The Impact of the Appalachian Mountains on Cyclonic Weather Systems. Part I: A Climatology

Abstract

A climatological study of cold fronts and cyclones crossing the Appalachian Mountains from the west through northwest has been performed. A sample size of 50 fronts and 40 cyclones was derived from the seven winter seasons (December through March) between 1979 and 1986. The analysis revealed that initially linear cold fronts are slowed and deformed by the mountains, with the mean frontal speed decreasing 40% and the fronts developing a pronounced ?kink? just east of the southern Appalachians. Furthermore, over the mountains the cross-front thermal and pressure gradients are enhanced by about 25% and 63%, respectively. Once past the mountains, fronts are weaker and often difficult to locate. In particular, the cyclonic wind shift is very ill defined, and the cross-front pressure gradient is reduced to 38% of the premountain value. Finally, small-scale (500-km diameter) ?lee lows? form on more than half of the fronts. Though typically benign, some of these disturbances may play a role in offshore cyclogenesis. Most cyclones undergo redevelopment across the Appalachians. During redevelopment, the primary cyclone is first deflected northward, then dissipates over West Virginia or Pennsylvania, while a secondary center forms approximately 350 km away in the lee. Two types of redevelopment are identified: the first occurs nearly due south of the primary when the associated cold front crosses the mountains, while the second takes place farther east near the coast when cold-air damming is present. The exact location of redevelopment is highly variable and apparently is a complex function of the distribution of upper-level ?forcing? and low-level thermal structure. On average the net effect of the Appalachians consists of a small and temporary but locally important course shift. A comparison of the results with theoretical work suggests that the neglect of diabatic processes and friction, and the use of relatively flat, symmetric topography and simple background flows, are probably responsible for most of the disagreement between observations and theory.