Desert awaits annual rainfall

Robert Henry Becker Arizona Summer Wildcat Monday's afternoon downpouring of rain flooded Sixth St. at Santa Rita Ave., slowing traffic, and made crossing the street dangerous for pedestrians and bicyclists. The monsoon season starts when the wind pulls moist air from the ocean which is then coupl ed with the hot desert air.

This week's rise in temperature and humidity are a signal for Tucsonans to carry an umbrella and await the inevitable monsoon season.

The National Weather Service is expecting the monsoon season to be in full bloom by the end of the week, with a chance of thunderstorms and highs around 100 degrees Fahrenheit.

A monsoon is a seasonal reversal in the prevailing wind direction, said Steven Mullen, associate professor of atmospheric sciences.

The directional change occurs twice a year, typically in the winter and summer, Mullen said.

In the winter, the prevailing winds blow across the land and continue off shore. In the summer, the wind changes direction and transports moisture inland, he said.

Mullen said the summer winds originate from the south/southeast, drawing moisture from the tropics, specifically from the Gulf of California.

These winds, he said, can be excessive in force, some as strong as hurricane winds.

Some, but not all, meteorologists are predicting that Arizona can expect a heavier monsoon season this summer based on the El Niño off the west coast of Baja, Calif.

El Niño, the process of the warming of sea surface temperatures, is believed by some scientists to influence weather patterns in the Southwest. It is monitored by satellite-derived data and sensors located on buoys.

Jim Meyer, meteorologist for the National Weather Service, said it is possible the monsoon will be more severe this year since the El Niño is warmer than it was this time last year.

The National Oceanic and Atmospheric Administration records show sea surface temperatures, off the west coast between April and June of 1996, were -0.3 degrees Celsius to -0.5 degrees Celsius. These temperatures were below normal.

The NOAA reports 1997's sea surface temperatures spanning the same months and location were above normal, between +1 and +2 degrees Celsius.

But Stephen Mullen, UA associate professor of atmospheric sciences, said the correlation between the El Niño and the monsoon season is weak because it only explains 10 percent to15 percent of the precipitation variance.

"The El Niño is not well understood physically," he said.

Meyer and Mullen both said projections for the monsoon season, which typically starts in the middle of July, can be better made once it is underway.

"It's my opinion that projecting summer rainfall three months in advance allows for little accuracy," Mullen said.

He said weekly weather predictions can be more accurate.

"We do have short range skills," he said.

If Arizona does see a heavier monsoon season, Mullen said, Tucsonans can expect higher humidity and lower temperatures than average because of cloud cover.

"People with swamp coolers won't be happy," he said.

Mullen said that, along with the monsoon, Tucsonans can also expect lightening-triggered fires, which are a problem each storm season.

A spokesperson for Coronado National Forest said 60 to 100 lightening-triggered fires occur per year in the mountain area surrounding Tucson.

"We've already had lightening fires off and on through this spring," said Rich Kvaly, fire and aviation staff officer for Coronado National Forest.

Kvaly said these are due to the unusually strong storms Tucson experienced earlier this spring.

The severity of the fire season depends on each fire's resistance to control, he said.

To prevent storm-related fires, Kvaly said that as of 8 a.m. yesterday, all forests began enforcing fire restrictions.

The restrictions limit campfires to only developed campground areas with grills. Visitors can smoke only in clear areas, buildings or their vehicles.

Kvaly said these restrictions are enforced every year and usually are lifted once the mountain tops accumulate about one inch of rainfall. This allows the ground to become sufficiently wet and decreases the danger.