Why Is a Monsoon Flooding Phoenix?

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Monsoon is a word that we usually associate with torrential rain battering the coastlines and islands of Asian countries. So it’s probably strange to hear it being used to describe the storms that currently are inundating Phoenix, Ariz., which is situated inland in one of driest states in the United States.

Only about eight inches of rain typically falls upon Phoenix in the course of the year, and as this chart shows, in recent years it’s been getting even more parched. So why is the Phoenix area suddenly threatened by flooding that’s closed highways, forced dramatic evacuations and washed away at least two homes?

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As it turns out, even though the area is bone dry for most of the year, the sudden rain isn’t a freak weather event. According to the Arizona State University’s School of Geographical Sciences and Urban Planning, the Arizona monsoon is a well-documented meteorological event that occurs during the summer, and is caused by a seasonal change in wind patterns.

Here’s how it works. During the winter, winds primarily blow toward Arizona from California and Nevada to the west and northwest. After summer begins, however, the winds shift, and moisture suddenly streams into Arizona, probably from the Gulf of Mexico and the Gulf of California. At the same time, the summer sun heats the desert around Phoenix intensely, lowering surface air pressure and causing rising air masses. The combination of those factors is perfect for creating thunderstorms that appear during what meteorologists call “bursts” and “breaks” in the weather system.

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Phoenix, incidentally, isn’t the only desert area that has monsoons. India’s more famous monsoons are caused by a similar weather process involving the country’s enormous Rajasthan Desert in the west. The word monsoon actually is derived from the Arabic word mausim, which means “season” or “wind-shift.”

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