Record Rainfall and Flooding in Texas and Oklahoma, May 2015; Extent and Historical Perspective

Abstract:

Heavy rains began in Texas and Oklahoma in early May 2015 and continued through the end of the month. Both states set all-time records for mean statewide precipitation; Texas - 227mm (8.93 in), Oklahoma - 357mm (14.06 in) -- for the period of record (1895-2015). These new statewide records were set despite the fact that the western portions of both Texas and Oklahoma received only modest rainfall.

Parameters used in this study to evaluate the magnitude and historical perspective of the May 2015 rainfall included daily and total storm precipitation, stream flow, changes in the Palmer Drought Severity Index (PDSI), and changes in reservoir water levels.

Although the Dallas-Fort Worth area and the cities of Austin, Houston and Oklahoma City sustained the most serious flood events, more than 100 localities in the two states reported some flooding. The region with the largest amounts of precipitation extended from north-central Texas northeastward into eastern Oklahoma. Cumulative May rainfall in this region exceeded 508 mm (20 in).

Provisional stream flow data for the river basins most affected -- Red River, Brazos, Colorado, and Trinity rivers -- reveal significant peaks, but the peaks generally are within the ranges of the historical record. With the exception of the Red River the most significant flooding relative to historic flood peaks, occurred on tributaries to the major rivers.

Comparison of the PDSI for the months of April and June reveals the dramatic impact of the precipitation during May. By the first week of June both states are classified as moderately moist - with the exception of the extreme northeastern corner of Oklahoma.

Changes in Reservoir levels (as a percent of capacity) between April and June was greatest for the Rolling Plains region (+ 15.5%), with lesser, but significant gains in South and Central Texas and the Central Oklahoma region.