As is typical for this time of year, moisture being brought up from the Sea of Cortez and the Pacific Ocean is interacting with the mountainous area of the Southwestern US. Called "Orographic Forcing", the mositure is pushed upward and storms are created. Due to slow wind speeds aloft, storms can sit over an area for a prolonged period of time, causing flash flooding concerns. Current flash flood watches and flood advisories have been posted for southern Nevada, southern California, and western Arizona. I pulled up a webcam image from the Las Vegas area, and you can easily see that the sky is filled with moisture and clouds.