From June 15 through September 30, the Southwest region of the United States is affected by the North American Monsoon System (NAMS). Last summer was the wettest on record for the Southwest, with the average rainfall during monsoon months being 7.93 inches!
The current weather setup over this region is supportive of a monsoon event in the Southwest. With an area of low pressure to the west and high pressure to the east, it results in a continuous surge of deep tropical moisture from the Gulf of Mexico. Combine this endless supply of moisture with energy from a disturbance passing through the region, you get those tropical-like downpours.
During the past two days, portions of Arizona, New Mexico, and west Texas have received over two inches of rainfall! Slow-moving showers and storms that developed moved through the states producing torrential rainfall; thanks to the deep moisture content in the atmosphere.
Although this rainfall was beneficial as the majority of the region is experiencing extreme to exceptional drought conditions, it has led to problems. When the ground is extremely dry, the rate water is absorbed is slower. This leads to an increased flash flooding threat for the area; Arizona saw this on Friday!