Mesoscale atmospheric processes during the September 2013 extreme rainfall and flooding in Colorado

From 9-16 September 2013, persistent heavy rainfall occurred across much of northern Colorado, and resulted in widespread flooding causing 9 fatalities and major damage to homes, businesses, and infrastructure. Although there is a long history of extreme rainfall and flash flooding in Colorado, past events have generally occurred on relatively small spatial and temporal scales and caused flooding in one or two basins. The September 2013 flood was therefore unusual, in that the heavy rains fell over a large swath of the northern Front Range and numerous creeks and rivers experienced record or near-record flooding. Although the proximate cause of the flooding was the persistent, widespread rainfall (with relatively modest rain rates), there were periods of deep convective storms that were associated with much heavier rainfall that greatly exacerbated the flooding in some areas. In particular, a mesoscale circulation that developed over the plains on the evening of 11 September 2013 and moved northwestward toward the foothills was associated with a band of convective precipitation with high rainfall rates in Boulder and the adjacent foothills. Using observations and numerical simulations, this presentation will provide an overview of this mesoscale vortex and other associated mesoscale processes, their influences on the timing and distribution of precipitation in northern Colorado, and the implications of these processes for the successful prediction of the extreme rainfall during this event.