Prefrontal Rainfall over North Central Teyas, May 12-13,1957 Robert 0. Cole and Dale

During the 24-hour period ending 1230 GMT May 13, 1957 moderate to heavy convective-type rain occurrecl over a large portion of north central Texas. In consideration of the type of prec.ipitation, amounts were not unusually large, ranging from 1 to 5 inches (fig. 1). However, the most interesting feature was that the rainfall area was well organized and locat,ed in an area where the surface isobars were curved anticyclonically (fig. 2) for the most part. Although cyclonic curvature usually connotes convergence and antic.yc1onic curvature diverL gence, Fletcher [2] has pointed to the esistence of heavy rainfall situations in Texas wherein there is low-level anticyclonic curvature and upper-level cyclonic curvature. Such was the case over north central Tesas May 12-13. In general precipitation is the result of saturated air being lifted and is closely associated with horizontal convergence in the low levels and divergence in the upper levels. In. addition to topography, the lifting of air can be accomplished by fronts, thermal instability, and isobaric convergence. This investigation has been limited to a synoptic analysis of features related to these lifting processes to determine why north c.entra1 Texas was a preferred area for moderate to heavy precipitation from 1230 GMT May 12 to 1230 GMT May 13. The synopt,ic analysis included the general features of the surface and upper-air patterns, stability factors, moisture considerations, upper-level wind field with vorticity concepbs, Llow-level wind distribution, and a system of trajectories for delineating low-level convergence.