To begin the North American monsoon, there needs to be two key features in place in the atmosphere.
The first is the Monsoon High.
As spring turns to summer an area of high pressure forms over the hot, dry Mexican Plateau, in northern Mexico.
Over the course of June this high pressure migrates north, and by July is ideally situated over the Four Corners area of the United States.
Clockwise circulation of wind around the center of the high assists in drawing in moisture from the Gulf of Mexico.
Since this moisture has to make its way over the Continental Divide it is mostly limited to the upper levels of the atmosphere, usually forming the first monsoon clouds in our skies.
At this point, there isn't much moisture near the surface of the Earth.
Much of the rain that falls from these first clouds, evaporates, creating cool, gusty winds and dry lightning.
The second feature stays a little closer to home.
The heat of summer in the Mojave Desert creates an area of low pressure with winds circulating counter-clockwise and inward towards the center of the low.
The low pressure draws tropical moisture northward from the Gulf of California and out of the East Pacific.
Once this moisture reaches land, it spreads into southeast Arizona.
This is the source of the much needed low level moisture, which is key for the rain to start soaking the ground.
Each of these features do migrate, weaken at times, and strengthen at times during the monsoon.
In fact, as the monsoon progresses the high pressure over the Four Corners gets absorbed into the Bermuda High.
This is a large area of high pressure that expands during the summer months to cover most of the eastern United States.
The center is usually near or directly over Bermuda in the Atlantic off the east coast of the U.S.
The placement and strength of all of these features on any given day is a key factor in our day to day forecasts for monsoonal thunderstorms.
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