Application of Temporal Patterns of Probable Maximum Precipitation for Dam Design in Wyoming

Prepared for

Wyoming State Engineer’s Office Herschler Building, Cheyenne, WY 82002

Funded by Federal Emergency Management Agency

National Dam Safety Program

Prepared by Applied Weather Associates, LLC

PO Box 175, Monument, CO 80132 www.appliedweatherassociates.com

Bill Kappel, Project Manager and Chief Meteorologist Geoff Muhlestein, Senior GIS Analyst

Doug Hultstrand, Senior Hydrometeorologist Dana McGlone, Staff Meteorologist

Kristi Steinhilber, Staff Meteorologist Jake Rodel, GIS Analyst

December 2015

Table of Contents

1.0 Introduction .......................................................................................................................... 1

2.0 Application of Temporal Patterns to PMP Depths ........................................................... 2

2.1 Example Applications of Local and General Storm Patterns ...................................................... 8 2.1.1 Example 1: General Storm East ..................................................................................................... 8 2.1.2 Example 2: Local Storm West ...................................................................................................... 14

3.0 Development of the Temporal Patterns ........................................................................... 18

3.1 Standardized Timing Distributions by Storm Type ................................................................... 18 3.1.1 Parameters ..................................................................................................................................... 19 3.1.2 Procedure used to calculate parameters ......................................................................................... 20

3.2 Results of the Analysis .............................................................................................................. 20

4.0 Description of Recommended Temporal Patterns Development .................................. 45

4.1 Local Storms West of the Continental Divide ........................................................................... 45 4.2 General Storms West of the Continental Divide ....................................................................... 46 4.3 Local Storms East of the Continental Divide ............................................................................ 46 4.4 General Storms East of the Continental Divide ......................................................................... 46 4.5 Hybrid Storms East of the Continental Divide .......................................................................... 47

References .................................................................................................................................... 48

Appendix A .................................................................................................................................. 49 

List of Figures

Figure 1 Local storm PMP temporal distribution curve for locations west of the Continental Divide ........ 3 Figure 2 General storm PMP temporal distribution curve for locations west of the Continental Divide for hours 25-48 (day 2) of the 72-hour total storm duration ............................................................................... 4 Figure 3 Local storm PMP temporal distribution curve for locations east of the Continental Divide ......... 5 Figure 4 General storm PMP temporal distribution curve for locations east of the Continental Divide for hours 25-48 (day 2) of the 72-hour total storm duration ............................................................................... 6 Figure 5 Hybrid storm PMP temporal distribution curve for locations east of the Continental Divide ...... 7 Figure 6 General storm 72-hour PMP temporal distribution curve representing the example basin east of the Continental Divide ................................................................................................................................ 13 Figure 7 Local storm 6-hour PMP temporal distribution curve representing the example basin west of the Continental Divide using a 10-minute time distribution ............................................................................. 17 Figure 8 SPAS Rainfall (R) versus time for all general storms ................................................................. 21 Figure 9 SPAS Rainfall (R) versus time for general storms east of the Continental Divide ..................... 22 Figure 10 SPAS Rainfall (R) versus time for hybrid storm type only ....................................................... 23 Figure 11 SPAS Rainfall (R) versus time for general storms west of the Continental Divide .................. 24 Figure 12 Normalized R (Rn) versus time for general storms east of the Continental Divide .................. 25 Figure 13 Normalized R (Rn) versus time for hybrid storm type only ...................................................... 26 Figure 14 Normalized R (Rn) versus time for general storms west of the Continental Divide ................. 27 Figure 15 Normalized R (Rn) versus shifted time (Ts) for general storms east of the Continental Divide .................................................................................................................................................................... 28 Figure 16 Normalized R (Rn) versus shifted time (Ts) for hybrid storm type only .................................. 29 Figure 17 Normalized R (Rn) versus shifted time (Ts) for general storms west of the Continental Divide .................................................................................................................................................................... 30 

Figure 18 Maximum 24-hour point rainfall versus time for general storms east of the Continental Divide (NRCS Type II curve included for comparison). ........................................................................................ 31 Figure 19 Maximum 24-hour point rainfall versus time for hybrid storm type only (NRCS Type II curve included for comparison). ........................................................................................................................... 32 Figure 20 Maximum 24-hour point rainfall versus time for general storms west of the Continental Divide (NRCS Type II curve included for comparison). ........................................................................................ 33 Figure 21 SPAS Rainfall (R) versus time for all local storms ................................................................... 34 Figure 22 SPAS Rainfall (R) versus time for local storms east of the Continental Divide ....................... 35 Figure 23 SPAS Rainfall (R) versus time for local storms west of the Continental Divide ...................... 36 Figure 24 Normalized R (Rn) versus time for local storms east of the Continental Divide ...................... 37 Figure 25 Normalized R (Rn) versus time for local storms west of the Continental Divide ..................... 38 Figure 26 Normalized R (Rn) versus time for local storms west of the Continental Divide without SPAS 1249 ............................................................................................................................................................ 39 Figure 27 Normalized R (Rn) versus shifted time (Ts) for local storms east of the Continental Divide ... 40 Figure 28 Normalized R (Rn) versus shifted time (Ts) for local storms west of the Continental Divide .. 41 Figure 29 Normalized R (Rn) versus shifted time (Ts) for local storms west of the Continental Divide without SPAS 1249 ..................................................................................................................................... 42 Figure 30 Maximum 6-hour point rainfall versus time for local storms east of the Continental Divide ... 43 Figure 31 Maximum 6-hour point rainfall versus time for local storms west of the Continental Divide without SPAS 1249 ..................................................................................................................................... 44 

List of Tables

Table 1 Local storm PMP temporal distribution table for locations west of the Continental Divide .......... 3 Table 2 General storm PMP temporal distribution table for locations west of the Continental Divide for hours 25-48 (day 2) of the 72-hour total storm duration ............................................................................... 4 Table 3 Local storm PMP temporal distribution table for locations east of the Continental Divide ........... 5 Table 4 General storm PMP temporal distribution table for locations east of the Continental Divide for hours 25-48 (day 2) of the 72-hour total storm duration ............................................................................... 6 Table 5 Hybrid storm PMP temporal distribution table for locations east of the Continental Divide ......... 7 Table 6 General storm PMP for a basin east of the Continental Divide ...................................................... 8 Table 7 General storm incremental example values for a basin east of the Continental Divide .................. 9 Table 8 General storm PMP applied to the temporal pattern for a basin east of the Continental Divide representing the largest 24-hour accumulation and distributed over Day 2 ................................................ 10 Table 9 General storm PMP distributed uniformly over the first 24-hour period (Day 1) during the 72-hour accumulation period ........................................................................................................................... 11 Table 10 General storm PMP distributed uniformly over the last 24-hour period (Day 3) during the 72-hour accumulation period ........................................................................................................................... 12 Table 11 Local storm PMP for a basin west of the Continental Divide ..................................................... 14 Table 12 Local storm incremental example values for a basin west of the Continental Divide ................ 15 Table 13 Local storm PMP applied to the temporal pattern for a basin west of the Continental Divide representing the largest 6-hour accumulation distributed during the first 6-hours of the storm in 10-minute increments ................................................................................................................................................... 16 Table 14 SPAS storm events used in Wyoming temporal distribution ....................................................... 19 

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1.0 Introduction

The information provided herein is a companion to the Wyoming Probable Maximum Precipitation (PMP) study completed by Applied Weather Associates (AWA) in December 2014 (Kappel et al., 2014). This study addresses the discussion from Section 13.3 of that report. In terms of temporal patterns for dam and spillway designs, Wyoming previously deferred to the Natural Resource Conservation Service (NRCS) and its guidelines. The NRCS design manual, TR-60, Earth Dams and Reservoirs, provides a cumulative temporal distribution curve (Type II) that has been used in Wyoming in the past (NRCS, 2005).

This study provides background regarding the information analyzed, the methods used to quantify and evaluate each storm's temporal accumulation pattern, the recommended temporal patterns that resulted from this evaluation, and how to apply the data.

AWA analyzed all storms in the Wyoming PMP study to develop temporal accumulation patterns associated with each storm type and general region. Investigations were completed by analyzing the rainfall accumulation of each storm and the time over which the main rainfall accumulated. Storms were grouped by geographic location (east versus west of the Continental Divide) and by storm type: local, general, and hybrid1. During these analyses, consideration was given to the synoptic meteorological patterns that created each storm type, access to moisture sources, and the general topographical setting. Discussions with the Wyoming State Engineers Office, Wyoming Water Development Office, and independent reviewer George Sabol (PhD, PE) also aided in the development of the recommended temporal patterns.

For PMP determination in basins west of the Continental Divide 25 square miles or smaller, only the local storm PMP is required. It is left to the analyst as to whether to evaluate the general storm PMP as this storm type will not control PMP at 25 square miles or less. Conversely, for basins larger than 50 square miles west of the Continental Divide, only the general storm PMP is required. For basins between 25 and 50 square miles west of the Continental Divide, both storm types should be evaluated. For all locations east of the Continental Divide, all storm types should be evaluated in all basins regardless of area size.

The PMP values derived during the Wyoming statewide study produced values that are considered possible at 100% from June 15 through September 15. If PMP values are required for time periods outside of that date range, seasonality adjustments should be applied. In these situations, it is assumed that the seasonally reduced PMP depths could occur with snow on the ground. Therefore, the rainfall depths should be added to any snowmelt contribution (separate analysis required) to produce a total runoff representative of the seasonally reduced PMP and the snowmelt. The seasonality adjustment maps are provided in Appendix G of the Wyoming statewide PMP Final Report. Further discussion on the application of the seasonality adjustments is provided in Appendix A of this document.

1 Hybrid storms are a storm type considered in the general storm category because of the overall rainfall accumulation pattern but with characteristics of local storms in that they contain a period of very intense rainfall accumulation. Because they show characteristics of both storm types, they are termed “hybrid.”

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2.0 Application of Temporal Patterns to PMP Depths

The following steps should be used to determine the temporal accumulation of PMP values. Section 2.1 provides example applications of these recommended temporal distributions:

1. Determine whether the basin is located east or west of the Continental Divide. 2. If west of the Continental Divide, determine basin area size.

a. If basin is 25 square miles or less, use the local storm PMP. b. If basin is between 25 and 50 square miles, use both local and general storm PMP. c. If basin is greater than 50 square miles, use general storm PMP. d. For the 6-hour local storm PMP.

i. Apply temporal pattern in Table 1 or Figure 1 to the local storm PMP depth. e. For 72-hour general storm PMP.

i. 1st 24 hours: Second Largest Rainfall Day uniformly distributed. ii. 2nd 24 hours: Apply temporal pattern in Table 2 or Figure 2 to the general storm

PMP depth. iii. 3rd 24 hours: Smallest Rainfall Day uniformly distributed.

3. If east of the Continental Divide. a. For 6-hour local storm PMP.

i. Apply temporal pattern in Table 3 or Figure 3 to the local storm PMP depth. b. For 72-hour general storm PMP.

i. 1st 24 hours: Second Largest Rainfall Day uniformly distributed. ii. 2nd 24 hours: Apply temporal pattern in Table 4 or Figure 4 to the general storm

PMP depth. iii. 3rd 24 hours: Smallest Rainfall Day uniformly distributed.

c. For 24-hour hybrid storm PMP. i. Apply temporal pattern in Table 5 or Figure 5 to the hybrid storm PMP depth.

For basins west of the Continental Divide, two scenarios are necessary; one using the local storm PMP with the local storm west temporal pattern, and the other using the general storm PMP with the general storm west pattern (one or both can be used depending on area size of the basin analyzed). For basins east of the Continental Divide, three scenarios are necessary; one using the local storm PMP with the local storm east temporal pattern, the second with the general storm PMP with the general storm east pattern, and the third using the hybrid storm PMP with the hybrid storm east pattern. The derivation of the values presented in Tables 1-5 started with visual inspections of the rainfall accumulation patterns of the analyzed storms. This was followed by the manual development of characteristic curves intended to best represent the rainfall accumulation patterns by storm type. An iterative approach was used along with manual manipulation of the values to obtain the recommended curves. The cumulative values were derived to ensure they started at zero and ended at one and produced an accumulation curve which matched the characteristic distribution. The incremental values were derived to ensure they produced an accumulation intensity that replicated the characteristic distribution.

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Table 1 Local storm PMP temporal distribution table for locations west of the Continental Divide

Figure 1 Local storm PMP temporal distribution curve for locations west of the Continental Divide

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Table 2 General storm PMP temporal distribution table for locations west of the Continental Divide for hours 25-48 (day 2) of the 72-hour total storm duration

Figure 2 General storm PMP temporal distribution curve for locations west of the Continental Divide

for hours 25-48 (day 2) of the 72-hour total storm duration

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Table 3 Local storm PMP temporal distribution table for locations east of the Continental Divide

Figure 3 Local storm PMP temporal distribution curve for locations east of the Continental Divide

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Table 4 General storm PMP temporal distribution table for locations east of the Continental Divide for hours 25-48 (day 2) of the 72-hour total storm duration

Figure 4 General storm PMP temporal distribution curve for locations east of the Continental Divide

for hours 25-48 (day 2) of the 72-hour total storm duration

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Table 5 Hybrid storm PMP temporal distribution table for locations east of the Continental Divide

Figure 5 Hybrid storm PMP temporal distribution curve for locations east of the Continental Divide

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2.1 Example Applications of Local and General Storm Patterns

Two examples are provided to show how the recommended temporal patterns are to be applied in a particular location. The first example represents a general storm east of the Continental Divide. The second example represents a local storm west of Continental Divide. The same process provided in the two examples below for each storm type are valid on either side of the Continental Divide. Therefore, the examples provided can be used for the appropriate storm type in combination with the appropriate temporal pattern shown in Figures 1-5 and Tables 1-5.

2.1.1 Example 1: General Storm East

Step 1: Derive general storm PMP for the basin. Table 6 provides the PMP data for the example basin as obtained from the PMP Evaluation Tool.

Table 6 General storm PMP for a basin east of the Continental Divide

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Step 2: Apply the 24-hour PMP value (9.83 inches) to the normalized Y-Axis in Figure 4 and Table 4. The result is the middle part (2nd day) of the total 72-hour curve (Table 8). Table 7 provides the 24-hour incremental rainfall values used in this example.

Table 7 General storm incremental example values for a basin east of the Continental Divide

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Table 8 General storm PMP applied to the temporal pattern for a basin east of the Continental Divide representing the largest 24-hour accumulation and distributed over Day 2

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Step 3: Derive the next largest 24-hour PMP depth by subtracting the largest 24-hour PMP (9.83 inches in this example) from the 48-hour PMP (16.43 inches) or the second largest 24-hour PMP increment. Apply this result (6.60 inches) uniformly over the first 24-hours (hours 0-24) of the 72-hour PMP accumulation period. The value for hours 0-24 is computed by dividing the second largest 24-hour increment by 24-hours. In this example, the value for 0-24 hours is 6.60 inches, which equals 0.275 inches per hour (Table 9). Refer to Table 7 which provided the 24-hour incremental rainfall values used in this example.

Table 9 General storm PMP distributed uniformly over the first 24-hour period (Day 1) during the 72-

hour accumulation period

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Step 4: Compute the remaining 24-hour rainfall amount representing hours 49-72. This equals the smallest 24-hour accumulation. This value is computed by subtracting the 48-hour PMP value (in this example 16.43 inches) from the 72-hour PMP value (in this example 16.72 inches). This results in a value of 0.29 inches. This is uniformly distributed over the final 24-hour period (hours 49-72) by dividing the value by 24. In this example, the hourly rainfall amount is 0.012 inches (Table 10). Again, refer to Table 7 which provided the 24-hour incremental rainfall values used in this example. Figure 6 displays the final 72-hour cumulative and incremental rainfall accumulation pattern using the example PMP discussed. Note, the discontinuity in this example between hour 24 and 25 does not represent an error in the data. It represents the transition from a uniform (idealized) distribution to a derived distribution using the actual storm data and rainfall accumulations. This is not uncommon in some rainfall accumulation patterns. Table 10 General storm PMP distributed uniformly over the last 24-hour period (Day 3) during the 72-

hour accumulation period

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Figure 6 General storm 72-hour PMP temporal distribution curve representing the example basin east

of the Continental Divide

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2.1.2 Example 2: Local Storm West

Step 1: Derive local storm PMP for the basin. Table 11 provides the PMP data for the example basin as calculated from the PMP Evaluation Tool. Note, the local storm is a 6-hour storm. The remaining 18 hours are standard output from the PMP Evaluation Tool and are for comparison purposes only and are not required for flood evaluations.

Table 11 Local storm PMP for a basin west of the Continental Divide

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Step 2: Apply the 6-hour PMP value (9.41 inches) to normalized Y-Axis in Figure 1 and Table 1. The result is shown in Table 13. The remaining values (if any) are for comparison purposes only because the local storm PMP should be modeled as a 6-hour event. Table 12 provides the incremental rainfall values used in this example. Figure 7 displays the resulting 6-hour cumulative and incremental rainfall accumulation pattern using the example PMP discussed and a 10-minute time distribution.

Table 12 Local storm incremental example values for a basin west of the Continental Divide

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Table 13 Local storm PMP applied to the temporal pattern for a basin west of the Continental Divide representing the largest 6-hour accumulation distributed during the first 6-hours of the storm in 10-

minute increments

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Figure 7 Local storm 6-hour PMP temporal distribution curve representing the example basin west of

the Continental Divide using a 10-minute time distribution

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3.0 Development of the Temporal Patterns

3.1 Standardized Timing Distributions by Storm Type

All Storm Precipitation Analysis System (SPAS) storms used to determine the PMP values for Wyoming were used to derive the preceding temporal accumulation patterns. The storms analyzed are provided in Table 14. The location of the storm center associated with each SPAS Depth-Area-Duration zone was used for the temporal distribution calculations. Hourly gridded rainfall data were used for all SPAS analyzed storms.

In terms of storm types, local storms are characterized by short duration (6-hours or less) and small area size high intensity rainfall accumulations. They are often not associated with large scale weather patterns and can be influenced by local moisture sources. General storms produce precipitation over longer durations (greater than 6-hours) and cover larger areas with comparatively lower intensity rainfall accumulations. General storms are produced by large scale synoptic patterns generally associated with areas of low pressure and fronts. In western Wyoming, general storms can also be associated with remnant moisture from decaying tropical systems originating from the eastern Pacific and Gulf of California. For regions east of the Continental Divide, some storms exhibit characteristics of both the local and general storm. These are termed hybrid storms and a unique temporal pattern has been derived to apply to this storm type.

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Table 14 SPAS storm events used in Wyoming temporal distribution

The Significant Precipitation Period (SPP) for each storm was selected by excluding relatively small rainfall accumulations at the beginning and end of the rainfall duration. Accumulated rainfall (R) amounts during the SPP were used in the analysis for the hourly storm rainfall. The total rainfall during the SPP was used to normalize the hourly rainfall amounts. The time scale (TS) was computed to describe the time duration when half of the rainfall accumulated (R). The procedures used to calculate these parameters are listed below.

3.1.1 Parameters

SPP - Significant Precipitation Period when the majority of the rainfall occurred R - Accumulated rainfall at the storm center during the SPP Rn - Normalized R T - Time when R occurred T50 - Time when Rn = 0.5

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Ts - Shifted time around when 50% of the rainfall occurred max24hr - Maximum 24-hour point rainfall at storm center location max6hr - Maximum 6-hour point rainfall at storm center location

3.1.2 Procedure used to calculate parameters

1. Determine the SPP. Inspect each storm's rainfall data for "inconsequential" rainfall at either the beginning and/or the end of the records. Remove these "tails" from calculations. Generally, use a criteria of less than 0.1 inches/hour intensity. No internal rainfall data are deleted.

2. Recalculate the accumulated rainfall records for R. 3. Plot the SPAS rainfall and R mass curves and inspect for reasonableness. 4. Normalize the R record by dividing all values by the total R to produce Rn for each hour,

Rn ranges from 0.0 to 1.0. 5. Determine T50 using the time when Rn = 0.5. 6. Calculate Ts by subtracting T50 from each value of T. Negative time values precede the

time to 50% rainfall, and positive values follow. 7. Determine max24hr and max6hr precipitation, convert accumulations into a ratio of the

cumulative rainfall to the total accumulated rainfall for that duration. 8. Prepare graphs of a) T vs R, b) T vs Rn, c) Ts vs Rn, and d) maximum point precipitation

for General (24-hour) and Local (6-hour) storm events.

3.2 Results of the Analysis

To further categorize the storms, the local and general storms were separated based on whether they occurred east or west of the Continental Divide. This delineation was applied because of the difference in the low-level moisture source region, with the Gulf of Mexico providing moisture east of the Continental Divide and the Gulf of California/Pacific Ocean providing moisture west of the Continental Divide. These differences result in different rainfall accumulation patterns. The effects of topographic differences within each of these regions (such as the Laramie Range or Big Horn Mountains to the east or the Tetons in the west) are captured in the PMP magnitudes but do not affect the temporal pattern of accumulation derived by storm type in each region. Included in this analysis is the storm type termed Hybrid. The hybrid storms exhibit characteristics of both general storms and local storms. The hybrid storm type only occurred east of the Continental Divide.

Following the procedures and description from the previous section, results are presented below for the each storm type. Figures 8-20 show graphs for general and hybrid SPAS storm events comparing (a) T vs R, (b) T vs Rn, (c) Ts vs Rn, and (d) maximum point precipitation for storm events east and west of the Continental Divide. Figures 21-31 show similar graphs except for the local SPAS storm events.

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Figure 8 SPAS Rainfall (R) versus time for all general storms

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Figure 9 SPAS Rainfall (R) versus time for general storms east of the Continental Divide

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Figure 10 SPAS Rainfall (R) versus time for hybrid storm type only

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Figure 11 SPAS Rainfall (R) versus time for general storms west of the Continental Divide

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Figure 12 Normalized R (Rn) versus time for general storms east of the Continental Divide

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Figure 13 Normalized R (Rn) versus time for hybrid storm type only

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Figure 14 Normalized R (Rn) versus time for general storms west of the Continental Divide

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Figure 15 Normalized R (Rn) versus shifted time (Ts) for general storms east of the Continental Divide

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Figure 16 Normalized R (Rn) versus shifted time (Ts) for hybrid storm type only

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Figure 17 Normalized R (Rn) versus shifted time (Ts) for general storms west of the Continental Divide

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Figure 18 Maximum 24-hour point rainfall versus time for general storms east of the Continental

Divide (NRCS Type II curve included for comparison)

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Figure 19 Maximum 24-hour point rainfall versus time for hybrid storm type only (NRCS Type II

curve included for comparison)

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Figure 20 Maximum 24-hour point rainfall versus time for general storms west of the Continental

Divide (NRCS Type II curve included for comparison)

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Figure 21 SPAS Rainfall (R) versus time for all local storms

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Figure 22 SPAS Rainfall (R) versus time for local storms east of the Continental Divide

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Figure 23 SPAS Rainfall (R) versus time for local storms west of the Continental Divide

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Figure 24 Normalized R (Rn) versus time for local storms east of the Continental Divide

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Figure 25 Normalized R (Rn) versus time for local storms west of the Continental Divide

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Figure 26 Normalized R (Rn) versus time for local storms west of the Continental Divide without SPAS

1249

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Figure 27 Normalized R (Rn) versus shifted time (Ts) for local storms east of the Continental Divide

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Figure 28 Normalized R (Rn) versus shifted time (Ts) for local storms west of the Continental Divide

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Figure 29 Normalized R (Rn) versus shifted time (Ts) for local storms west of the Continental Divide

without SPAS 1249

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Figure 30 Maximum 6-hour point rainfall versus time for local storms east of the Continental Divide

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Figure 31 Maximum 6-hour point rainfall versus time for local storms west of the Continental Divide

without SPAS 1249

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4.0 Meteorological Description of Recommended Temporal Patterns

Each of the recommended temporal patterns were derived through visual inspection, comparisons, and discussions with review members. This was completed by separating each storm by storm type (local, general, hybrid) and location (east or west of the Continental Divide), then plotting the values as shown in Section 3. The recommended temporal patterns reflect the meteorological conditions which produce each storm type. Therefore, it is useful to understand the meteorology associated with each temporal pattern.

4.1 Local Storms West of the Continental Divide

Local storms west of the Continental Divide showed a more intense accumulation pattern versus those east of the Continental Divide. This reflects the moisture available to the storm and the meteorological environment associated this storm type. In almost all cases, the most intense rainfall occurred within 2 hours or less. This is due to a lack of sustained low-level moisture inflow. The main low-level moisture source for extreme local storms west of the Continental Divide is the Gulf of California. Moisture originating from the Gulf of California must cross several mountain barriers and originates from a relatively smaller moisture source as compared to storms east of the Continental Divide. In addition, the response time for many of the basins that are susceptible to the local storm PMP west of the Continental Divide is often shorter than 1 hour. Therefore, the overall 6 hour accumulation period was split into 10-minute increments. These increments were derived during the Arizona statewide PMP study using storms with NEXRAD weather radar data. Use of NEXRAD allowed the temporal accumulation to be analyzed at 5-minute increments. Many of the same storms analyzed during that study were used for the development of the Wyoming PMP.

The shorter and steeper accumulation pattern analyzed during this study matches the pattern that was found during similar investigations in the Arizona statewide PMP study. Because of the similarity of accumulation patterns, it is recommended that the same local storm temporal pattern used for the Arizona statewide PMP study be used for local storm west of the Continental Divide in Wyoming. Note, the paragraph below is from the Arizona PMP report, Section 8.8.2, and describes the process used in that analysis (Tomlinson et al., 2013).

NEXRAD rainfall data from storms analyzed by SPAS was used to develop a recommended temporal distribution for local storm PMP design events in Arizona. The NEXRAD data indicates that for the vast majority of these storms, periods of high intensity rainfall occur in three hours or less. For practical and administrative reasons however, the existing practice of modeling 6-hour local PMP design events is maintained. To decide on an appropriate recommendation, a “hydrologic subcommittee” was formed of various members of the project team, Review Committee, ADWR, and the Flood Control District of Maricopa County, each having experience and expertise in performing hydrologic studies in Arizona. After evaluation of temporal patterns from all of the analyzed storms having NEXRAD data, the hydrologic subcommittee defined and recommended a 6-hour distribution (designated as “LS-6”) modified from a storm observed in July 1998 in Navajo County at Joseph City near Holbrook. The LS-6 distribution recommended for local storm PMP design events in Arizona is shown in Figure 8.6" (Figure 1 of this report).

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The local storm PMP is temporally distributed within the PMP Evaluation Tool by applying the ratio of incremental rainfall to total 6-hour PMP rainfall at each 10-minute time step to each grid cell. The distribution incremental ratios and the cumulative ratios are listed in Table 8.2 (Table 1 of this report)

4.2 General Storms West of the Continental Divide

General storm patterns analyzed west of the Continental Divide showed a consistent pattern of accumulation through 24 hours. The rainfall accumulation pattern is nearly continuous through the main 24-hour period. This is reflective of the meteorological environment associated with the general storm type west of the Continental Divide. This is different than the NRCS Type II curve which produces a more intense accumulation during the middle of the 24 hour period. The moisture for these events is supplied by the Pacific Ocean, however the low-level moisture is significantly altered by interactions with terrain before it reaches Wyoming. The lift required to generate the rainfall is driven by the local topographic interactions as well as the thermodynamics associated with cold air aloft and frontal interactions at the surface. Because these factors can remain uniform over a large area during the storm events, general rainfall can occur for a significant period of time with similar intensities. The PMP for general storms west of the Continental Divide is provided for 72 hours.

4.3 Local Storms East of the Continental Divide

Local storm patterns analyzed east of the Continental Divide showed a consistent pattern of accumulation through 6 hours, with more intense rainfall during the middle 2 hours. This is expected given the meteorological environment which results in this type of storm event for regions east of the Continental Divide. In this region, low-level moisture is supplied from the Gulf of Mexico, often on the low-level jet or pulled into strengthening low pressure to the lee of the Rocky Mountains. Because there is no intervening topography between the Gulf of Mexico and eastern Wyoming, high quantities of low-level moisture can be continuously supplied over the 6-hour period during extreme rainfall scenarios. Regional topography in eastern Wyoming (e.g. Laramie Range, Big Horn Mountains) can affect the low-level moisture feed by providing a barrier to leeward locations to the north and west. This is reflected in the relatively lower depths of PMP values in these regions versus those in the direct path of the moisture. However, this does not affect the temporal rainfall accumulation pattern. This is why the local storm accumulation patterns show a more uniform pattern through the 6-hour period versus locations west of the Continental Divide. The recommended local storm east of the Continental Divide temporal accumulation pattern reflects a combination of these scenarios.

4.4 General Storms East of the Continental Divide

General storm patterns east of the Continental Divide showed a consistent pattern of accumulation through 24 hours, with slightly more intense rainfall during the most intense 6 hours. This is similar to the general storm patterns analyzed for areas west of the Continental Divide. This is different than the NRCS Type II curve which produces a more intense accumulation during the middle of the 24 hour period. Like local storms for regions east of the Continental Divide, low-level moisture is from the Gulf of Mexico, pulled into a strengthening low pressure system to the lee of the Rocky Mountains. In addition, general storms incorporate moisture in the mid and upper levels which comes from the Pacific Ocean. Finally, because the

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lift for general storms is a combination of both dynamic lift mechanisms (e.g. topography, convergence) and thermodynamic lift mechanisms (cold air above warm air), the location of surface low pressure and low pressure at 500mb (approximately 16,000 feet) are important in generating extreme rainfall. The lift required to convert the moisture in the atmosphere into rainfall is supplied by the overall synoptic meteorological environment. However, because the thermodynamic contrast is not as extreme as during a local storm, the intensity of the rainfall accumulation is reduced. The overall pattern leading to rainfall can last longer than a local storm over a given area because a storm’s overall movement may be blocked or slowed by the upstream atmospheric pattern and/or continued strengthening of the storm. This allows heavy rain to accumulate for a longer period of time compared to a local storm. Several of the general storms analyzed show a more intense accumulation pattern within a portion of the main 24-hour accumulation period, while others were uniform throughout the 24-hours. Therefore, the recommended general storm east of the Continental Divide temporal accumulation reflects a combination of these scenarios. Finally, because the majority of the rainfall accumulates within 24 hours, the remaining 48 hours should be treated with a uniform pattern.

4.5 Hybrid Storms East of the Continental Divide

The hybrid storm pattern varied significantly from the general storm pattern. Hybrid storms accumulate most of their rainfall (greater than 80%) in a 12-hour period. Therefore, hybrid storms last longer than a local storm, yet they are shorter and more intense than a general storm. The meteorological pattern leading to these events is controlled by components of each storm type. Hybrid storms receive their low-level moisture from the Gulf of Mexico. The moisture is pulled into an area of low pressure to the lee of the Rocky Mountains and/or concentrated along a front in the region. The lift for these events is a combination of both dynamic mechanisms (topography, convergence) and thermodynamic mechanisms (cold air above warm air). The location of low pressure, both at the surface and at 500mb (approximately 16,000 feet), is also important in generating extreme rainfall. The low-level moisture from the Gulf of Mexico is forced to rise because of the overall synoptic meteorological environment and rising terrain. The intensity of rainfall accumulation is more similar to a local storm than a general storm. This is because the rainfall accumulates with high intensity but lasts longer than a local storm. This allows heavy rain to accumulate for a longer period of time, extending from 6 to 12 hours. This pattern is more similar to the NRCS Type II curve than a general storm, but accumulates more total rainfall in the peak 12-hour period. Initially, these storms were considered general storms. However, the rainfall accumulation patterns are different from the general storm type. For these reasons the PMP temporal pattern is provided for 24 hours.

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References

Kappel, W.D., Hultstrand, D.M., Muhlestein, G.A., Steinhilber, K., McGlone, D., Parzybok, T.W, and E.M. Tomlinson, December 2014: Statewide Probable Maximum Precipitation (PMP) Study for Wyoming.

Natural Resources Conservation Service (NRCS), Conservation Engineering Division. (2005, July). Earth Dams and Reservoirs, TR-60.

Tomlinson, E.M., Kappel, W.D., Hultstrand, D.M., Muhlestein, G.A., S. Lovisone, and T.W. Parzybok, July 2013: Statewide Probable Maximum Precipitation Study for Arizona.

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Appendix A

Application of the Seasonality Adjustment for the All-Season PMP for Use in Rain-on-Snow PMF Evaluations

Introduction

This provides more guidance regarding the application of the seasonality maps for use in deriving the PMF from a rain-on-snow scenario. This text can be added to Page 106 of the Wyoming PMP report and replace the footnote on that page or added as an addendum to the report. Background and Recommended Implementation

The PMP datasets provide all-season PMP depths valid for June 15th through September 15th. During this timeframe, rain-on-snow events representing PMP level rainfall are not considered possible and therefore only rainfall should be considered in the PMF development using the all-season PMP data. In order to determine PMP values outside of the all-season timeframe and for use in calculating total runoff that results from seasonally adjusted PMP rainfall in combination with snowmelt, the seasonality maps provided in Appendix G of the Wyoming statewide PMP report should be applied.

To apply a cool-season (rain-on-snow) PMP adjustment, multiply the basin average PMP by the seasonal adjustment factor for the appropriate time of year. The monthly seasonal adjustment factors can be obtained from the maps in Appendix G or the Seasonal Adjustment Factor GIS gridded datasets. The maps represent the 15th of each month. To determine the value for other days, a simple interpolation between the bounding months can be used. For large basins, a gridded basin average may be taken by extracting the values with the same process used for PMP extraction. To adjust PMP for a specific date, the adjustment factor can be interpolated between the two bounding months. Snow water equivalent (SWE) depths (or other means of determining snow melt contributions) can then be applied to seasonally adjusted PMP according to hydrologic guidance. The combination of the seasonally adjusted PMP rainfall with the runoff from snowmelt would result in the total runoff for the given location. If this value is greater than the all-season PMP, it may be used as the controlling event for the PMF.