After a wet El Nino winter and spring, Arizona is drying out again. Monsoon rainfall for Arizona has been dismal so far and it’s looking dry as we wrap up August too. According to the National Weather Service, El Nino is officially over.
Arizona and New Mexico typically receive up to half of their annual rainfall during the summer monsoon season. The monsoon season suppresses much of the hot summer temperatures, replenishes water resources, and nourishes the vegetation. The monsoon arrives with much flash and fanfare, with its trademark thunderstorms and flooded streets in southwestern cities. Monsoon rainfall events tend to be short and spotty, with intense, local storms drenching some neighborhoods but not others. The water the storms bring quickly flows off the landscape into streets and rivers, with much of the remnant moisture evaporating in the summer sun. But this season, it apparently came and went with little activity.
The monsoon is driven by the sun heating up the land and the Pacific Ocean at different rates, with land surfaces warming more quickly than the ocean. The warm land creates low-pressure zones as hot air rises. Once this pattern establishes across the region, the winds shift to fill in the vacuum. With this shift in the winds, the monsoon begins in northern Mexico in May. The moisture-laden monsoon air travels north to Arizona and New Mexico, encouraged by the pressure difference between the hot, parched southwestern air and the cooler Mexican air. In 2008, National Weather Service officials decided to consider June 15–September 30 as the U.S. Southwest monsoon season in Arizona, although the thunderstorms that bring the rain may form in different times and places across the region.
Heading into winter, there is a slight chance El Nino could redevelop, but more likely we’ll stay in neutral conditions, which means we once again do not have an clear indicator as to how our winter weather will play out.