Sept. 30, 2004
Arizona Daily Star
The 2004 monsoon officially ended Thursday as Tucson’s fourth driest since 1895.
A paltry 2.43 inches of rain fell at the city’s official measuring site at Tucson International Airport, where the 30-year average is 6.06 inches.
Tucson’s monsoon, marked by a wind shift that taps moisture south and east of here, ends Sept. 30 and starts, on average, July 3. This year it began July 8, the first of three consecutive days when the dew point — a measure of humidity — averaged 54 degrees or above.
When the monsoon is clicking, a high pressure system sets up around the Four Corners and its clockwise rotation pulls moisture in from the Gulf of Mexico. But this summer the high was positioned farther south than normal, said Jeff Davis, a National Weather Service meteorologist.
“That kept normal and above-normal precipitation down toward the international border and on into Mexico,” Davis said. “We’re on the northern fringes of the Mexican monsoon, so we see fluctuations from year to year.”
The airport, located near the center of the Tucson basin, often receives less rain than other parts of the metro area, especially those at higher elevations. The monsoon is also inherently spotty since the heaviest rain typically falls from scattered thunderstorms that can flood one neighborhood as the sun shines in the next.
“Just looking at one site doesn’t necessarily give you the whole picture,” Davis said.
Most of Southern Arizona also had below-average rainfall this summer, though both Safford and Clifton had wetter-than-normal monsoons.
The monsoon usually accounts for about half of Tucson’s average annual rainfall of 12.17 inches. But experts consider winter precipitation more vital to the region’s water supply since it is snowmelt that primarily fills the Southwest’s rivers, reservoirs and underground aquifers.
In Tucson, eight of the past 10 years have seen below-normal rainfall and 2004 seems destined to continue the trend. Since Jan. 1, the airport has received 5.85 inches, less than two-thirds of the 30-year average of 9.26 inches.
Onset of a weak El Niño pattern — marked by warmer sea surface temperatures in the Pacific — has forecasters predicting this winter will be wetter than normal in the Southwest.
But the region’s current drought, described by some climate experts as the worst since the 16th century, is expected to persist through the end of the year, according to the federal Climate Prediction Center.
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