipitation Study for Virginia

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Probable Maximum Precipitation Study for Virginia Probable Maximum Precipitation Study for Virginia Prepared for Virginia Department of Conservation and Recreation 600 East Main Street, 24th Floor, Richmond, VA 23219-2094 (804) 371-6095 www.dcr.virginia.gov Prepared by Applied Weather Associates, LLC PO Box 175, Monument, CO 80132 (719) 488-4311 www.appliedweatherassociates.com Bill Kappel, Project Manager and Chief Meteorologist Doug Hultstrand, Senior Hydrometeorologist Jacob Rodel, Staff GIS Analyst Geoff Muhlestein, Senior GIS Analyst Kristi Steinhilber, Staff Meteorologist Dana McGlone, Staff Meteorologist Bryon Lawrence, Staff Meteorologist November 2015 Notice This report was prepared by Applied Weather Associates, LLC (AWA). The results and conclusions in this report are based upon best professional judgment using currently available data. Therefore, neither AWA nor any person acting on behalf of AWA can: (a) make any warranty, expressed or implied, regarding future use of any information or method in this report, or (b) assume any future liability regarding use of any information or method contained in this report. ii Acknowledgements The Virginia Department of Conservation and Recreation (Department) and the Virginia Soil and Water Conservation Board (Board) would like to express sincere appreciation and thanks for the hard work and dedication of the entire staff of Applied Weather Associates, LLC. The Department and the Board would also like to acknowledge with much appreciation the review and feedback of the study’s independent Technical Review Panel comprised of John Harrison, PE, Schnabel Engineering, Inc., Mathew Lyons, PE, USDA Natural Resources Conservation Service, Dr. Arthur Miller, AECOM, and Stephen Rich, CCM, Southeast Weather Consulting. Furthermore, the Department and the Board would like to express gratitude and thanks to staff members from the Federal Energy Regulatory Commission, Ken Fearon, Elise Dombeck, and Kevin Griebenow. iii Table of Contents Table of Contents iv List of Figures vi List of Tables viii Executive Summary x Glossary xv 1. Introduction 1 1.1 Background 2 1.2 Objective 6 1.3 Approach 6 1.4 PMP Analysis Domain 8 1.5 PMP Analysis Grid Setup 8 2. Weather and Climate of the Region 10 2.1 Air Mass Source Regions 13 2.2 PMP Storm Types 14 2.2.1 General Storms 14 2.2.2 Tropical Storms 15 2.2.3 Hybrid Storms 16 2.2.4 Local Storms (Thunderstorms and Mesoscale Convective Systems) 16 3. Topographic Effects on PMP Rainfall 18 3.1 Orographic Effects 21 4. Dew Point and SST Climatology Background 24 4.1 Use of Dew Point Temperatures 24 4.2 Use of Sea Surface Temperatures 25 5. Extreme Storm Identification 27 5.1 Storm Search Area 27 5.2 Data Sources 27 5.3 Storm Search Method 29 5.4 Developing the Short List of Extreme Storms 38 6. Storm Maximization 45 6.1 Storm Representative Dew Point Determination Process 46 6.1.1 Storm Representative Dew Point Determination Example 48 6.1.2 Rationale for Adjusting HMR 51 Persisting Dew Point Values 52 6.1.3 Background on Adjusting HMR 51 Persisting Dew Point Values 53 6.2 Storm Representative Sea Surface Temperatures (SSTs) Calculation Example 54 7. Storm Transpositioning 56 8. Development of PMP Values 61 8.1 Available Moisture at Source and Target Locations 62 8.2 In-Place Maximization Factor 63 iv 8.3 Moisture Transposition Factor 63 8.4 Orographic Transposition Factor 64 8.4.1 OTF Calculations for Smethport, PA and Simpson, KY 64 8.5 Total Adjusted Rainfall 66 8.6 Sample Calculations 67 8.6.1 Example of Precipitable Water Calculations – Reducing MTF Factor 68 8.6.2 In-place Maximization Factor 69 8.6.3 Moisture Transposition Factor 69 8.6.4 Supplemental Calculation – Enlarging MTF Factor 70 8.6.5 Orographic Transposition Factor 72 8.6.6 Total Adjustment Factor 73 8.6.7 Supplemental Calculation Orographic Transposition Factor – Enlarging Factor74 8.6.8 Total Adjustment Factor 75 8.7 PMP Calculation Process 76 8.7.1 PMP Evaluation Tool Description and Usage 76 8.8 Temporal Distribution of PMP Values 82 9. Procedure for Calculating Basin-Specific PMP 84 9.1 Basin Average PMP Calculation 85 10. PMP Sensitivity and Comparisons 88 10.1 Evaluation of Basin-Specific PMP 95 10.2 Comparison of the PMP Values with Precipitation Frequency 97 10.3 Annual Exceedance Probability of Short List Storms 98 10.4 Comparison of the PMP Values with HMR PMP Values 101 10.4.1 Discussion of Comparison Results 114 11. Sensitivity Discussions Related to PMP Derivations 118 11.1 Assumptions 118 11.1.1 Saturated Storm Atmosphere 118 11.1.2 Maximum Storm Efficiency 118 11.2 Parameters 119 11.2.1 Storm Representative Dew Point and Maximum Dew Point 119 11.2.2 Sensitivity of the Elevation Adjustment Factor to Changes in Storm Elevation119 12. Recommendations for Application 121 12.1 Site-Specific PMP Applications 121 12.2 Climate Change Assumptions 121 12.3 Future Work Requirements 122 Bibliography and Reference List 123 v APPENDICES Appendix A: Virginia Probable Maximum Precipitation Maps Appendix B: 100-year Return Frequency Maximum Average Dew Point and SST Climatology Maps Appendix C: Orographic Transpositioning Factor (OTF) Maps Appendix D: PMP Evaluation Tool Python Script Appendix E: PMP Version Log: Changes to Storm Database and Adjustment Factors Appendix F: PMP Storm Data (Separate Binding) Appendix G : Storm Precipitation Analysis System (SPAS) Description Appendix H: Point OTF Evaluation for PMP Calculations – Use of Single Point vs Areal- Average Precipitation Climatology Values Appendix I: HMR Storm Separation Method (SSM) Appendix J: Supplemental Digital Data DVD Appendix K: Review Board Letters List of Figures Figure 1.1 Hydrometeorological Report coverages across the United States ................................ 3 Figure 1.2 Example of HMR 51 72-hour 200-square mile PMP map showing the stippled regions (from Schreiner and Riedel, 1978). .................................................................................... 4 Figure 1.3 Virginia PMP project domain. The overall project domain extends beyond the state boundaries in some areas to ensure all drainage areas are included in the analysis. ...................... 5 Figure 1.4 Locations of AWA PMP studies as of November 2015 ............................................... 5 Figure 1.5 PMP analysis project domain ....................................................................................... 8 Figure 1.6 PMP analysis grid placement over the North Anna basin ............................................ 9 Figure 2.1 Synoptic weather features associated with moisture from the Atlantic Ocean .......... 11 Figure 2.2 Locations of surface features associated with moisture advection from the Gulf of Mexico into Virginia and surrounding regions ............................................................................. 12 Figure 2.3 Air mass source regions affecting the Virginia basin (Ahrens, 2007) ....................... 14 Figure 3.1 Topography across the analysis domain ..................................................................... 19 Figure 3.2 Elevation contours at 500 foot intervals over the state of Virginia ............................ 20 Figure 4.1 Maximum dew point climatology development regions and dates ............................ 25 Figure 5.1 Virginia storm search domain .................................................................................... 29 Figure 5.2 Storm locations for storms on the short storm list ...................................................... 41 Figure 5.3 Storm locations for local storms on the short storm list ............................................. 42 Figure 5.4 Storm locations for general storms on the short storm list ......................................... 43 Figure 5.5 Storm locations for tropical storms on the short storm list ........................................ 44 Figure 6.1 Dew point values used to determine the storm representative dew point for Tamaqua, PA, June 2006, SPAS 1047 storm event. ...................................................................................... 48 Figure 6.2 HYSPLIT trajectory model results for the Madisonville, KY March, 1964 (SPAS 1278) storm ................................................................................................................................... 50 Figure 6.3 Surface stations, 24-hour average dew points, and moisture source region, along with HYSPLIT trajectory model results for the Madisonville, KY March, 1964 (SPAS 1278) storm. 51 vi Figure 6.4 Daily SST observations used to determine the storm representative SST value for the SPAS 1276, June 1972 storm event. ............................................................................................. 55 Figure 7.1 Transposition zones used to define transposition limits for individual storms .......... 57 Figure 7.2 Orographic Transposition Factors for Halifax, VT October 2005, SPAS 1201. ........ 60 Figure 8.1 Comparison of total adjusted values of the Smethport, PA, Simpson, KY, and Holt, MO storms without constraint compared to the world record rainfalls. ....................................... 65 Figure 8.2 Example of a storm adjustment factor feature class table .......................................... 67 Figure 8.3 Location of Warner Park, TN, April 2010 (SPAS 1208) transposition to grid point #4,898............................................................................................................................................ 68 Figure 8.4 Location of Wellsboro, PA, May 1889 (SPAS 1339) transposition to grid point #13,697.........................................................................................................................................
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