Discrete frontal propagation over the Sierra-Cascade Ranges and western Great Basin

On 25 Mar 2006, a complex frontal system moved rapidly across the Sierra–Cascade Mountains and intensified dramatically over the Great Basin where it produced one of the strongest cold-frontal passages observed in Salt Lake City, UT over the past 25 y. Observational analyses and numerical simulations by the Weather Research and Forecast (WRF) model illustrate that the frontal system moved discretely across the Sierra–Cascade Mountains and western Great Basin, with an apparent motion that was much faster than anticipated by advection alone.

Although topographic and diabatic effects contribute to the discrete frontal evolution, large-scale processes play an essential role. Prior to the event, a broad large-scale upper-level trough and low-level baroclinic zone were draped over central California and northern Nevada. As an upper-level cyclonic potential vorticity (PV) anomaly moved through the large-scale trough, cyclogenesis ensued near the poleward edge of the low-level baroclinic zone, leading to the development of a surface occluded front. While the surface cyclone moved across Oregon and northern California, the upper-level cyclonic PV anomaly coupled with the low-level baroclinic zone over northern Nevada, forming the new surface cold front over the Great Basin and in advance of the occluded front.

The WRF simulation clarifies the importance of the synoptic pattern, orographic effects, and diabatic processes in the discrete frontal evolution, which occurs in a manner that is markedly different from that described in previous studies of discrete frontal propagation or front-mountain interactions. Given the difficulties of identifying and tracking surface fronts over the Great Basin, forecasters should consider the possibility of discrete frontal evolution in similar synoptic situations.