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The approximate cause of the huge tornado outbreak of April 3-4, 1974, in roughly layman's language, was the result of a number of important factors coming together at the right critical time. A very vigorous upper level atmospheric disturbance, along with a strong polar
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jet stream lead to the amplification and general strengthening of a low pressure region to the east of the Rocky Mountains. As this low pressure region, with its counterclockwise rotation moved eastward, it became more and more intense and caused a great deal of warm, moist air to move into the Ohio river valley from the Gulf of Mexico. The natural rising of the surface, warm, moist air was inhibited by an upper atmospheric layer of warm dry air from the dry southwest region of the country. |
| Radar Photo 15:11 GMT |
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| At the interface of the lower, warm, moist air and the upper, dry air was a large region subsidence, where the air was sinking onto the lower moist air. This large sinking air mass region acted like a lid on the lower, moist air to keep it from rising and dissipating energy by forming normal thunderstorms.Throughout the morning of April 3, sunlight continued to heat the moist surface air, increasing its normal tendency for upward convection. Eventually in the early to mid afternoon the pent-up upward convection could not be contained any longer over many locations from Illinois to Ohio to Alabama and Georgia in the south. |
Radar Photo 17:54 GMT |
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In many locations over this broad geographic area, the rising, moist air had enough energy to punch through the "lid" holding it down. As the energetic, moist air rapidly rose, friction and other perturbations led to localized rotation and tornado development. Nearly 150 tornados developed over this region of the US.
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Radar Photo 21:00 GMT |
This tornado outbreak was relatively rare. Several factors came together at the right time to allow for the magnitude of this tornado outbreak. These factors include: the location and energy of the upper level wave (disturbance) with respect to the surface low pressure region; the relatively warm Gulf region providing an abundance of warm, moist surface air; the extent of the upper level dry air and the resulting subsidence (lid effect); and the extent of the solar heating. Take away one or more of these factors and the enormity of the outbreak would most likely have been greatly reduced. It is near speculation as to how frequently such a confluence of factors might occur. One such guess might be several hundred to five hundred years.
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To download the NOAA report on this day click here. Warning this an 8 meg Adobe Acrobat file that is extremely technical.
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