Minutes of the January 24, 2011 Meeting
Smoky Mountain AMS Chapter
Around ten people first met for dinner at Dazzos Pizza in downtown Knoxville, and then 15 to 20 people convened for the meeting on the Ag campus of the University of Tennessee. The speaker was David Gaffin from the National Weather Service in Morristown, who spoke on "An examination of varying supercell environments over the complex terrain of the eastern Tennessee River Valley". The following was an abstract of his talk:
"The climatological minimum in tornado frequency over the eastern Tennessee River Valley is often attributed to the complex terrain of the region. While tornadoes are relatively uncommon in the eastern Tennessee River Valley (compared to areas further west and south), several significant tornado outbreaks have occurred in this area. Five events that produced supercells across the eastern Tennessee River Valley during the past decade were examined to gain insight into why there were varying levels of tornado-producing efficiency. This study determined that the three events with a high tornado-producing efficiency were characterized by tornado-producing supercells tracking near a well-defined, preexisting west-to-east oriented quasi-stationary frontal boundary. The other two events with a low tornado-producing efficiency had supercells that were located and moved further away from a south-to-north oriented cold front. The west-to-east propagating supercells likely encountered lower LCL heights (and stronger helicity values) for a longer distance along the west-to-east oriented quasi-stationary frontal boundaries in the high tornado-producing efficiency events. In the low tornado-producing efficiency events, the west-to-east propagating supercells encountered higher LCL heights (and weaker helicity values) the further they propagated away from the south-to-north oriented cold fronts, especially across the Great Tennessee Valley where the lowering elevation from the Cumberland Plateau aided in the higher LCL heights. Another common trait observed in four of the five events was the location of strong equivalent potential temperature advection near the locations of the observed supercells and tornadoes. During the one non-tornado-producing event, the combination of high LCL heights, a lack of low-level directional wind shear, and the location of the supercell paths away from a well-defined surface boundary likely caused the lack of tornado development despite the presence of numerous supercells. The two outbreak events experienced the strongest low-level helicity values in the study, although the low-level instability values were the weakest. While the strongest low-level wind shear was present with the two outbreak events, the higher low-level instability during the other three non-outbreak events may have compensated for the weaker (but still significant) low-level wind shear."
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