Evolution of the surface meteorological fields on May 3, 1999

For many years forecasters have analyzed the evolution of surface meteorological fields to gain insight into severe weather outbreaks. Numerous studies have shown relationships between surface features and convective initiation and organization. Structures of interest include dryline bulges, outflow boundaries, thermal and moisture ridges, and spatial and temporal patterns of small low pressure centers.

The evolution of the surface features prior to and during the early stages of the Oklahoma/Kansas tornado outbreak of May 3, 1999 likely was influenced by antecedent rainfall. From April 29 to May 1, 1999 several rounds of thunderstorms occurred over the Texas panhandle and west Texas north of Midland. On May 3 the dryline moving out of the Texas panhandle was retarded in its eastward progress relative to the dryline advance farther south out of southwest Texas. Also, the airmass west of the dryline was not as dry over the panhandle as farther south. It is surmised that the soil moisture available in the panhandle from the previous days' rainfall slowed the vertical mixing process, whereas farther south the much drier soil permitted much greater mixing of moisture and momentum. Thus, a surge in the dry line out of southwest Texas became the focusing mechanism for convective initiation on May 3, 1999.

Moreover, on May 2, 1999 a mesoscale convective system (MCS) persisted over north central Texas leading to the development of a cool, moist airmass just south of the Red River. By midday on May 3, 1999 this airmass was moving north westward into southwest and south central Oklahoma. The thunderstorms that formed late in the afternoon of May 3, 1999 soon approached the boundary on the southwest side of this modified airmass. The presence of the boundary and its progress into central Oklahoma may be important in explaining the strength and longevity of the tornadoes that struck in the Oklahoma City area.

The surface features in this event were well sampled in both the traditional synoptic scale data sets and in the Oklahoma Mesonet. Four analyses of traditional fields were conducted; a manual analysis of the synoptic data, a manual analysis supplemented by mesonet data, an objective analysis of the synoptic data, and an objective analysis supplemented by mesonet data. The four analyses are compared. In addition the analyses on non-traditional fields sampled in the Oklahoma Mesonet are discussed.