A Case Study Diagnosis of the Formation of an Upper-Level Cutoff Cyclonic Circulation over the Eastern United States

The synoptic-scale evolution during the formation phase of a midtropospheric cutoff cyclonic circulation over the eastern United States is diagnosed within the potential vorticity framework using the GALE (Genesis of Atlantic Lows Experiment) case of 18–19 January 1986. The study examines 1) the precursor flow evolution prior to cutoff cyclone formation; 2) the wind, mass, and potential vorticity evolution during the 2-day period encompassing cutoff formation; and 3) the relative contribution of upper-versus lower-tropospheric forcing on the quasigeostrophic height tendency field prior to and during cutoff formation. The primary large-scale features prior to cutoff cyclone formation are an amplifying ridge over the western United States and eastern North Pacific and a diffluent trough over the central United States. The primary smaller-scale feature prior to cutoff formation is a short-wave trough-jet streak system that propagates through the longer-wave-amplifying ridge, and then intensifies upon arriving in northwesterly flow downstream of the ridge axis. The intensification of this shorter-wavelength system is associated with increases in stratospheric potential vorticity at levels considered to be well within the middle and upper troposphere. Major midtropospheric cyclogenesis then ensues as the jet propagates toward the base of the diffluent trough while further intensifying. The circulation then “closes off” at 500 hPa within the base of the amplifying trough as stratospheric potential vorticity values descend to near 620 hPa, and become increasingly confined to the base of the trough. The subsequent intensification of the cutoff circulation is accompanied by sustained potential vorticity and temperature increases well above the level of the extruded tropopause. This intensification phase is also accompanied by an increasingly isolated distribution of stratospheric potential vorticity, and by the formation of an isolated warm pool, in the mid-and upper troposphere above the circulation center. These features are consistent with calculations showing that the primary mass loss required to support the formation and subsequent intensification of the cutoff circulation is confined to the upper troposphere. A quasigeostrophic height tendency diagnosis suggests that the advection of potential vorticity at and above the 500-hPa level drives the process of upper-level trough amplification and cutoff cyclogenesis in this case. The quasigeostrophic height tendency patterns are also entirely consistent with the observed mass and wind-field tendencies, and with previous observational and theoretical analyses regarding the invertibility principle of potential vorticity.