Reanalysis of a Record-Breaking Storm in the Colorado Rockies-Combining Meteorology with Hydrology to Find the Answer

A localized thunderstorm produced extreme rainfall and flooding in July 1999 at a high elevation, interior location of Colorado, near the town of Saguache. This storm was utilzied for PMP development during the Colorado-New Mexico Regional Extreme Precipitation Study (CO-NM REPS) and was controlling of PMP depths for the local storm type in mountainous regions in that study. A partnership between Colorado State University (CSU), Colorado Department of Water Resources (DWR) and Applied Weather Associates (AWA) was formed to investigate high-elevation extreme precipitation and runoff events with the goal of updating hydrologic guidelines in Colorado. AWA’s Storm Precipitation Analysis System (SPAS) rainfall data were used as input for flood runoff reconstruction to determine if the rainfall as originally analyzed would produce the flooding observed in the location. For the Saguache July 1999 storm, the result of this investigation demonstrated that the original total storm rainfall amounts were likely accurate, however, the exact location, spatial extents, and timing needed to be updated. Based on initial modeling, a detailed investigation took place to more accurately quantify the rainfall and flooding associated with this storm. This was completed by performing bucket survey type analyses, talking with witnesses of the event, and investigating radar scan data to derive additional information to update the original storm analysis. These updates were applied to the hydrologic model for sensitivity analysis and ultimately were applied to the CO-NM REPS study and PMP outputs. This presentation will detail the on the ground “bucket survey” that was completed in cooperation with CSU and DWR to derive new data, how those additional data were applied to update the original storm analysis, how the updated storm analysis was applied to the CO-NM REPS tool, and the results of those updates on PMP and flood runoff investigation.