Unexpected Heavy Snowfall Generated by Wave Activity across the Southern Appalachian Region

On 26 March 1999, an unexpected and unusual heavy snowfall, defined by the National Weather Service as four (six) or more inches of snow occurring in a twelve (twenty-four) hour or less time period, occurred across the southern Appalachian region producing eight to twelve inches (20 to 30 cm) of snow across the Smoky Mountains and four to six inches (10 to 15 cm) across portions of southwest North Carolina, northeast Tennessee and southwest Virginia. This snowfall appeared to be the result of a combination of mountain waves generated by the Smoky Mountains and gravity waves generated by convective activity.

Snowfall began in northeast Tennessee the morning of 26 March 1999 as the result of mountain wave activity, as a strengthening southerly wind flow became perpendicular to the Smoky Mountains with the approach of a strong cut-off 500 mb low. A saturated inversion layer was observed just above the mountain ridges between 850 and 600 mb, which likely generated and maintained mountain waves across northeast Tennessee. Convective activity later developed upwind of the Smoky Mountains, across northeast Georgia and northwest South Carolina along an inverted surface trough. This convective activity generated gravity waves along the windward side of the Smoky Mountains which amplified the mountain waves on the leeward side.