How did the July 5 Phoenix Sandstorm Happen?

thecrimsoncrow.com

On the night of July 5, there was a monster sandstorm that hit Phoenix, Arizona, and made the city, according to witnesses, turn from "day to night". Many described it as something they had never seen it before. So, what caused it?

There were several strong storms in the area on the night of July 5. All thunderstorms have updrafts, which is rising warm air, and downdrafts, which are sinking cool air. These downdrafts can also be called outflows, as they hit the ground and spread out in all directions.
This particular outflow was strong, and threw up massive amounts of dust. As the outflow boundary continued north from Casa Grande, with continued to throw up more and more dust. Cloud tops were likely over a thousand feet high, judging by pictures taken during the sandstorm.