HRRR model performance for the September 2013 northeastern Colorado floods
First the observations for this period will be reviewed to document the evolution of the small-scale circulation feature, which developed during the afternoon over the western Denver suburbs and then drifted northwards and stalled. It appears to be much like the well-documented “Denver Cyclone”, except in this case played an important role in precipitation distribution and not severe weather. Also available during this period is total lightning data from the Colorado Lightning Mapping Array (CO LMA), which helped to reveal where the most intense updrafts were developing in the foothills. Numerical model forecasts for this event, as documented in the recently released National Weather Service (NWS) Assessment Report, were not particularly accurate in their details (although certainly gave a longer-range indication of a wet period), and tended to focus precipitation away from the foothills as one got closer to the event. 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 displacement 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 these forecasts came too late to be of utility to forecasters at the Boulder NWS Weather Forecast Office (WFO). The evolution of the HRRR forecasts for 11 September 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 for 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.