Downslope Windstorms in the Front Range: A 21-Year Climatological Analysis

A 21-year climatology of downslope windstorms in Boulder, Colorado is derived from data measured by a meteorological tower at the National Renewable Energy Laboratory's Flatirons Campus (formerly the National Wind Technology Center). Downslope windstorms occur regularly in the Front Range, often exacerbating wildfires and causing structural damage. Wind speed, wind direction, and windstorm duration criteria are imposed on meteorological data for classifying downslope windstorm events at a 1-minute and hourly temporal resolution. Windstorm trends are investigated daily, monthly and yearly. Over this period, 1172 downslope windstorms were classified, averaging 56 windstorms per year with a standard deviation of 8.7 windstorms per year. Downslope windstorms were found to exhibit significant seasonal patterns with January being the peak month for downslope windstorm occurrences, as well as windstorm intensity and duration. Annual windstorm frequencies were fit with generalized least squares and generalized linear models to investigate temporal trends between 2002-2022. Annual hours of strong westerly winds as well as sustained 1-minute wind speeds at the 90th, 95th, and 99th percentile all exhibited significant decreases during this period. When applied to MERRA2 reanalysis data, a similar annual trend is observed in the number of windstorm hours, while a contrasting trend is observed in annual windstorm frequency. To the best of our knowledge, this is the first study to classify downslope windstorm events using 1-minute temporal resolution meteorological data east of the Rocky Mountains.