The weather experts have been weighing in with radar and satellite images to explain the origin of Tuesday evening's haboob that buried Phoenix in a 100 mile wide dust storm.
Jesse Ferrell at the Weather Matrix blog suggests the event was caused by the collision of two outflow boundaries. "The way outflow boundaries are caused is illustrated thusly: Basically, cold air crashes down out of dying thunderstorms, hits the ground, spreads out, picking up dust on its periphery. As you probably noticed above, two outflow boundaries came together near Phoenix, but I'll await the NWS's report as to whether or not the two intercepting had something to do with the extreme winds. Sometimes when outflow boundaries interact, nothing happens; sometimes storms are formed (as shown SW of Phoenix in the radar loop), and sometimes they dissipate each other."
Check out the Weather Matrix blog for a variety of 3D graphics, videos, and technical analyses of the storm.