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

On 25 Mar 2006, a complex frontal system moved rapidly across the Sierra- Cascade Mountains and intensified over the Great Basin where it produced one of the strongest cold-frontal passages observed in Salt Lake City, UT during the past 25 y. Observational analyses and numerical simulations by the Weather Research and Forecast (WRF) model illustrate that the frontal system propagated discretely across the Sierra- Cascade Mountains and western Great Basin. This discrete propagation occurs in a synoptic environment that features a low-level baroclinic zone that extends across northern California and Nevada in advance of a broad upper-level trough. The eastward movement of a cyclonic potential vorticity (PV) anomaly into the upper-level trough initiates Pacific cyclogenesis and the formation of an occluded front in the polar airmass rearward of the low-level baroclinic zone. Continued eastward migration of the upperlevel cyclonic PV anomaly then encourages the formation of an intense cold front ahead of the landfalling occlusion and along the pre-existing low-level baroclinic zone over northern Nevada. This new cold front forms in a region of confluence to the lee of the Sierra Nevada and becomes the dominant surface front of the system. Characteristics of the event are similar to that described in previous studies of front-mountain interactions, but the roles of the non-uniform large-scale environment and three-dimensional topography appear to be unique to the case and region. Although convective precipitation is observed and likely helps sharpen the cold front, diabatic effects associated with moist processes are not necessary for the discrete propagation. Given the difficulties of identifying and tracking surface fronts over the Great Basin, forecasters should consider the possibility of discrete frontal propagation in similar synoptic situations.