Thursday, 2 August 2001
Synoptic and mesoscale analysis of the 9 August 2000 Appalachian-crossing derechos (formerly poster P3.6)
Early in the morning on 9 August 2000, organized convection developed across Illinois and Indiana on the northeast side of a subtropical ridge within northwest flow, and in the vicinity of a surface warm front. Subtropical moisture contributed to relatively high convective instability and precipitable water. The line of convection developed into a bow echo as it moved into east-southeast into Ohio and then across the northern half of West Virginia and all but the southwestern part of Virginia by the afternoon hours. This mesoscale convective system (MCS) produced primarily straight-line wind damage all along its path, qualifying it as a “derecho” event. This MCS also left an outflow boundary in its wake, which was the focus for mesoscale low-level convergence and moisture pooling, resulting in high instability persisting into the late evening hours. A second MCS, with an east-west oriented convective line, developed along the warm front in central Indiana and Ohio in the late afternoon, and raced toward the south-southeast, the eastern portion of which tended to follow the old outflow boundary as it approached the Appalachians in the form of a large scale bow echo. This MCS produced a couple of tornadoes in the early stages, and many more wind damage reports, particularly in the vicinity of the old outflow boundary. Western portions of the MCS moved southward into Kentucky, and then eventually back toward the southwest into Tennessee before weakening, while the eastern part of the convective line turned more toward the south as it entered the Appalachian chain. The convective line remained severe as it moved into the North Carolina and Virginia Piedmont areas, before finally weakening well after midnight. The position of the upper-level ridge and large scale influence of the coriolis force likely played a role in this right-turning evolution.
This study documents the evolution of this relatively uncommon Appalachian-crossing pair of derechos, and also briefly compares a couple of previous events to help determine factors that influence the longevity of severe organized convection moving through the Appalachians. The study also demonstrates how integrated precipitable water can fluctuate over a mesoscale time frame in advance of these convective lines, as measured by a GPS sensor located in southwest Virginia (providing 30 minute temporal resolution).
Supplementary URL: http://www.nws.noaa.gov/er/rnk/research.html