Analyzing and Forecasting Rocky Mountain Lee Cyclogenesis Often Associated with Strong Winds

Since numerical forecast models often err in predicting the timing and location of lee cyclogenesis, a physically based method to diagnose such errors is sought. A case of Rocky Mountain lee cyclogenesis associated with strong winds is examined to explore the transformation from a stationary lee trough to a mobile midlatitude cyclone (hereafter, departure). Up to 12 h before departure, a pronounced surface pressure trough travels eastward across western North America at an average speed of 22 m s21. Several methods are employed to examine the structure and evolution of the pressure field: total sea level pressure, time series at individual stations, isallobars, and bandpass filtering. Bandpass filtering of the observed sea level pressure data is useful for clarifying the movement of the mobile trough through the complex terrain. Quasigeostrophic height-tendency diagnostics show that the mobile pressure trough is related to the traveling midto upper-tropospheric vorticity maximum that is responsible for departure. At many stations, surface temperature changes associated with this pressure trough are not consistent with those commonly associated with surface frontal passages. To test the hypothesis that mobile pressure troughs are associated with departure, a five-winter climatology of 111 southern Alberta lee cyclones is constructed. Sixty-two percent of these events feature an upstream pressure minimum 3–9 h prior to departure, in a manner resembling the case study. Seventy-six percent of these 111 events are associated with reports listed in Storm Data, indicating the potential severity of these storms.