An Unusual Summertime Downslope Wind Event in Fort Collins, Colorado, on 3 July 1993

Abstract

An unseasonal, severe downslope windstorm along the eastern foothills of the Colorado Rocky Mountains is described. The storm, which occurred on 3 July 1993, produced wind guts in Fort Collins, Colorado, over 40 m s?1 and resulted in extensive tree and roof damage. The synoptic pattern preceding the wind event resembled a pattern typical of that for a Front Range late fall or wintertime wind storm, including a strong south?southwest-oriented height gradient at 700 mb and a strong west to east sea level pressure gradient across the Front Range. A particularly interesting facet of the event was that one small geographical area in and near Fort Collins experienced wind gusts nearly 40% stronger than any other location involved in the event. The mesoscale forecast version of the Regional Atmospheric Modeling System (RAMS) with 16-km grid spacing over Colorado was run for the storm. Consistent severe winds were not predicted by the model in this configuration. Increasing resolution in postanalysis to a 4-km grid spacing along the Front Range resulted in severe downslope winds but of too strong a magnitude. The addition of explicit, bulk microphysics moderated the forecast wind strengths to observed magnitudes. That is, both a grid spacing of ?4 km and the use of explicit bulk microphysics were required to produce an accurate representation of the downslope winds observed.