Norwegian-Type and Cold Front Aloft–Type Cyclones East of the Rocky Mountains

Abstract

Conventional data and mesoscale model simulations are used to analyze two cyclones that developed east of the Rocky Mountains in June and November 1998. Both cyclones formed when a Pacific cold front overtook a lee trough/dryline east of the Rockies. In one case the leading edge of the Pacific cold front was on the surface, as depicted in the classic Norwegian model of a cyclone. In the other case, which is referred to as a cold front aloft (CFA) cyclone, the leading edge of the Pacific cold front was aloft and in advance of the lee trough. The lifting and severe weather associated with the Pacific cold front was along the leading edge of this front in both the Norwegian-type and CFA-type cyclones. To obtain an estimate of how often CFA cyclones, with a coincident CFA rainband, occurred in the central United States during the period 15 September 1994 through 15 September 1995, 70 cyclones that maintained closed surface low pressure centers for at least 24 h, and produced precipitation in the United States east of the Rockies, were analyzed. Analysis of these cyclones revealed that 46% were CFA type, 23% were Norwegian type, and the remaining 31% could not be readily classified into either type. Lee troughs were common features east of the Rocky Mountains during this period. They were present in 62% of the 70 cyclones analyzed. The 1000?500-hPa thickness field is suggested as a useful tool in locating the leading edge of a Pacific cold front, and in determining whether a cyclone is a Norwegian type or CFA type. The issue of how the frontal structures in CFA-type cyclones should be analyzed on surface weather charts is discussed, and some suggestions offered.