Every year, southern Colorado and the southwest United States experience the monsoon. This means more rain for typically dry areas, but what causes it?
It all comes down to a change in the wind pattern. Off-season, an area of high pressure sits over Mexico, with the jet stream over the central U.S. This keeps moist air to the south and pulls in an overall dry flow to the western U.S.
During monsoon season, the high pressure moves over the south central U.S. and an area of low pressure forms off the California coast. This subtropical high and monsoon low funnel in moisture between them and push it into the southwestern U.S., bringing more frequent storms to these dry areas, along with the risk of flash flooding.