Ed Tomlinson, PhD Bill Kappel Doug Hultstrand Applied Weather Associates, LLC Monument, Colorado

1. Review the Extreme Precipitation Analysis Tool (EPAT)Phase 2: Independent Meteorological Review Ed Tomlinson, PhD Bill Kappel Doug Hultstrand Applied Weather Associates, LLC Monument, Colorado EPAT Phase 2 60% Meeting Colorado Springs, Colorado December 16, 2013

2. Applied Weather Associates, LLP Established 1996 Ed Tomlinson, PhD • President and Chief Meteorologist Bill Kappel • Vice-President and Senior Meteorologist Eight meteorologists and a GIS specialist Applied Weather Associates, LLC PO Box 680 Monument, Colorado 80132-0680 719/488-9117 E-mail: edtomlinson@appliedweatherassociates.com billkappel@appliedweatherassociates.com Web-site: www.appliedweatherassociates.com

3. Applied Weather Associates, Personnel • Chief Meteorologist • Ed Tomlinson, PhD • Senior Meteorologists • Bill Kappel • Hydrometeorologist • Doug Hultstrand • GIS Specialist • Geoff Muhlestein

5. Probable Maximum Precipitation Definition: The theoreticallygreatest depthof precipitation for a given durationthat is physically possibleover a given storm area at a particular geographic location at a certain time of year (HMR 59, 1999)

6. Tasks • Task 1 Familiarization with EPAT • Review the EPAT v4.2 program and EPAT Phase I deliverables • Develop an understand how EPAT works • Identify known errors and concerns • Provide a top level review of the GIS code

7. Tasks • Task 2 Evaluate the EPAT Storm Library • Evaluate spatial and temporal characteristics of the storm data included in the EPAT Storm Library • Evaluate completeness, reliability and accuracy

8. Tasks • Task 3 Evaluate In-place Maximization and Transposition Procedures • Independently evaluate the in-place maximization factors for each storm in the EPAT Storm Library • The storm transposition procedure will be reviewed • Access the reliability of horizontal and vertical adjustments

9. Tasks • Task 4 Evaluation of the EPAT Procedures • Evaluate all EPAT procedures • Current PMP science and practice • Evaluate accuracy and reliability of calculations • In-place maximization factors • Storm transposition limits • Moisture transposition factors • Storm orientations • Moisture inflow vectors • Storm isohyetal patterns. • EPAT procedures that are current and applied appropriately will be identified • Identify outdated and incorrect procedures • Recommend updated procedures

10. Tasks • Task 5 EPAT Rio Grande Dam Drainage Basin Evaluation • Evaluate EPAT v4.2 rainfall depth, area and duration results • Provide comparisonswith information from NOAA Atlases 2 and 14, HMR 49 and HMR 55A • Determine PMP values using storms in the AWA storm analysis database from site-specific PMP studies in Colorado and from the Arizona statewide study • Appropriate storms will be transpositioned for surrounding regions • Run the PMP Evaluation Tool developed for the Arizona statewide PMP study • Compare with EPAT results and discuss differences • Comparison of the storms used to derive PMP • Comparisons of the various maximization factors, and other parameters of each storm where the same storm was used in both analyses

11. Tasks • Task 6 Recommendations for EPAT Modification • Recommend appropriate procedures and GIS applications of those procedures for continued use • Identify outdated and/or incorrect procedures • Recommend potential EPAT modifications • Improve the science • Improve the reliability • Recommend additions of state-of-the-science procedures • Recommend continued use of the EPAT GIS architecture and software will be made where appropriate • Recommend improvements where reliability and efficiency can be achieved

12. Review the Extreme Precipitation Analysis Tool (EPAT) • EPAT • Objective • Objective tool • Alternative assessment to PMP • Alternative methodology for calculation of PMP • Provide refined assessment with respect to HMRs • Refine HMR and site-specific procedures • Area Sizes: 1 – 500 square miles • Elevations: Above 5,500 feet • Provide site-specific PMP assessments • Scientifically reproducible extreme precipitation estimates • Incorporate technological improvements • Spatial and temporal attributes • Use Doppler radar (NEXRAD) • Technical advances in meteorology and statistics • AWA Review Goals • Identify procedures and evaluate consistency with PMP procedures • Evaluate non-consistent procedures for technical and scientific reliability

13. Review the Extreme Precipitation Analysis Tool (EPAT) • Site-specific PMP (SSPMP) studies became leading method to determine PMP 1995-2005 • SSPMP studies • Procedural inconsistencies • Outdated methods • Needed updated storm library • Did not incorporate advances such as updated maximum dewpoint climatologies • Did not include Doppler radar • Developed to use explicit storm transpositioning • No discussion on use of the Storm Separation Method • Envisioned to provide a new standard of practice • Include new science • Climate change not included • …climate is changing…the quantification of future climate change is still not refined enough to enable inclusion in deterministic PMP techniques

14. Review the Extreme Precipitation Analysis Tool (EPAT) • Five primary procedures • In-place maximization • Transposition • Storm placement and orientation • Elevation adjustment • Areal point reduction

15. Review the Extreme Precipitation Analysis Tool (EPAT) • Storm Library • Has meteorological data from most significant storms • Two types • Local • Isolated in both time and space • 6 hours or less • 500 square miles or less • General • Longer than 6 hours • Storm area greater than 500 square miles • EPAT made exceptions for Local Storms • Some EPAT Local Storms were general storms with embedded convection • States that HMR series used convective portions of General Storms as Local Storms • States that both types need deep moisture • Consistent with storm maximization procedure • 76 storms in the library

16. Review the Extreme Precipitation Analysis Tool (EPAT) • Bunching of significant storms along the Front Range and Palmer Divide • Result of meteorological mechanisms • A factor of remote sensing capabilities • Local Storms • Generate most, if not all, of their rainfall in 6 hours or less • Consistent with HMR 55A • EPAT uses storms that “expand beyond the 6-hour time frame • In some cases, longer than 24 hours • Given the requirement of rainfall values to meet or exceed specific depth-duration frequency (DDF) criteria over this relatively short period of time, storms must have produced relatively high rainfall during the event. • Most local storms … involved some form of deep, moist convective atmospheric processes.

17. Review the Extreme Precipitation Analysis Tool (EPAT) • Local Storms (continued) • Significant meteorological factor: winds in the sub-cloud layer • Low level moisture advection • Observation has shown that if the winds in the sub-cloud layer > 25 knots, rainfall production …can approach a doubling of rain rate • Not part of PMP evaluations • Identifies winds associated with a storm • Sub-cloud winds • Not used to evaluate moisture advection • Cloud layer winds • Can produce train-echo effect • Produce inverse wind shear • Storm orientation results from cloud layer winds

18. Review the Extreme Precipitation Analysis Tool (EPAT) • Temporal characteristics of Local Storms • Temporal distribution of rainfall of a storm used for the maximized and transpositioned storm • If NEXRAD was available, 5-minute rainfall was produced using a dbZ conversion to a scaled format from 6 down to 0 • A number of geographic points were selected and averaged • NRCS SCS 24-hour Type II rainfall distribution initially implemented • In some storms, rainfall rates exceeded world records • The use of SCS Type II appeared to be too conservative • After using SCS Type II, if there was no “specific dialogue returned to HDR”, SCS Type II distributions were retained

19. Review the Extreme Precipitation Analysis Tool (EPAT) • Temporal characteristics of Local Storms • Alternate temporal distribution used • Henz Convective Storm Model (HCSM) • Used to develop peak hourly rainfall rates • HCMS was used in concert with “some level of reasonable deduction” to distribute rainfall in a manner consistent with information available to HDR staff. • EPAT peaks rain rates were reviewed where HMR and SSPMPP studies had been performed • In all cases the magnitude of the peak rainfall intensity for the EPAT controlling local storm was less than the HMR or SSPMP temporally distributed storm • The difference is a direct derivative of using data from the denser rain gage networks found in urban areas and analyses of WSR-88D reflectivity fields • EPAT has bounding limits for peak rainfall intensities from world and US records

20. Review the Extreme Precipitation Analysis Tool (EPAT) • Spatial Profiles • Storm Total Precipitation used for some storms • GIS interpolation techniques used to produce smoothed spatial isopleths • Produced by storm reconstruction by HDR or NOAA National Storm Data files

21. Review the Extreme Precipitation Analysis Tool (EPAT) • General Storms • Storms caused by large-scale vertical motion associated with synoptic scale weather features • Duration: >6 hours • Storm area: >1,000 square miles • Usually steady rain • Produces far greater volume of precipitation/runoff than local storms • EPAT classified some HMR general storms as local storms • East of the Divide, EPAT General Storms were strong slow-moving extra-tropical storms that induced a persistent inflow of cool and nearly saturated airmass into the topography of the foothills. • Key is the multi-day nature of a slow-moving upper-level storm system from the Pacific • EPAT has only four General Storms east of the Divide • General Storms cover a large spatial region • For East Slope general storms, there was no clear discernible pattern

22. Review the Extreme Precipitation Analysis Tool (EPAT) • EPAT Storm Library

23. Review the Extreme Precipitation Analysis Tool (EPAT) • In-Place Maximization • Average dewpoint values sometimes used • Professional opinion was used for appropriate lengths of duration for some storms • Appears that maximum 12-hour persisting dewpoints used for maximization • Documentation is provided to the best extent possible given data availability • In-place maximization is not automated in EPAT • Sources of in-place maximization is in Appendix A, if available • Surface dewpoint values used to determine Precipitable Water Index (PWI) • The in-place maximization step begins with a determination of the surface moisture that was attributed to the production of peak rainfall in the development of the original storm through examination of historical meteorological observations. The surface dewpoint value is then used to determine the …PWI… • The next step involves determining the climatological maximum PWI for the location where the storm originally took place and adjusted 15 days toward the time of seasonal maximum moisture availability

24. Review the Extreme Precipitation Analysis Tool (EPAT) • In-Place Maximization • In-place maximization factor computed at 1,000 mb, not at the storm/barrier elevation • In-place maximization factor limited to 1.5 in accordance with HMR 51 • The in-place maximization factor was computed using dewpoint information at the storm’s original location at 1,000 mb • For many historical storms analyzed by other agencies, … PWI variables were used • 1,000mb values were used • Calculations referred to 12-hour maximum persisting dewpoint as defined by Henz and Tomlinson, 2003 • Surface data were used and leveraged by a comprehensive database of upper-air observations • Observations from areas where inflow moisture was likely flowing into a storm was interrogated for determing the representative dewpoint for the event • For local storms, observations in the general area were sufficient • NOAA-NCDC data can now be obtained in a very minimal period of time than was possible when the EPAT V4.2 database was initially constructed • From 1998 onward, hourly weather observations were available on-line

25. Review the Extreme Precipitation Analysis Tool (EPAT) • In-Place Maximization • For many events, short duration dewpoints were derived consistent with the duration of the storm. This reduction in period was consistent with the treatment of Local Storms • Use of shorter duration persisting dewpoints is not consistent with EPAT’s use of 12-hour persisting dewpoints • Average (not persisting) dewpoints were used in many storms • It was the professional opinion of EPAT authors to use average dewpoint for appropriate lengths of duration for storms without maximization data already available. • Seasonal adjustment was to add/substract 15 days towards the more moist time of year • Used August 15th as the maximum • Climatological (maximum 12-hour persisting) dewpoint maps are available in HMR 55A east of the Divide • HMR 50 has maps for the southwestern US (12-hour persisting) • Persisting maximum dewpoint maps were converted to PWI values • A raster interpolation of PWI contour lines was created to maintain surface continuity • Figure 37 has different shapes for maximum dewpoint map and PWI map

26. Review the Extreme Precipitation Analysis Tool (EPAT) • Transposition • Transposition uses storm original location and transposition location (no inflow vectors) • Uses HMR 50 or HMR 55A maximum dewpoint maps (1,000 mb 12-hour persisting) • HMR 55A limited the maximum transposition factor to 1.2. EPAT does not limit. • HMR 55A uses barrier moisture depletion in transpositioning and EPAT does not

27. Review the Extreme Precipitation Analysis Tool (EPAT) • EPAT Climate zones • Climate zones are used for transpositioning • NOAA/NCDC defined climate divisions • Agricultural (namely crop) usage • Large river basin delineations • Occasionally geopolitical boundaries • Meteorological factors involved with extreme precipitation storms are not completely aligned with meteorological variables that tie locations together from a climatic standpoint. • A map was created that provided some spatial definition for storm analysis and categorization. • Some guidance was taken from HMR 55A • Need was identified to consider elevation differences across the state • The final version of the climate division map had a fine scale analysis • Some zones represent differences in available moisture but EPAT does not use storm elevation ormoisture barrier depletion

28. Review the Extreme Precipitation Analysis Tool (EPAT) • Storm placement • For storm placement and orientation, prior SSPMPs have maximized rainfall but did not take into consideration storm-specific meteorological characteristics. • Extreme rainfall storms must have persisting inflow of low level moisture • It may not be scientifically justified for a storm to be manipulated to ‘best fit” the greatest volume in the basin. If the storm is placed over the basin that does not maintain the original inflow of low-level winds with respect to the original orientation, then the proper spatial coverage of the storm is potentially compromised • The EPAT process with respect to orientation is anchored in the premise that the two-dimensional orientation of length and width of storms are unique to the low-level and upper-level wind flow. Original premise come from Henz, 2003 in work on the Cherry Creek PMP study. • Low-level winds are subjected to terrain limitations. • Low-level winds are assumed to follow the lowest portion of the basin

29. Review the Extreme Precipitation Analysis Tool (EPAT) • Storm placement • Cross barrier, low-level flow would either not be sustained and/or bring about subsidence (sinking and drying) (called barrier moisture depletion that EPAT does not include) • In order to maintain the integrity of the storm’s spatial characteristics, the storm’s low-level wind vector is required to stay parallel to the lowest part of the basin. • Henz, 2003 in the Cherry Creek SSPMP study documented that rainfall patterns were associated with cloud-layer winds and sub-cloud layer inflow winds. Steering winds established the longitudinal axis of the rainfall pattern while the sub-cloud layer in-flow winds feeding the storm updraft were 40-90 degrees to the right. The storm’s low-level wind vector is mandated to stay parallel to the lowest portion of the basin (thalweg) and the storm orientation is perpendicular. • The storm pattern orientation in a basin is scientifically oriented across the basin in a consistent manner. • EPAT is to find the location in the basin that creates the greatest amount of volume • Orientation of storms is constrained by the basin thalweg rather than random or orientations that maximize rainfall. • Developers used scientific judgment to implement the constraint mechanism

30. Review the Extreme Precipitation Analysis Tool (EPAT) • Storm placement • Rainfall adjusted for elevation • No adjustment are made below 6,000 feet elevation following HMR 57 and 59 for local storms • HMR 57 and HMR 59 both apply 9%/1,000 feet above 6,000 feet elevation for local storms • EPAT uses 9%/1,000 feet for both local and general storms • EPAT use mean elevation for storm elevation, both original location and transpositioned basin location • Elevation reductions (on a percentage-per- 1,000-foot basin) were employed in the SSPMP documents such as Barr Lake and Black Hollow SSPMP

31. Review the Extreme Precipitation Analysis Tool (EPAT) • Areal reductions • Applied to events that only have one observation • Uses graphs from HMR 49 or curves in HMR 55A • Did not develop reduction factors for Colorado storms • Used for eight storms

32. Schedule • Task