The summer Southwest Monsoon begins in Mexico as a dome of high pressure builds over that country; usually developing by June, the clockwise circulation around this dome brings in moisture from the Gulf of Mexico and the Gulf of California, producing intermittent heavy rains, especially across the central ranges. By early-mid July, this high pressure moves northward, into the southern Plains of the U.S.; reinforced by a heat-induced low over the Mojave and Sonoran Deserts, the moisture flow pushes into southeastern California, Arizona and New Mexico. By mid-late July, this monsoon moisture extends northward into Utah and Colorado; any instability related to fronts or upper air disturbances will trigger intense thunderstorms with torrential rain. The monsoon season generally lasts through August and may persist into September.
Monsoons, though generally thought of as heavy rains, are actually the changing wind patterns which produce conditions as described above; in fact, there are seasonal monsoons across the globe that, depending upon wind direction and topography, induce wet or dry seasons. The rainy summer season in America's Southwest is vital to the ecology of our desert and semiarid regions; without these seasonal rains, most of the Southwest would resemble the Sahara.
The summer monsoon reached Denver yesterday afternoon in spectacular fashion. Lines of thunderstorms, characterized by intense lightening and torrential downpours, crossed the Metro Area over a period of three hours. Almost all areas received at least 2 inches of rain, while the southern and western suburbs were deluged with 4 inches. More rain was forecast for this afternoon and similar events will occur sporadically into early September.