A multiscale analysis of a severe weather-producing Great Lakes cutoff cyclone: Part I: Antecedent conditions, cutoff formation, and vortex breakdown

Beginning on 15 May 2006, Typhoon Chanchu recurved in the western Pacific Ocean and underwent extratropical transition. The upper-level outflow from Chanchu strengthened the mid-latitude jet stream and resulted in an episode of downstream development that propagated across the ocean basin. The energy associated with this downstream development contributed to the cyclonic wave breaking of a pre-existing, quasi-stationary trough in the extreme eastern Pacific Ocean. As this trough crossed into the lee of the Rocky Mountains and cutoff, a surface cyclone formed beneath an upper-level potential vorticity (PV) anomaly. This cutoff cyclone progressed eastward across the northern United States and at 1200 UTC 25 May, was positioned over Wisconsin. A dry slot extended from the southern flank of the cutoff cyclone and wrapped cyclonically around the cyclone to its northwestern periphery. Subsequently, vortex breakdown occurred as multiple mesoscale short-wave troughs developed along this dry slot and encircled the center of the cutoff cyclone. This vortex breakdown was likely associated with barotropic instability, and the multiple mesoscale short-wave troughs that formed strengthened as they rotated cyclonically around the cyclone.

This presentation will provide both an observational and a WRF-model simulated investigation of the complex flow evolution that led to the development of the cutoff cyclone and the subsequent vortex breakdown. Observational data will highlight the multiscale processes and interactions that resulted in the formation of a distinct PV anomaly and cutoff cyclone to the lee of the Rockies, while simulated data will explore the development and progression of the embedded mesoscale short-wave troughs around the center of the cutoff cyclone. These troughs appear to have played a significant role in a convective outbreak in the Great Lakes region. Important mesoscale aspects of this convective outbreak will be discussed in the companion (part II) presentation.