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The 1974 Super Outbreak was the second-largest tornado outbreak on record for a single 24-hour period, just behind the 2011 Super Outbreak. It was also the most violent tornado outbreak ever recorded, with 30 F4/F5 tornadoes confirmed. From April 3 to April 4, 1974, there were 148 tornadoes confirmed in 13 U.S. states and the Canadian province of Ontario. In the United States, tornadoes struck Illinois, Indiana, Michigan, Ohio, Kentucky, Tennessee, Alabama, Mississippi, Georgia, North Carolina, Virginia, West Virginia, and New York. The entire outbreak caused more than $600 million (1974 USD) in damage in the United States alone, and extensively damaged approximately along a total combined path length of .〔 At one point, as many as 15 separate tornadoes were ongoing at the same time.〔 The Super Outbreak of April 3–4, 1974 remains one of the most outstanding severe convective weather episodes of record in the continental United States. The outbreak far surpassed previous and succeeding events in severity, longevity and extent, with the notable exception of the 2011 Super Outbreak, which lasted from April 25–28. With a death toll of 319, this outbreak would not be surpassed until the 2011 Super Outbreak, which killed 324 people. ==Meteorological synopsis== A powerful spring-time low pressure system developed across the North American Interior Plains on April 1. While moving into the Mississippi and Ohio Valley areas, a surge of very moist air intensified the storm further while there were sharp temperature contrasts between both sides of the system. Officials at NOAA and in the National Weather Service forecast offices were expecting a severe weather outbreak on April 3, but not to the extent that ultimately occurred. Several F2 and F3 tornadoes had struck portions of the Ohio Valley and the South in a separate, earlier outbreak on April 1 and 2, which included three killer tornadoes in Kentucky, Alabama, and Tennessee. The town of Campbellsburg, northeast of Louisville, was hard-hit in this earlier outbreak, with a large portion of the town destroyed by an F3. Between the two outbreaks, an additional tornado was reported in Indiana in the early morning hours of April 3, several hours before the official start of the outbreak. On Wednesday, April 3, severe weather watches already were issued from the morning from south of the Great Lakes, while in portions of the Upper Midwest, snow was reported, with heavy rain falling across central Michigan and much of Ontario. By 12 UTC on April 3, a large-scale trough extended over most of the contiguous United States, with several modest shortwaves rotating around the broad base of the trough. The mid-latitude low-pressure center over Kansas continued to deepen to , and wind speeds at the 850-mb level increased to () over portions of Louisiana, Mississippi, and Alabama. Due to significant moisture advection, destabilization rapidly proceeded apace; the warm front near the Gulf Coast dissipated and then redeveloped northward over the Ohio River valley. Consequently, CAPE levels in the region rose to 1,000 j/kg. However, a warm temperature plume in the elevated mixed layer kept thunderstorms from initiating at the surface. Meanwhile, a large mesoscale convective system (MCS) that had developed overnight in Arkansas continued to strengthen due to strong environmental lapse rates. Later in the day, strong daytime heating caused instability to further rise: by 18 UTC, CAPE values in excess of 2,500 j/kg were present over the lower Ohio and the Mississippi Valley. As wind speeds in the troposphere increased, Large-scale lifting overspread the warm sector. At the same time, the forward-propagating MCS spread into the Tennessee and Ohio valleys, where it evolved into the first of three main convection bands that produced tornadoes. This first convective band moved rapidly northeast, at times reaching speeds of about ().〔 However, thunderstorm activity, for the moment, remained mostly elevated in nature.〔 By 1630 UTC, the large MCS began to splinter into two sections: the southern part slowed, lagging into southeast Tennessee, while the northern part accelerated, reaching Pennsylvania by 1930 UTC. The split was related to several factors, including a band of subsidence over eastern Kentucky and western West Virginia; local downslope winds over the Appalachians; and an inversion over the same area. These factors allowed the northern part of the MCS to accelerate due to efficient ducting, while the southern part slowed as the boundary layer warmed and moistened.〔 Numerous surface-based supercells began to develop in the southern area, beginning with one that produced an F3 tornado at about 1630 UTC near Cleveland, Tennessee.〔 Meanwhile, a new band of scattered thunderstorms developed at 1500 UTC over eastern Arkansas and Missouri; over the next four hours, this band became the focus for several intense supercells, starting in eastern Illinois and southern Indiana.〔 In the wake of the MCS, backing low-level winds, rapid diurnal destabilization, and perhaps cool, mid-level advection had occurred over the warm sector, weakening the convective inhibition (CINH) layer, and favorable wind profiles bolstered helicity to over 230 m²/s²—a combination of factors conducive to tornadogenesis.〔 Consequently, the storms increased in intensity and coverage as they moved into Illinois, Indiana, and northern Kentucky, producing several tornadoes, including the first F5 tornado of the day, at 1920 UTC, near Depauw, Indiana.〔 Several of the storms to form between 1920–2020 UTC became significant, long-lived supercells, producing many strong or violent tornadoes,〔 including three F5s at Depauw; Xenia, Ohio; and Brandenburg, Kentucky. These storms formed the second of three convective bands to generate tornadoes.〔 While violent tornado activity increased over the warm sector, a third band of convection developed at about 16 UTC and extended from near St. Louis into west-central Illinois. Based upon real-time satellite imagery and model data, differential positive vorticity advection coincided with the left exit region of an upper-level jet streak that reached wind speeds of up to (), thereby enhancing thunderstorm growth.〔 Storms grew rapidly in height and extent, producing baseball-sized hail by 1720 UTC in Illinois and, shortly thereafter, in St. Louis, Missouri, which reported a very severe thunderstorm early in the afternoon that, while not producing a tornado, was the costliest storm to hit the city up to that time.〔 By 1950 UTC, supercells producing F3 tornadoes hit the Decatur and Normal areas in Illinois. As thunderstorms moved into the warmer, moister air mass over eastern Illinois and Indiana, they produced longer-lived tornadoes—one of which began near Otterbein and ended near Angola in Indiana, a distance of .〔〔 Meanwhile, by 00 UTC the southern half of the first convective band became indistinguishable from new convection that had formed farther south over Alabama and Tennessee in connection with convective band two. In this area, increasing west-southwesterly wind shear at all levels of the troposphere, juxtaposed over near-parallel outflow boundaries, allowed successive supercells, all producing strong, long-tracked tornadoes, to develop unconstrained by their outflow in a broad region from south Tennessee into eastern Mississippi.〔 These storms, forming after 23 UTC, produced some of the most powerful tornadoes of the outbreak, including two F5s near Tanner and an extremely potent F5 that devastated Guin in Alabama.〔 Michigan was not hit as hard as neighboring states or Windsor, with only one deadly tornado that hit near Coldwater and Hillsdale, killing people in mobile homes; however, thunderstorm downpours caused flash floods, and north of the warm front in the Upper Peninsula, heavy snowfall was reported. Activity in the south moved towards the Appalachians during the overnight hours and produced the final tornadoes across the southeast during the morning of April 4.〔 A series of studies by Dr. Tetsuya T. Fujita in 1974–75—which were later cited in a 2004 survey by Risk Management Solutions—found that three-quarters of all tornadoes in the 1974 Super Outbreak were produced by 30 'families' of tornadoes—multiple tornadoes spawned in succession by a single thunderstorm cell.〔 The majority of these were long-lived and long-tracked individual supercells. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1974 Super Outbreak」の詳細全文を読む スポンサード リンク
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