The Influence of Terrain during the 27 April 2011 Super Tornado Outbreak and 5 July 2012 Derecho around the Great Smoky Mountains National Park.

Gaffin, David

The complex terrain around the Great Smoky Mountains National Park appeared to have influenced the evolution of two significant convective events and the resulting location of the extensive tree damage. During the Super Tornado Outbreak of 27 April 2011, an EF4 tornado moved across the Tennessee side of the Park creating a long path of fallen trees. The initiation of this violent tornado coincided with the opening of the southeast-to-northwest oriented Little Tennessee River Valley. Several other tornado tracks during the Super Outbreak initiated near the openings of other southeast-to-northwest oriented valleys located farther northeast up along the southern Appalachian Mountain range. It appeared that the strong southeasterly boundary-layer winds observed on 27 April 2011 may have accelerated through these valleys (due to constricted flow), and could have encouraged tornadogenesis within the northeast-propagating supercells. Another significant blowdown of trees occurred in the Park on 5 July 2012, as a derecho moved southwest across the Great Tennessee Valley. There were several reports of strong northeasterly winds in excess of 60 mph at several observation sites within the Park. Northeasterly winds are an unusual direction to observe high winds in the southern Appalachian region, which may have contributed to the significance of the tree damage. The most significant tree damage occurred within the Laurel Creek valley and near Cades Cove, where pockets of damage were observed mainly in areas that had a good open view to the northeast direction. On the Tennessee side of the Park, there are no mountain barriers to slow down or block winds from a northeasterly direction. Chilhowee Mountain normally blocks strong westerly winds from reaching the Laurel Creek and Cades Cove valleys, while Hannah Mountain likely slows down or blocks any winds from the southwest (the predominant wind direction in the area). The strong northeasterly winds observed on July 5th likely flowed unimpeded into the northeast-to-southwest oriented valleys of the southwestern portion of the Park (and possibly even accelerated in the valleys due to constricted flow).