The North American monsoon, the fickle phenomenon that is the summer rainy season in the Southwest, is forecast to be more vigorous than average, with a strong beginning and end. 

During most years, the July through September rainy season forecast for Arizona and New Mexico is no better than a coin flip. But not this summer, when increasing confidence has caused forecasters to paint a more optimistic picture – good news for a region that has been caught in the throes of severe drought for more than 18 months.

"The ecosystem is so tuned up to summer moisture that an early, consistent monsoon can stimulate a robust growing season and provide short-term drought relief," said Mike Crimmins, a climate extension specialist at the University of Arizona.

Crimmins, along with other UA and national experts, share the methodologies and challenges in forecasting the upcoming monsoon season. 

Forecasting challenges

Forecasting the monsoon is no easy task. Experts at the National Oceanic and Atmospheric Administration's Climate Prediction Center, known as the CPC, mine 41 different analysis tools, from global climate models that incorporate atmospheric physics to historical relationships between rainfall and the state of El Niño-Southern Oscillation, a natural force that influences climate and weather around the globe.

In May, conflicting evidence in many of these tools created doubt about the strength and onset of the 2012 summer rains, resulting in an "equal chances" forecast that the monsoon would be above, below or near average.

"I tried hard to put something on the map because we know most people think [equal chances] is a non-forecast," said Jon Gottschalck, head of forecast operations at the CPC. "When I was making [the forecast in May], the signals were all over the place."

Uncertainty in the monsoon is the norm. The CPC has stamped an equal chances forecast on the Southwest in 12 of the last 17 years. Part of the forecasting challenge lies in geography: Arizona and New Mexico sit on the northern fringes of the core North American monsoon region, which is centered over the Sierra Madre Occidental in northwest Mexico. As a result, many climate factors come into play and cause high year-to-year and month-to-month variability. 

"Sea surface temperatures in the Pacific and Atlantic oceans, snow cover in the Rocky Mountains, the state of El Niño-Southern Oscillation, dry conditions in the Midwest and tropical storm activity have all been stated to influence the monsoon during different times and places of the season," Crimmins said.