Topographic distortion of a cold front over the Snake River Plain and central Idaho Mountains

The topographic distortion of a cold front over the Snake River Plain (SRP) and central Idaho Mountains on 3 December 1998 is described using high-den sity surface observations from MesoWest, a collection of meteorological net works over the western United States. Although relatively unperturbed upstr eam of central Idaho, the cold front became distorted as it was deflected a nd accelerated up the low-elevation SRP, where a pronounced frontal bulge d eveloped. The speed of the cold front over the SRP was comparable to the ma gnitude of the postfrontal winds that, due to terrain channeling, were orie nted normal to the front. Meanwhile, the front advanced more slowly over th e central Idaho mountains and southwest Montana, becoming increasingly diff use over the former. Eventually, cold air surrounded the central Idaho Moun tains and the two portions of the cold front merged over eastern Idaho. The cold front intensified as it moved from the eastern to central SRP, wit h rapid changes in temperature and pressure observed at locations in the so uthern half of the SRP. Intensification of the cross-frontal temperature gr adient in this region appeared to be the result of confluence between south erly prefrontal winds, which experienced downslope warming to the lee of th e Jarbidge-Caribou Highlands, and terrain-channeled postfrontal winds. Alth ough the rapid changes in temperature and pressure suggested that the front developed the local structure of a gravity current, the frontal motion ove r the SRP was not consistent with gravity current theory and instead appear ed to be the result of advection of the front by the terrain-induced flow f ield. The case study illustrates the value of high-density and multielevation Mes oWest observations for advancing knowledge of frontal evolution over the we stern United States and improving operational surface analyses. Such observ ations aid in the identification of large-scale airmass and circulation cha nges that can be masked by boundary layer processes, valley inversions, and local and mesoscale terrain-induced wind systems.