Powerful downdrafts created when thunderstorms near Marana and Oro Valley collapsed became the wind that pushed huge dust storms into Phoenix and Yuma on Tuesday, the National Weather Service and an Arizona State University professor said.

In http://www.azcentral.com/news/articles/2011/07/07/20110707arizona-dust-storm-mess-health-fears.html" target="_blank">The Arizona Republic today, ASU professor of geographical sciences Randy Cerveny likened the storm to "a bomb blast," which swept up more and more dust as the winds raced across extremely dry deserts. 

Most of the dust moved northwest through the Phoenix metropolitan area, but another part of the storm blew all the way through Yuma across the Colorado River into California.