Heavy Monsoon in American Southwest

Southwestern Monsoon brings nearly double normal rain
Standing water can bring mosquitoes
B. POOLE Published: 07.31.2008

It has been a wet, blustery monsoon, and that’s just the way a University of Arizona researcher called it in June.

On June 4 – weeks before the rains came – atmospheric sciences professor Christopher Castro called for an early start to the rainy season with more expected than average in June and July.

“Our early June outlook validated very well with respect to all points,” Castro wrote in a Web presentation Wednesday in which weather experts took stock of this year’s monsoon.

The numbers back up Castro’s assessment, said Erik Pytlak, a meteorologist with the National Weather Service in Tucson.

Through Tuesday, every National Weather Service data collection point across southeastern Arizona showed above normal rainfall since June 15, the start of the monsoon, Pytlak said.

“In some cases double normal,” he said.

At Tucson International Airport, where Tucson measurements are taken, 3.58 inches of monsoon had fallen by Tuesday. Normal is 2.07 inches.

It was wetter in other spots, as compared to normal. Picacho Peak has had 2.08 inches of rain (normal there is 0.8 of an inch), and Nogales has had 7.9 inches (normal there is 4.31), according to the weather service.

Though the official start of the monsoon is now June 15 regardless of the dew point, the previous marker of the start, the rains came earlier than normal, Castro said.
“It’s pretty safe to say the onset was in late June,” he said.

Again, the numbers bear that out, Pytlak said.

“We really zoomed above normal (for rainfall) on the 22nd of June. That’s really when the storms started forming in earnest over our mountains,” he said.
Under the old system, the average start of the monsoon – marked by three days of average dew point of 55 degrees or higher – was July 3.

Dew point is the temperature to which air must cool for water vapor to condense into water.

The reason for our steady march of storms up from Mexico lies at the tip of Baja California, where cool ocean temperatures have been causing moisture to flow into the U.S., Pytlak said.

That daily flow of moisture could be interrupted this week because the ocean is warming at the south end of the gulf, he said.

Gregg Garfin, a climate investigator with the Climate Assessment for the Southwest, housed at UA, offered an outlook for the rest of the monsoon, which ends Sept. 30.
Based on historical data, Garfin called for equal chances of above average and below average rain through October and a slight chance of above-average rain in November, December and January.

Castro offered his forecast after analyzing computer models, which scientists are trying to tailor to the tough-to-model conditions of the Southwest monsoon. The models show promise, Castro said.

“I believe we can achieve better seasonal forecasts” within the next five years through the use of the models, he said.

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