Heavy Rainfall in Texas Associated with Predecessor Rain Events Produced by Eastern Pacific Basin Tropical Cyclones

Long-time weather forecasters in Texas will attest that the potential for heavy rainfall increases when upper level moisture associated with tropical cyclones in the eastern Pacific basin streams northeastward across the state. In reality this conceptual model is flawed, as upper level moisture from these tropical cyclones plays a minimal role in generating intense convection. Instead the convection is produced dynamically through the interaction of the tropical cyclone with the mid-latitude westerlies, a phenomenon first coined as a predecessor rain event (Cote 2007). Predecessor rain events (PREs) have received some attention in the research and operational community recently in order to better anticipate heavy rainfall far removed from the parent tropical cyclones. However, research remains focussed on PREs occurring over the continental United States associated with recurving Atlantic Basin tropical cyclones.

This study will examine 58 tropical cyclones that tracked near the Pacific coastline of Mexico to determine the synoptic pattern and other conditions that led to the development of 14 PREs in Texas that produced extreme rainfall and flooding. By examining null events and six additional cases where rainfall was not heavy enough to qualify it as a traditional PRE, insights into the minimum thresholds of moisture, instability, and other ingredients required for a PRE to develop in Texas can be gleaned. Finally, this study will present a simple method to forecast the orientation of the heavy rainfall axis and region of Texas impacted based on the historical patterns and trends. The examination of trends within this database could be used to help improve forecasting of Atlantic basin PREs.