Severe weather during the North American Monsoon

During the summer months, the southwest United States receives up to 60% of its annual precipitation in association with the North American Monsoon. Most this precipitation falls from convective storms that form over the strongly heated elevated terrain during the afternoon hours and propagate towards the lower elevation deserts after sundown. Embedded in an environment with typically weak vertical wind, marginal instability, and a deep, well-mixed boundary layer, the majority of these thunderstorms are poorly organized and well below severe limits. However, on approximately one-third of days during the monsoon, severe weather (most likely in the form of wind, but also hail and isolated tornadoes) is reported over Arizona. These high impact events are often poorly forecast and can cause major damage to the growing population and infrastructure of the region.

The goal of the current study is to conduct an updated climatology of severe weather in the southwestern United States, taking advantage of the recent high resolution model output and reanalysis datasets. These synoptic scale analyses will be used in conjunction with lightning data from the National Lightning Detection Network, surface, upper air and radiosonde observations. We will compile and examine all severe weather events in July and August 1991-2005 over Arizona (947 events on a total of 386 days). The flow regimes and synoptic patterns most often associated with severe weather will be investigated and composites created. We will also calculate a composite severe weather sounding and determine how often this sounding is observed during the 15 year period. Illustrative case studies of both severe weather outbreaks and null cases will be examined in detail.