Nearly 50 years of observations of hook echoes and their associated rear-flank downdrafts (RFDs) are reviewed. Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis remains poorly understood. Despite many strong similarities between simulated and observed storms, some possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional numerical models.

http://tornado.sfsu.edu/geosciences/classes/m835/pdfs/HookEcho.pdf

Hook Echoes and Rear-Flank Downdrafts: A Review

The Super Outbreak of tornadoes over the central and eastern United States on 3–4 April 1974 remains the most outstanding severe convective weather episode on record in the continental United States. The outbreak far surpassed previous and succeeding events in severity, longevity, and extent. In this paper, surface, upper-air, radar, and satellite data are used to provide an updated synoptic and subsynoptic overview of the event. Emphasis is placed on identifying the major factors that contributed to the development of the three main convective bands associated with the outbreak, and on identifying the conditions that may have contributed to the outstanding number of intense and long-lasting tornadoes. Selected output from a 29-km, 50-layer version of the Eta forecast model, a version similar to that available operationally in the mid-1990s, also is presented to help depict the evolution of thermodynamic stability during the event.

Revisiting the 3-4 April 1974 Super Outbreak of Tornadoes

The outbreak of tornadoes from the Mississippi River to just east of the Appalachian Mountains on 2–5 April 1974 is analyzed using conventional techniques and the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5). The MM5 was run for 48 h using the NCEP–NCAR reanalysis dataset for initial conditions. It is suggested that the first damaging squall line within the storm of 2–5 April 1974 (herein referred to as the Super Outbreak storm) was initiated by updrafts associated with an undular bore. The bore resulted from the forward advance of a Pacific cold front into a stable air mass. The second major squall line within the Super Outbreak storm, which produced the strongest and most numerous tornadoes, was directly connected with the lifting associated with a cold front aloft. This second squall line was located along the farthest forward protrusion of a Pacific cold front as it occluded with a lee trough/dryline. An important factor in the ...

A New Look at the Super Outbreak of Tornadoes on 3–4 April 1974

In this study of the 19 April 1996 Illinois tornado outbreak, 109 cells were tracked using radar data to understand the transition of the cell configuration from a considerable number of initial cells to a small subset of supercells after several hours of evolution. Of these 109 cells, 85 developed along three synoptic boundaries (dryline, warm front, and dryline–warm front occlusion) between 1940 and 2230 UTC. A large majority of these 85 cells formed in a 1-h period between 2040 and 2140 UTC. With a considerable number of cells initiating within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells. Cell-type initiation analysis revealed that storms that would become supercells were initiated, on average, 17 min before nonsupercell storms. Cyclonic supercells, with mean storm life spans of 214 min, had much longer lives than nonsupercell storms. Anticyclonic supercells resulting...

The 19 April 1996 Illinois Tornado Outbreak. Part I: Cell Evolution and Supercell Isolation

This article is one in a series of papers from the Symposium on the Mystery of Severe Storms: A Tribute to the Work of T. Theodore Fujita. It reviews Fujita's vast contributions to the understanding of tornadoes and tornadic thunderstorms and briefly summarizes a few complementary studies on these topics by other authors. This article also reviews Dr. Fujita's applications of photogrammetry to the study of thunderstorms and tornadoes. Comments about Ted Fujita's influence on the authors' careers are interspersed throughout the paper and in the acknowledgments.

Tornadoes, Tornadic Thunderstorms, and Photogrammetry: A Review of the Contributions by T. T. Fujita

Radar observations of tornado families occurring on April 3, 1974 are discussed. Of the 93 tornadoes included in the sample, 81% were associated with hook-like echoes with appendages at least 40 deg to the south of the echo movement. At least one tornado was associated with 62% of the hook-like echoes observed. All of the tornadoes with intensities of F 4 and F 5 were produced by hook-like echoes; the mean intensity of all tornadoes associated with this type of echo was F 3, while the mean intensity of the remaining tornadoes was F1. The tornadic hook-like echoes moved to the right of the non-tornadic echoes forming a tornado line in advance of the squall line. Some tornadoes were associated with 'spiral' echoes.

G. S. Forbes

Papers

Relationship between tornadoes and hook echoes on April 3, 1974