From the Summit County Citizens Voice (Bob Berwyn):
Flashes of lightning, loud thunder and heavy rains Friday marked the start of Colorado’s summer monsoon season. The one- to 2-month period when moist and warm air flows northward from the subtropics may not be as dramatic as the Asian monsoon, but it’s still a critical piece of the state’s overall weather picture, providing abundant moisture just at the time when forests and fields are reaching their driest point.
This seasonal switch in the dominant atmospheric circulation pattern over Colorado often makes July the wettest month of the year in Summit County, which is slightly amazing, considering that February and March often dump several feet of snow in the area. But you have to consider the fact that, when you melt down all that snow, it often only is the equivalent to an inch or two of water, while July delivers, on average, 2.32 inches of moisture.
The U.S. southwestern monsoon season occurs when winter and spring’s jet stream-driven westerlies retreat to the north. Instead of being dominated by incoming cyclonic storms off the Pacific, the weather in the Southwest and the Rockies is influenced by the clockwise rotation of air around a big area of high pressure parked in the center of the country, often over Texas. The rotation draws moist air northward from the Gulf of Mexico, the Gulf of California and the eastern Pacific.
According to the National Weather Service, the pattern is also driven by an eastward shift of a big high pressure system over the Pacific Ocean, which also helps displace the westerlies that prevail for much of the year and reinforces southerly winds that carry moisture into the desert Southwest and Colorado.