A Synoptic and Mesoscale Investigation of the Extreme Precipitation Event over the Front Range of Colorado on 10-13 September 2013

10-13 September, 2013 featured a sustained precipitation event centered around and to the north of Denver, CO along the Front Range of the Rocky Mountains. The event was unusual in that the precipitation, which included both stratiform and convective types, occurred in large quantities over a relatively long period of time. Using satellite, radar and weather analysis data, we will show that the intensity and longevity of the event is due to a concatenation of mesoscale and synoptic features.

A nearly stationary upper-level low across the western U.S. provided upper-level lift as well as mid-to-upper-level moisture from the Pacific while a stalled front over Northeastern Colorado provided surface convergence for mid-to-low-level moisture from the Gulf of Mexico. Persistent east-to-southeast winds over the Palmer Divide provided sufficient flow for the Denver Cyclone to set up. This contributed to the surface convergence found along the stationary front, as well as along the topographic gradient provided by the Front Range of the Rocky Mountains. The combination of the stationary front, Denver Cyclone and southwest winds aloft produced an environment with sufficient shear to maintain vortices normally associated with severe weather. These vortices allowed the strongest convective elements to tilt with height leading to efficient, long-lasting moderate rainfall which was the hallmark of this event.