Forecasting downslope windstorms at Boulder, Colorado: The empirical-statistical approach revisited

Despite major advances in theoretical understanding of downslope windstorms over the past 2 decades, operational skill in forecasting these events, particularly those occurring over the eastern slopes of the Rocky Mountains, remains a challenge. In part we suspect this is because there is rarely a mean-state critical layer in the larger (synoptic and mesoalpha) scale flow in these areas when conditions are otherwise favorable. The existence of steepening or breaking vertically propagating mountain waves and an associated windstorm extending to the foot of the mountains appears to depend on less obvious characteristics of the wind hodograph and stratification upstream of the efficacious mountain range.

With support from a COMET (Cooperative Program for Operational Meteorology, Education and Training) partners grant, and informed by theoretical advances of recent years, we are revisiting the empirical, statistical approach to the problem. The goal is to build a forecast aid that, given a forecast of the upstream hodograph and stability upstream of the Front Range of Colorado by one of the NCEP (National Centers for Environmental Prediction of the US National Weather Service) operational numerical weather prediction models, would provide forecasters with an objective probability of gust speed exceeding certain thresholds during a certain time period. We are using a "perfect prog" approach, which will accomodate the rapid time scale of evolution of the operational numerical models relative to the typical return period of highly significant events. For our "training" dataset we have chosen 10-plus years of relatively homogeneous peak gust information normalized to a representative location in Boulder. Our choice of predictors is influenced by the successes and failures of earlier more limited studies as well as by current theoretical understanding of the phenomenon.

In our presentation we will provide further details of our methodology and to the extent possible describe how our results may be practicably applied as well as how they relate to current physical understanding.