The April 15, 1973 Frio County F4 Tornado

Morning upper air analyses indicated the presence of a trough extending from the Four Corners Region north-northeast into central Canada. Troughs, which stick out as "dips" in the height field illustrated in upper air charts, help to lift warm and unstable air into thunderstorms. Reanalysis confirms that the morning area of troughing had reached the Front Range of the Rockies by the evening of April 15. With falling heights overspreading the Rio Grande by this time, it's likely that this area of troughing played at least a partial role in the development of severe thunderstorms in the Frio County vicinity on the evening of April 15.

The search for potential influences next takes us to the upper portions of the troposphere, very near the level that commercial airliners fly. Tied to the disturbance approaching from the west, a southwesterly jet stream overspread the Southern Plains during the afternoon hours on April 15. The strong winds embedded within the feature certainly helped to provide wind shear to help thunderstorms organize and rotate in the mid and upper levels. Areas of acceleration, or divergence, contained within the jet stream likely acted as secondary sources of lift for any developing thunderstorms. It's likely that the approaching jet stream played at least a partial role in the development of severe storms over Frio County on the evening of April 15.

Without a more concentrated source of lift in the lower atmosphere, storms will struggle to develop despite the presence of even the strongest jet streams. In many situations, fronts, drylines, and other types of surface boundaries are the sources of said focused lift. The previously mentioned cold front and dryline were much too far from Frio County to provide any meaningful lift. However, an area of sharp surface temperature differences became evident by the mid to late afternoon hours in the vicinity of the eventual tornado track. This feature, which can be referred to as a differential heating boundary, was readily apparent in the 4:00 PM CST April 15 surface observations. A nearly 20 degree temperature difference between Cotulla and San Antonio was evident at this time. While very difficult to prove in the absence of radar data, the presence of such a pronounced differential heating boundary in the near vicinity of the future tornado's path points to its likely involvement in storm formation and maintenance in the Frio County area on the afternoon of April 15.

With surface and upper level influences on storm formation identified, the focus turns to why high-end tornadoes were possible during the evening hours of April 15, 1973. Tornado chances tend to increase when winds turn with height in the atmosphere. This turning allows thunderstorms to rotate as they grow, potentially leading to tornadogenesis. Oftentimes, a vertically-turning wind field occurs when there are strong south or southeasterly low level winds beneath an enhanced west or southwesterly upper level jet stream. Reanalysis confirms the presence of strong southerly low level flow across the Southern Plains throughout the morning and afternoon hours on April 15. The combination of these winds and the jet stream overspreading the region would have created a vertically-turning wind profile supportive of tornadoes. Surprisingly, the corridor of strongest low level winds vacated to East Texas by the time of the tornado, leaving Frio County wedged between heightened values over the Sabine Basin and much weaker flow to the west. Such an evolution would typically trend an environment away from tornado production.