An Unexpectedly Heavy and Complex Snowfall Event across the Southern Appalachian Region

Abstract

On 26 March 1999, an unexpectedly heavy and complex snowfall event occurred across the southern Appalachian region. This event produced 20?30 cm (8?12 in.) of snow across the Smoky Mountains and 10?15 cm (4?6 in.) across other portions of southwest North Carolina, northeast Tennessee, and southwest Virginia. This snowfall event was complex in that several different lifting mechanisms combined to produce unexpectedly heavy amounts, especially in a narrow band across the Great Tennessee Valley. Lift from frontogenesis, orography, cold air damming, and mesoscale waves contributed to the snowfall amounts across the entire region. An interesting aspect of this snowfall was the banded enhancements observed during the initial stage of the event. These banded enhancements, observed by both satellite and radar, were determined to be the result of mesoscale waves. These waves developed around 0900 UTC in the lee of the Smoky Mountains as a strengthening southerly flow above 850 hPa became nearly perpendicular to the Smokies. A moist stable layer just above the mountain ridges (between 850 and 650 hPa) provided a sufficient duct for mountain waves to form across northeast Tennessee. Convective activity later developed around 1200 UTC across northeast Georgia along an inverted surface trough. This convective activity appeared to have helped trigger additional waves across western North Carolina. It appeared that the waves contributed to the heavy snowfall amounts by providing additional lift to the larger-scale lift present, which together maximized the release of the conditional instability across the region. After 1400 UTC, wave activity appeared to diminish across the southern Appalachian region as the larger-scale lift overwhelmed the waves.