PMP studies have been performed throughout the United States as well as in other countries for decades in an attempt to estimate the greatest amount of precipitation that can be expected in any specified location for a defined period of time. The National Weather Service defines PMP as “theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographic location at a certain time of the year.” For some geographic locations, it is best to use an “all season” PMP which applies to any month and not just the “warm season”. Therefore, this study was based on the greatest PMP value year-round rather than in any specific season with appropriate “seasonal adjustments” as calculated using previous HMR guidance.
The initial steps required to develop the PMP for the Ball Mountain watershed were consistent with those in both orographic and non-orographic region. Specifically, extreme storms that were determined to be transpositional to the region were maximized and given both horizontal and vertical transposition adjustment factors. Each of these steps relied upon the use of dew point climatology as a proxy for moisture (or precipitable water) availability. The topographic effects, however, necessitated the use of several additional reports as guidance to quantify the impact of orographic enhancement of extreme rainfall events for the study.
By implementing the techniques outlined in NOAA Technical Memorandum NWS HYDRO 41 (Probable Maximum Precipitation Estimates for the Drainage Above Dewey Dam, Johns Creek, Kentucky), HMR 55A (Probable Maximum Precipitation Estimates – United States Between the Continental Divide and the 103rd Meridian), and HMR 59 (Probable Maximum Precipitation for California), the orographic component of PMP for the region was calculated and a basin-specific estimate of PMP was determined. These steps revolved around the development of a K-factor (orographic enhancement factor) specific to the Ball Mountain watershed. The K-factor calculation relied upon a combination of historical storm intensity observations (M-factor) and a ratio of precipitation frequency depths in the orographic and non-orographic region surrounding the basin (T/C ratio).
Depth-Area-Duration (DAD) data from all transposed extreme storms were adjusted based upon the storm maximization factor, horizontal and vertical transposition adjustment factors, and orographic enhancement factor, combined into a single plot, and enveloped to ensure conservatism in the final depths both spatially and temporally. These site specific PMP depths were then placed into the HMR 52 program to create the Probable Maximum Storm (PMS) for the Ball Mountain watershed.
The results of a 72-hour PMP depth of 21.15” was a 13.9% reduction from the 24.57” calculated in the 1990 Ball Mountain Lake Spillway Design Flood Review, which relied on the generalized PMP estimates from HMR 51. Somewhat surprisingly, the 60-hour PMP depth of 20.52” was a reduction of only 0.4% from HMR 33. A detailed analysis of the change in PMP depths identified the unintended implicit transposition of the extreme storm at Smethport, Pennsylvania in June 1942 as the likely cause of an over-estimation of PMP depths for southern Vermont in HMR 51.
This study was executed using programs such as Microsoft Excel and ESRI ArcGIS to make a variety of computations. Ongoing development of the HEC-MetVue (Meteorological Visualization Utility Engine) program as well as the enhancement of an Extreme Storm database by USACE has the potential to dramatically decrease the time and effort required to execute similar studies in the future.