Factors Contributing to Heavy Rainfall during the North American Monsoon

The North American monsoon is an important component of the hydrological cycle for the southwest United States and northwest Mexico. These areas receive roughly 50% of their annual rainfall during the monsoon season, with the annual percentage increasing southward as winter storms contribute less to annual totals. As with other global monsoons, the communities within the North American monsoon region have grown to depend on these rains for societal needs such as agriculture, irrigation and power generation. While the North American monsoon is the result of a seasonal shift in the global circulation bringing moisture into Mexico and the southwestern United States, monsoon rains arrive as a series of short-lived mesoscale convective events, often associated with extreme weather conditions such as high winds, heavy rain and flooding, and lightning. This presentation summarizes some of our results regarding the physical mechanisms governing synoptic scale (~1000 km) - to - mesoscale (~100 km) interactions over northwest Mexico and the southwest United States that lead to heavy precipitation in these regions. The variability in column water vapor and its relationship to heavy rainfall based on in situ observations from surface Global Positioning System (GPS) receivers will also be discussed.