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The 2013 Monsoon: Waiting on the Rain

By Stephanie Doster | June 20, 2012 

The timing and strength of the 2013 monsoon is proving difficult to predict, but medium-range weather forecasts are suggesting a more or less on-time and wetter-than-average start to the Southwest’s summer rainy season.

The National Weather Service (NWS) is forecasting equal chances for a dry, average, or wet summer through September, but outlooks for June 28 to July 4 suggest the rains will start on the early side of average and will be wetter-than-average, John J. Brost, science and operations officer with the NWS Tucson office, said during a monsoon briefing Thursday.

A June 20 online NWS discussion forecast hot and dry conditions into next week, followed by a chance of thunderstorms later during the week of June 24.

The monsoon officially begins June 15 and ends September 30. The average onset date is July 3 in Tucson and July 7 in Phoenix. The earlier the rains begin, the more likely the region is to log average or above-average rainfall totals.

“Nine of the last 12 monsoons have been below average and drought conditions are intense and widespread in Arizona and New Mexico,” said Zack Guido, associate staff scientist for the Climate Assessment for the Southwest (CLIMAS). “An earlier onset tips the odds for the Southwest to experience an average or above-average monsoon.”

Forecasting the monsoon is tricky. Experts analyze large-scale circulation patterns that involve sea surface temperatures and ocean currents, study analogous patterns from the past, and use computer models to issue outlooks.

“There is no consensus among all of these toward a really dry or really wet monsoon this year. It’s a mixed signal,” Brost said, explaining the equal chances forecast through September. “But hot is a good bet.”

The rain that fell in and around Tucson on June 15 likely was simply a precursor to the monsoon, Brost said.

The El Niño-Southern Oscillation, a natural see-saw in sea surface temperatures and surface air pressure between the east and west tropical Pacific Ocean, and Pacific Decadal Variability (or Oscillation), a pattern of ocean temperature anomalies in the northeast and tropical Pacific, are two sources of large-scale climate variability that influence the onset of the North American monsoon and early monsoon precipitation, said Chris Castro, associate professor in the UA’s atmospheric sciences department. 

“Right now, a developing La Niña [which can boost summer precipitation] and enhanced tropical convection in the western Pacific suggest an early onset and normal to wetter-than-normal conditions through mid- to late July,” he said. “The situation is similar to last year, when the first monsoon rains in Tucson occurred in late June.”

Between June 15 and September 30, 2012, Yuma received 2.25 inches of rain—0.96 inches more than average. Rainfall at the Tucson and Phoenix airports totaled 6.02 inches and 3.00 inches, respectively. The average rainfall for those sites during that time period is 6.08 inches in Tucson—about half of the city’s annual rainfall—and 2.71 inches in Phoenix, according to the NWS.

The briefing was sponsored by CLIMAS.

During the briefing, Mike Crimmins, associate professor and extension specialist with the UA’s department of soil, water and environmental science and Arizona Cooperative Extension, discussed how different aspects of the monsoon—the onset date, temperature, intensity, and other variables—affect different stakeholders in different ways.

Crimmins and Guido have launched a project to more specifically characterize the monsoon so they can discuss it in more detail and link it to impacts such as wildfire and flooding that are important to stakeholders.  

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