Topographic Distortion of a Cold Front over the Snake River Plain and Central Idaho Mountains

Abstract

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-density surface observations from MesoWest, a collection of meteorological networks over the western United States. Although relatively unperturbed upstream of central Idaho, the cold front became distorted as it was deflected and accelerated up the low-elevation SRP, where a pronounced frontal bulge developed. The speed of the cold front over the SRP was comparable to the magnitude of the postfrontal winds that, due to terrain channeling, were oriented normal to the front. Meanwhile, the front advanced more slowly over the central Idaho mountains and southwest Montana, becoming increasingly diffuse over the former. Eventually, cold air surrounded the central Idaho Mountains 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, with rapid changes in temperature and pressure observed at locations in the southern half of the SRP. Intensification of the cross-frontal temperature gradient in this region appeared to be the result of confluence between southerly prefrontal winds, which experienced downslope warming to the lee of the Jarbidge–Caribou Highlands, and terrain-channeled postfrontal winds. Although the rapid changes in temperature and pressure suggested that the front developed the local structure of a gravity current, the frontal motion over the SRP was not consistent with gravity current theory and instead appeared to be the result of advection of the front by the terrain-induced flow field. The case study illustrates the value of high-density and multielevation MesoWest observations for advancing knowledge of frontal evolution over the western United States and improving operational surface analyses. Such observations aid in the identification of large-scale airmass and circulation changes that can be masked by boundary layer processes, valley inversions, and local and mesoscale terrain-induced wind systems.