The Desert Southwest of the United States, centred upon and surrounding Arizona, is aptly named, comprising three named deserts:

  • The northern Sonoran, wrapping around the northern Bay of California, with its trademark cartoon-favourite cactus, the saguaro;
  • the Mojave desert of southern California which includes Death Valley, the site of the highest reliably recorded temperature on earth;
  • and the northern Chihuahuan, spreading from Mexico and including the famous town of Roswell, New Mexico, the spiritual home of UFO folklore

Like all deserts, it is not an entirely dry place. In July and August, rain often falls. In fact, this is the area of the North American Monsoon. It is not as strong or persistent as its Indian counterpart but it shares most of the basic characteristics.

The wind direction changes to bring moisture from the Gulf of California into Sonora and Arizona. Upper-level moisture is also transported into the region, mainly from the Gulf of Mexico by easterly winds aloft. The following rains often occur as thunderstorms and, as in the South Asian Monsoon, come in bursts.

Half of the annual rains in this part of the US come from this monsoon season and it has just started in earnest. On Friday, August 5, the Phoenix area was hit by thunderstorms and flash flooding, causing power outages in northern Phoenix, and several areas along Interstate 17 were briefly closed by flooding. Nearly 70mm of rain fell in Scottsdale in just three hours.

This week, Tropical Storm Javier, currently travelling up the Pacific coast of Baja California, will inject an unusually large amount of moisture into the atmosphere. The possible consequent rainfall in Arizona could well be at record levels.

Phoenix has an average August rainfall catch of 25mm, similar to July - these are the two “wettest” months for Arizona’s capital city. This week could generate storms bringing twice the monthly average in just a few hours.

Source: Al Jazeera