Monsoon season

Start of Arizona’s monsoon will now be determined by calendar
Published on Tuesday, March 11, 2008

PHOENIX (AP) — Arizona’s monsoon, that annual period marked by sometimes violent thunderstorms, powerful downpours and flash floods, will now be more predictable — at least in some ways.The National Weather Service this year is abandoning its decades-old system of using dew points to mark the onset of summer thunderstorms. Instead, the agency will simply set a date to mark the season’s start.

“We want to get the focus away from how we determine when it starts and when it ends and put the focus on awareness that our most violent weather occurs during the season,” said Tony Haffer, meteorologist-in-charge at the Weather Service office in Phoenix.

The Weather Service announced the official span of the rainy season on Monday: June 15 to Sept. 30. On average, the monsoon starts in late June to early July and typically ends in early to mid-September.By setting dates, the Weather Service and public-safety agencies can better prepare people for the risks of summer thunderstorms, Haffer said.

The calendar-based system more closely resembles the widely accepted hurricane season. Haffer said hurricanes can develop outside that season, just as monsoon storms will likely develop outside their new season.The change also reflects advances in weather-forecasting technology. Haffer said the old dew-point system was based on equipment and knowledge available about 50 years ago.

Dew point is a measure of the amount of moisture in the air, specifically the temperature at which water vapor will condense into water.The system also varied around the state. Tucson used a dew-point temperature of 53 degrees to mark the start of the season, adding to the confusion.

The new dates will apply statewide.A monsoon is defined as a shift in the prevailing winds, in this case from the west and northwest to the south and southeast. The shift brings moisture from the Gulf of California and the Gulf of Mexico.In Arizona, the highest rainfall amounts during the monsoon occur in the mountains, and in the southeast. The driest areas are along the Colorado River valley in the far west.

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