On Summer Cyclogenesis in the Lee of the Rocky Mountains
About twice in an average summer season, it is observed that sea-level cyclones of extraordinary intensity develop in the lee area of the northern Rocky Mountains. These cyclones differ from the customary frontal waves in that the onset of the development is sudden, the development period is short and the system remains quasi-stationary until full maturity is reached. In common with the frontal waves and most minor disturbances in the same region is the feature that the geneses are preceded by a forward march of an upper cold low or trough from the northeastern Pacific Ocean. An analysis of individual cases shows that the sea-level development begins at the time when an area of positive vorticity advection aloft spreads over the eastern slopes of the mountain range. The movement of the upper cold low is found to be strongly influenced by the behavior of the next cold upper low or trough upstream.