Life cycle and mesoscale frontal structure of an Intermountain cyclone

High resolution analyses and surface observations are used to examine the structure and evolution of an Intermountain cyclone and its attendant surface fronts. The cyclone, known locally as the “Tax Day Storm,” produced the lowest sea level pressure on record and strongest cold frontal passage (based on the 2 h temperature fall) observed in the past 25 years at the Salt Lake City International Airport (KSLC).

Three major surface features develop downstream of the Sierra Nevada prior to the incipient cyclogenesis. The first is a barrier parallel lee trough that forms in response to strengthening cross-barrier flow. The second is a decaying elongated trough draped over the northern Great Basin. The third is a confluence zone that extends downstream over the Great Basin normal to the Sierra Nevada. Strong contraction (i.e., deformation and divergence) within this Great Basin Confluence Zone (GBCZ) forms an airstream boundary that is initially nonfrontal, but becomes the locus for surface frontogenesis as it collects isentropes and cooler air from northern Nevada. As the elongated trough rotates into phase with the GBCZ and developing surface front, cyclogenesis occurs as an upper-level cyclonic potential vorticity anomaly traverses the Sierra Nevada and quasi-geostrophic forcing for ascent spreads over the Great Basin. Differential diabatic heating further sharpens the front over northern Utah where dramatic frontal distortions are produced by the local orography.

This analysis further establishes the critical role that the GBCZ plays in Intermountain frontogenesis and identifies it as an important mesoscale feature accompanying Nevada cyclogenesis. Recognition of this role may improve short-range forecasting and help advance our understanding of orographically modified frontal and cyclone evolution.