Nonclassic Evolution of a Cold-Frontal System across the Western United States during the Intermountain Precipitation Experiment (IPEX)

A cold-frontal passage through northern Utah was studied using observations collected during Intensive Observing Period 4 of the Intermountain Precipitation Experiment (IPEX) on 14{15 February 2000. To illustrate some of its nonclassic characteristics, its origins are considered. The front developed following the landfall of two surface features on the Pacific coast (hereafter, the cold-frontal system). The first feature was a surface pressure trough and wind shift associated with a band of precipitation and rope cloud with little, if any, surface baroclinicity. The second, which made landfall 4 h later, was a wind shift associated with weaker precipitation that possessed a weak temperature drop at landfall (1C in 9 h), but developed a stronger temperature drop as it moved inland over central California (4{6C in 9 h). As the first feature moved into the Great Basin, surface temperatures ahead of the trough increased due to downslope flow and daytime heating, whereas temperatures behind the trough decreased as precipitation cooled the near-surface air. Coupled with confluence in the lee of the Sierra Nevada, this trough developed into the principal baroclinic zone of the cold-frontal system (8C in less than an hour), whereas the temperature drop with the second feature weakened further. The motion of the surface pressure trough was faster than the post-trough surface winds and was tied to the motion of the short-wave trough aloft. This case, along with previously published cases in the Intermountain West, challenges the traditional conceptual model of cold-frontal terminology, structure, and evolution.