Clear Lake Texas Extreme Rain Event Case Study

On May 12th 2015 between 7-12 inches of rain fell in 3-5 hours between 9PM-2AM CDT in a localized area of Clear Lake City, Texas, a suburb 15NM southwest of Houston, Texas. This 100-year rainfall led to flash flooding resulting in one fatality, property damage, major traffic delays and over 20 water rescues during the morning commute into the city of Houston. A supercell developed earlier in the day along the Galveston Bay which then moved only ~20 NM in 10 hours. This case is unique in that several additional precipitation bands developed just off the Gulf Coast with a NE movement of ~15 kt, overtaking the existing supercell, seemingly without diminishing its intensity. This begs the question; why did storms in a similar environment have such different characteristics?

This study explores this question using observed and model soundings as well radar data from surrounding radars. Radar data indicated the Clear Lake supercell's heights extended roughly twice as high as the precipitation bands from the Gulf of Mexico. This likely allowed the supercell to experience a deeper mean layer shear thus exhibiting a different storm motion as it was affected by the increased vertical shear. A combination of linear updraft-in-shear effects on new updraft growth and a slower storm motion over the deeper cloud depth are hypothesized to have contributed to the supercell's longevity and slower motion. The resulting near-stationary movement combined with the rainfall from the more transient shallow precipitation bands to cause historically high rainfall amounts to fall in such a short period of time.