The 08 May 2009 Missouri Derecho: Radar Analysis and Warning Implications over Parts of Southwest Missouri

A historic wind storm with embedded tornadoes occurred during the morning of 08 May 2009 from southeast Kansas through southern Illinois. Incredible wind damage occurred with this system in which many homes, businesses in smaller communities, and farmsteads were damaged or destroyed by damaging winds or tornadoes. Individual large swaths of damaged trees extending from 75 to 100 km in length and 30 to 40 km wide were documented during damage assessments. Within these swaths, complete deforestation was revealed owing to intense microburst or individual burstswaths within the microbursts. Surface wind gusts were estimated over 50 to 55 m s-1 within these swaths. Most of the individual tornadic damage tracks occurred from near Springfield, Missouri into south central Missouri. The overall area of wind damage extended from southeast Kansas through parts of the southern third of Missouri and into southern Illinois. Since this was a large and prolonged wind storm, the study of this event could be divided into four or more time periods. This study will briefly examine the environmental characteristics prior to the event arriving over a part of southwest and south-central Missouri. However, we will closely examine the storm and mesovortex evolution of this event during the period of 1230 to 1400 UTC covering the area south and east of Springfield, Missouri. We will show two contrasting convective modes in which the reflectivity pattern north of the apex of the bow revealed a “leading stratiform” pattern, while the “leading convective line – trailing stratiform” pattern maintained its identity near and south of the apex of the bow. We will discuss why the reflectivity convective line segment north of the apex of the mature bow echo not only weakened after 1253 UTC, but also fragmented with weak mesovortices identified along a north-south convergence axis. In contrast, three strong mesovortices formed near and just south of the apex of the large bow after 1302 UTC along the gust front of the leading convective line. With time these strong mesovortices migrated along the convergence zone north of the apex into the leading stratiform region. Insights will be provided as to why rapid mesovortex development and intensification occurred near and just south of the apex. We believe that the shear vector orientation with respect to the convection at several levels was nearly perpendicular across the area north of the apex of the bow. The orientation and magnitude of these shear vectors was indicative of an environment more favorable for the development of persistent mesovortices, potentially including supercell-like structures. Meanwhile, shear vector orientation was nearly parallel to the convection south of the apex, leading to less intensification and organization within this segment of the storm complex. We will show that the original three strong mesovortices which formed along the leading edge near the apex migrated northeastward towards the western flank of the “leading stratiform” region. These vortices may have been associated with supercell-like structures along the western flank of the “Leading Stratiform” region, and were anchored to the surface convergence zone. Last we plan to discuss the challenges and difficulties to warning operations and implications with this rapidly moving convective system.