Foehn Winds That Produced Large Temperature Differences near the Southern Appalachian Mountains

Abstract

Abstract A 30-yr (1971–2000) study was conducted to 1) determine the typical characteristics and synoptic conditions of foehn wind events that produced large temperature differences near the southern Appalachian Mountains and 2) further examine those events that produced dewpoint rises. Foehn wind events were identified on both the western and eastern sides of the southern Appalachians by comparing temperatures between a station immediately adjacent to the southern Appalachians (foehn station) and two nearby stations located farther from the mountains (nonfoehn station). A foehn wind event occurred when the following criteria were met: the daily maximum or minimum temperature at the foehn station was at least 3°C (∼5°F) higher than the nonfoehn station, winds of at least 2.5 m s−1 (∼5 kt) at the foehn station must have been from a downslope direction, and warming of at least 3°C (∼5°F) must have occurred during the period of downslope winds. Foehn wind events on the western side typically occurred as southeasterly winds developed ahead of a low pressure system over the mid–Mississippi River valley, while foehn wind events on the eastern side were typically the result of northwesterly winds behind the passage of a shallow cold front. The relatively warm 850-hPa air mass, which was only a couple of degrees cooler than the surface temperatures, was likely the source region of the foehn winds on both sides, although additional flow through gaps likely contributed to the less than theoretically possible warming observed at the foehn stations. Composite soundings of the foehn wind events revealed a vertical temperature and wind profile that was conducive to large-amplitude mountain wave formation, especially on the western side of the southern Appalachians. An interesting (and possibly unique) finding observed with these southern Appalachian foehn wind events (compared with other documented foehn wind events) was the rise in surface dewpoints at the foehn stations during nearly three-fourths of the events on the western side and around one-third of the events on the eastern side. A comparison of the dewpoint-rise composites to the composites from all foehn wind events near the southern Appalachians revealed that the rise in surface dewpoints was mainly the result of a drier initial surface air mass (compared with the 850-hPa air mass) and not necessarily a more moist 850-hPa air mass.