"The Paroxysmal Precipitation of the Desert": Flash Floods in the Southwestern United States

The 14 September 2015 flash flood in southern Utah resulted in 20 fatalities, making it the most deadly natural disaster in Utah history. The majority of the fatalities (14) occurred near Hildale, Utah in the Short Creek watershed. Indirect discharge measurements in the Short Creek watershed include a 266 cms peak from the Maxwell Canyon tributary of Short Creek at a drainage area of approximately 5.3 sq. km., a value that exceeds envelope curve peaks for Utah. The 14 September 2015 flash flood reflects features common to other major flash flood events in the region. The flood was produced by a hailstorm that intensified rapidly and interacted with complex terrain. We examine the hydrometeorology and hydroclimatology of extreme floods in the arid/semi-arid watersheds of southern Utah through detailed analyses of the 14 September 2015 storm and comparison of the storm with other flash flood producing storms in the region. The Southwestern US is affected by infrequent, but severe floods associated with thunderstorms during the North American Monsoon. We develop characterizations of rainfall and thunderstorm variability over the study region during the North American Monsoon season using radar observations from flash flood producing storms and observations from the National Lightning Detection Network. Lagrangian properties of flash flood producing storms are examined using 3-D reflectivity fields and the TITAN storm tracking algorithms. We examine the storm environment of flash flood producing storms through simulations using the Weather Research and Forecasting (WRF) model.