Probable Maximum Precipitation Study for Pennsylvania Prepared for Pennsylvania Department of Environmental Protection 400 Market Street, Harrisburg, PA 17105-8460 (717) 787-8568 https://www.dep.pa.gov/Business/Water/Waterways/DamSafety/Pages/default.aspx 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 Bryon Lawrence, Staff Meteorologist March 2019 Pennsylvania Probable Maximum Precipitation Study 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 Pennsylvania Probable Maximum Precipitation Study Acknowledgements The Pennsylvania Department of Environmental Protection (PA DEP) would like to express sincere appreciation and thanks for the hard work and dedication of the staff of Applied Weather Associates. In addition, the efforts of Aterra Solutions to investigate and more accurately quantify the runoff resulting from the Smethport, PA July 1942 storms were invaluable to the success of this study. The potential benefits of reevaluating the Smethport, PA July 1942 storm were recognized as part of work funded by the St. Mary’s Water Authority which was completed by Applied Weather Associates in 2007. PA DEP 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.; Dr. Barry Keim, Louisiana State Climatology/Professor at LSU; Dr. Arthur Miller, AECOM; and Dr. Kyle Imhoff, Pennsylvania State Climatologist/Professor Penn State University. Furthermore, the PA DEP would like to express gratitude and thanks to staff members from the Federal Energy Regulatory Commission, Ken Fearon, Devan Mahadevan, Nicholas Agnoli, and Matt Lehrer for their support, discussion, insight, and suggestions. Finally, PA DEP would also like to recognize several others that attended and participated in the many review meetings. Their contribution in this process and anticipated involvement in the future is much appreciated. iii Pennsylvania Probable Maximum Precipitation Study Table of Contents Executive Summary ....................................................................................................................... xi Glossary ....................................................................................................................................... xvi List of Acronyms ........................................................................................................................ xxii 1. PMP Development Background ............................................................................................. 1 1.1 Background ...................................................................................................................... 2 1.2 Objective ........................................................................................................................ 10 1.3 PMP Analysis Domain ................................................................................................... 10 1.4 PMP Analysis Grid Setup .............................................................................................. 10 2. Methodology ......................................................................................................................... 12 3. Weather and Climate of the Region ...................................................................................... 15 3.1 Regional Climatological Characteristics Affecting PMP Storm Types ......................... 18 3.2 Storm Types ................................................................................................................... 18 3.2.1 Local Storms ........................................................................................................... 19 3.2.2 General Storms........................................................................................................ 21 3.2.3 Tropical Storms ....................................................................................................... 22 4. Topographic Effects on Precipitation ................................................................................... 23 5. Data Description and Sources ............................................................................................... 26 5.1 Use of Dew Point Temperatures .................................................................................... 26 5.2 Use of Sea Surface Temperatures .................................................................................. 28 6. Data Quality Control and Quality Assurance ....................................................................... 30 7. Storm Selection ..................................................................................................................... 31 7.1 Storm Search Process ..................................................................................................... 31 7.2 Short Storm List Development ....................................................................................... 34 7.3 Catskill, NY July 1819 Storm Event Discussions .......................................................... 34 7.4 Final PMP Storm List Development .............................................................................. 35 8. SPAS Analysis Results ......................................................................................................... 43 8.1 SPAS Data Collection .................................................................................................... 43 8.2 SPAS Mass Curve Development.................................................................................... 44 8.3 Hourly and Sub-Hourly Precipitation Maps................................................................... 44 8.4 Standard SPAS Mode Using a Basemap Only ............................................................... 44 8.5 SPAS-NEXRAD Mode .................................................................................................. 44 8.6 Depth-Area-Duration Program ....................................................................................... 45 8.7 Comparison of SPAS DAD Output Versus Previous DAD Results .............................. 45 iv Pennsylvania Probable Maximum Precipitation Study 8.8 Utilization of Hydrologic Investigation for Smethport Reanalysis ................................ 46 9. Storm Adjustments................................................................................................................ 53 9.1 In-Place Maximization Process ...................................................................................... 53 9.2 Storm Representative Dew Point Determination Process .............................................. 54 9.2.1 Storm Representative Dew Point Determination Example ..................................... 56 9.2.2 Storm Representative Sea Surface Temperatures Calculation Example ................ 60 9.3 In-Place Maximization Factor (IPMF) Calculation ....................................................... 61 9.4 Transposition Zones ....................................................................................................... 62 9.4.1 Updated Transposition Limits for the Smethport July 1942 Storm ........................ 65 9.5 Moisture Transposition Factor ....................................................................................... 66 9.6 Moisture Transposition Factor Calculation Example..................................................... 67 9.7 Geographic Transposition Factor ................................................................................... 67 9.7.1 NOAA Atlas 14 Smoothing .................................................................................... 68 9.8 Geographic Transposition Factor (GTF) Calculation .................................................... 72 Total Adjustment Factor (TAF) ............................................................................................ 73 9.9.............................................................................................................................................. 73 10. Development of PMP Values............................................................................................. 74 10.1 PMP Calculation Process............................................................................................ 74 10.1.1 Sample Calculations................................................................................................ 74 10.1.2 Sample Precipitable Water Calculation .................................................................. 76 10.1.3 Sample IPMF Calculation ....................................................................................... 76 10.1.4 Sample GTF Calculation......................................................................................... 77 10.1.5 Sample TAF Calculation........................................................................................