Inverted troughs and cyclogenesis over interior North America: a limited regional climatology and case studies

Typically, the inverted trough separates a polar or arctic air mass dammed up along the eastern foothills of the Rockies from an older modified polar air mass over the plains. Inverted troughs are favored when anticyclonic conditions prevail at the surface across south-central Canada and the northern plains states beneath a confluent flow aloft. A composite analysis shows that an inverted trough is most likely when a band of meridionally oriented ascent in the lower and middle troposphere persists along the eastern slopes of the Rockies beneath confluent flow aloft. Cyclones without an inverted trough tend to occur when the synoptic-scale ascent region moves rapidly eastward away from the mountains, so that surface pressure falls immediately to the east of the mountains with attendent cold-air damming cannot be sustained. The life cycle of both cyclones departs significantly from the simple conceptual ideas illustrated in the Norwegian cyclone model. -from Authors