Hydrometeorology and Hydroclimatology of Flash Floods in the Arid/Semi-Arid Southwestern US

We examine the hydrometeorology and hydroclimatology of flash floods in central Arizona during the North American Monsoon (July-September) period. Thunderstorm systems during the North American Monsoon are the dominant flood agents that determine the upper tail of flood frequency over the study region, and also shape the envelop curve of floods for small watersheds (drainage areas less than 250 km²). In this study, we present characterizations of flood-producing storms that control the upper-tail properties of floods in central Arizona. Our study is also motivated by improving procedures for the computations of Probable Maximum Precipitation (PMP) in arid/semi-arid regions. We developed a 30-year (i.e., 1988-2017) storm catalog over central Arizona based on high-resolution (i.e.,15-min) rainfall observations from a dense network of rain gauges, cloud-to-ground lightning strikes together with stream gauging observations. Preliminary analyses show that the storm catalog is mainly composed of two different types of flood-producing storms. One produces widespread flooding with relatively small flood peak magnitudes, while the other is localized but produces more intense rainfall intensity and flood peaks. We characterize the synoptic environment for the storms in the catalog based on analyses of NCEP North American Regional Reanalysis fields. The key question addressed is whether or not flash-flood producing storms in arid/semi-arid regions outbreak in a recurring storm environment, serving as the basis for PMP analyses. For a collection of the most intense storm events (e.g., 19 August 2014, 7 September 2014), one notable synoptic feature is the strong upper-level trough off the coast of California. Structure and evolution properties of flood-producing storms are further investigated using the storm tracking algorithm TITAN and 3D radar reflectivity fields.