A Study of Mesoscale Processes Contributing to the Formation of the 11 September Heavy rainfall during the Colorado flood of 2013

The great Colorado flood that occurred the second week of September 2013 was unprecedented and caused loss of life and significant property damage. While the rainfall and flooding covered a large area in northern Colorado over a week period, three episodes of heavy rainfall struck the Front Range of the Rockies with the most intense episode occurring in Boulder County the night of 11 September. The large-scale environment brought abundant moisture from the Gulf of Mexico and is believed to be the critical for the record amount of rainfall. Locally heavy rainfall, however, was driven by mesoscale processes. This study will examine the formation mechanisms of these processes on the night of 11 September using high resolution analyses from the Variational Dopper Radar Analysis System (VDRAS). VDRAS produces fine-scale analyses by assimilating observations from several WSR-88D radars in the Front Range region and surface networks using a 4-dimensional data assimilation technique. Results suggest that, in addition to the large scale forcing over the Rocky Mountains, the Palmer Divide south of Denver and the Cheyenne Ridge north of Boulder played an important role in the formation of the mesoscale heavy rainfall system.