Experts Still Unsure When Monsoons Will Begin


Parts of the Rim Country enjoyed a sample of the monsoon season last week. Whether or not it will be officially be declared "monsoon season," is anyone's guess, according to the University of Arizona Cooperative Extension Climate Science Applications Program.

The Arizona Cooperative Extension Service brochure, "Arizona and the North American Monsoon System" details the complex alignment of climate factors that create the healthy rains the state generally gets between July and August.


A dark cloud hung over Rumsey Park Wednesday evening, but never dropped any rain.

"The mechanism that produces summer precipitation is not associated with large-scale jet streams or strong low pressure systems, but from convective thunderstorms that arise through the combination of solar heating and moisture ... A subtle change in circulation patterns during the summer opens up a flow of moisture from the south that dramatically increases convective thunderstorm activity across the state. That subtle change in circulation patterns is the North American Monsoon," the brochure reads.

The moisture that comes into the state is primarily from the tropical Pacific Ocean up through western Mexico.

On occasion, moisture is also brought in from the Gulf of Mexico, if it is at elevations of more than 10, 000 feet.

A high-pressure system set up over the Four Corners area is the magnet pulling the atmospheric circulation.

It's the mechanism that can also limit the production of the monsoon season, depending upon its strength and position.

With a favorable position and strength, it can pull moisture into the state with an east-to-southeast flow.

If the high-pressure system is too far west, the winds have a northerly flow, making them dry.

During a "good" monsoon season, a large portion of the state can get as much as half its annual rainfall, according to extension service information.

While the rain is welcome, it can be so heavy in such a short period of time (often spanning less than an hour) that flash flooding becomes a serious threat.

Lightning is also a problem, possibly resulting in forest fires and often in power outages.

"During the monsoons, almost all our power outages are due to lightning," said Jan Parsons, manager for the Payson district of APS. She said it was lightning that caused the outage Monday afternoon, which left nearly 3,000 residents in the Rim Country without power for four hours.

The sometimes torrential rains of the monsoon season can also interfere with driving --causing sight distance limitations, rockslides and in the lower elevations, hazardous dust storms.

Staying safe while traveling

The Arizona Department of Transportation offers the following information on staying safe while traveling during a monsoon storm:

  • Pull off the roadway
  • Stop
  • Turn off all vehicle lights
  • Set your emergency brake and take foot off the brake
  • Buckle up

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