Abstract
Abstract A diagnostic analysis is made for the rapid development of two subsynoptic scale cyclones that coexisted over Canada in the spring season, using the Level IIIb First GARP Global Experiment dataset assimilated by the European Centre for Medium Range Weather Forecasts and National Weather Service operational data. Both cyclones started developing at 0000 UTC 25 April 1979. When development began, these cyclones wore separated from each other by a distance of only about 1300 km. Nonetheless, the physical processes leading to their initial cyclogenesis are found to be different. One of the cyclones remained a weak, shallow surface low for about 48 h after it formed in a zone of developing upper-level westerly waves. It started developing only when it drifted into a region of the horizontal advection of upper-level potential vorticity anomaly associated with a strong tropopause fold. In contrast, the other cyclone formed in a region of strong surface frontogenesis caused primarily by the velocity defo...
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