What is in this article?:
- 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.
Monsoon by month
The strings guiding Douglass' forecast are tethered to conditions in the Midwest and Pacific Ocean. Research in the late 1980s, for example, found a strong correlation between dry conditions in the Midwest, centered over Iowa, and wet weather over the Southwest and northern Mexico during the initial weeks of the summer rainy season.
"If you're interested in what's going on in the southwestern U.S., you also better be interested in what's going on in the Midwest," Douglass said. "And if you start looking at lags, it's the Midwest that seems to be behaving first."
The Midwest has been dry for the past three months, with many parts of the region receiving less than 70 percent of average. The sea surface temperatures in the tropical Pacific Ocean have been recently warming and also favor an early and wet onset.
"This year, sea surface temperatures look nearly identical to last year," said Christopher Castro, an assistant professor of atmospheric sciences at the UA. "This would suggest an early to average start with average to above-average precipitation from late June to early July."
El Niño also figures into the mix. Sea surface temperatures are currently near average, but many El Niño-Southern Oscillation forecasts project El Niño will develop by late summer.
During El Niño summers, conditions in August tend to be dry, Douglass said. Generally, this occurs because the subtropical jet intensifies as sea surface temperatures warm. This, in turn, pulls the monsoon high south, exposing southern Arizona and New Mexico to the dry northern side of this high.
As for September, El Niño conditions tend to increase the frequency of tropical Pacific Ocean storms, which can squeeze moist air from the Gulf of California into the Southwest. The intensifying subtropical jet and waning solar radiation also creates a conduit that helps steer storms into the region.
But there are no guarantees that El Niño will evolve in this manner, Douglass said. It's a forecast based solely on historical data.
Nonetheless, after two consecutive dry winters that sandwiched a lackluster monsoon for many parts of the Southwest, optimism is a welcome guest. If the forecasts prove accurate, the monsoon will help squelch dry conditions that have been plaguing the region.
The recent thunderstorms in southern Arizona on June 16 had a monsoon flavor, suggesting an early and strong beginning. The storms could also be a false start, as they sometimes have been in the past, proving again that the monsoon is a fickle phenomenon.