Examination of the predictability of the September 2013 northeastern Colorado floods by the HRRR model

In mid September 2013 much of northeastern Colorado experienced an historic flood event. Although rains occurred over a prolonged period from 9 through 15 September, a key part of the event happened during the evening of 11 September, when a small scale circulation stalled over far southeastern Boulder County, sending convective cells west-northwestward into the foothills. About half the total precipitation in the nearby foothills and Front Range for the entire event fell during this six to ten hour period on the evening into the night of the 11th, and while significant rains also fell on the following day, it is likely that the magnitude of damage would have been significantly lowered without the rains on 11 September. This study focuses on this key date of 11 September and the predictability of the circulation that led to the heavy rainfall in terms of a high-resolution (3 km horizontal grid resolution) convection-resolving model, the HRRR.

Although not a “severe” event in terms of the normal severe weather parameters of hail, wind or tornadoes, convective cells were a key part of the precipitation, and in fact we will document the type of convection involved on the evening of the 11th in part with total lightning data from the Colorado Lightning Mapping Array (CO LMA). But the main emphasis will be on the smaller scale lower-level based circulation that developed during the afternoon and drifted northward before stalling in an ideal position to send cells westward into the foothills. This circulation appears to be akin to the well-documented “Denver Cyclone”, which is often associated with severe weather near the Front Range, but in this case was important in the distribution of heavy rainfall. As the just-released NWS Assessment report details, operational numerical model forecasts for this event, while indicating a likely wet period with substantial lead time, were not particularly impressive even on the shorter time scales. The type of circulation that was noted on 11 September is on a scale that might not be readily resolved by the larger scale models like the GFS or ECMWF. In the case of the HRRR, the model did forecast the circulation, but it was displaced to the east. Because of this heavy rains were not predicted for the foothills and nearby Front Range but instead over the adjacent plains, and below levels that were felt by forecasters to be of great concern. Eventually the HRRR solutions (the HRRR is run hourly out to 15 h) did more accurately position the circulation and as a result shifted the heavy rains into the foothills, but generally this came too late to be of utility to forecasters at the Boulder NWS WFO. The evolution of the HRRR forecasts for 11 Sep will be detailed and compared to the observed flow and reflectivity. But an additional aspect of this study will be to rerun the HRRR using data assimilation that was not available in the real-time HRRR runs in September 2013 and document whether this made a difference in the position of the circulation and distribution of the heavy rainfall.