On the Predictability of Thunderstorms over the Southwestern United States

Thunderstorms that occur in response to the North American monsoon are an important climatological feature of Arizona's summer wet season and contribute a large portion to the region's annual rainfall. As such, the accurate prediction of these storms is necessary to public safety. The purpose of this study is to explore the practical predictability of monsoon thunderstorms over the southwestern United States in the presence of typical errors in the preconvective environment. The Weather Research and Forecasting (WRF) model is run with 1-km horizontal grid spacing and a control run of a central-eastern mountain and Sonoran desert regime thunderstorm is produced using a horizontally homogenous environment. One-hour forecast errors from the 13-km Rapid Update Cycle (RUC) model valid at 0000 UTC from July-September 2011 are used to define typical environmental errors. Thirty runs initialized with environmental perturbations defined by these forecast errors are produced. The runs are analyzed to ascertain how quickly the simulated storms differ from one another using the control sounding as truth. Every perturbed simulation produces a thunderstorm. A thunderstorm is defined in this study as having a radar signature of 30 dBZ or greater and the lifetime is defined as how long the storm can maintain that level of reflectivity. The environmental errors cause the simulations to differ from one another and the control run in terms of the storm's lifetime, reflectivity, wind speed, and 5-minute rainfall accumulations. A high level of confidence can be placed in the one-hour forecast since every perturbed simulation produces a thunderstorm. The environmental errors cause the simulations to differ from one another in terms of the storm's lifetime, occurrence, location, reflectivity, wind speed, and 5-minute rainfall accumulations. Storm lifetime, occurrence, and location are somewhat sensitive to environmental errors. The ensemble members do not differ drastically from one another in these categories. The smaller the feature of the storm, the more the practical predictability decreases, as occurs for the location of the 5-minute rainfall accumulations and lowest level winds. Thus, the smaller the feature of the storm, the greater effect environmental errors have on the predictability of that feature.