PALM SUNDAY TORNADOES OF APRIL 11, 1965

Abstract

An extensive aerial survey was made over a large portion of the area affected by the outbreak of tornadoes on Palm Sunday on Apr. 11, 1965. The destruction from the tornadoes extended over parts of six Midwestern States. Aerial and ground damage surveys were combined with eyewitness reports to determine the exact location and time of each tornado occurrence and its path. Radar pictures of the squall line clouds were used to verify the direction and speed of the tornado-producing clouds. Almost simultaneously with the first tornado touchdown in eastern Iowa, TIROS IX took pictures of the Midwest United States that showed a large tongue of cloud-free dry air behind the cold front. The vertical structure of the cold dome is discussed in connection with its role in the development of the tornadoes. Two predictive parameters, namely, the best lifted index (BLI) and material differential advection (MDA) were developed and evaluated with data gathered on this outbreak of tornadoes. The wind speed of a tornado in relation to its parent tornado cyclone is discussed in terms of an anemometer trace showing a peak gust speed of 151 mi hr–1. An indirect wind-speed estimate was also attempted by examining characteristic cycloidal marks left on the fields along the tornado paths. The ground speeds computed ranged from 166 to 180 mi hr–1 for one tornado.