Monsoon 2011 is just around the corner

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PHOENIX - At the height of the summer, the North American monsoon impacts about 20 million people in the US and Mexico. As dramatic as it is, it is one of the least-understood weather patterns in the US.  It was it 1949 that our summer thunderstorm season was first described as a monsoon.

Every June, the winter storms, bringing rain and snow become a distant memory. The land has become thirtsy under the glare of the relentless sun.  It is time for the giant heat engine that is the monsoon to get cranked up.

In the upper levels of the atmosphere, a strong high pressure system builds to the east of Arizona. The clockwise rotation of the high brings moisture into the state from the Gulf of California and Gulf of Mexico. The first thunderstorms usually arrive around Independence Day. But by then, the season is already in full force in the mountains.

The rain comes fast and furious, dumped from clouds towering 50,000 feet into the sky. You’d think something as big and powerful as the monsoon would be dependable, but the AZ monsoon can be rather fickle.

Over the past decade, the amount of monsoon rain phoenix received has varied greatly….from nearly 6 inches in 2008 to less than an inch in ’07 and ’09. Last summer, we received slightly less than average rain.

And while the valley gets about 1/3 of its rain from the summer storms, Tucson and Alpine, for example, get more than half of their annual precipitation in the summer while in northern parts of Mexico, the monsoon accounts for 70% of the annual total.

Even with all the variability, the North American monsoon has never been a “no-show,” but in the years where we get little rain the monsoon is sometimes dubbed, “the non-soon.”