Humid, overcast days are a reminder that the Valley is already a month into the monsoon season.

But shouldn't the area have seen more activity by now?

Arizona and Mexico can receive up to half of their annual rainfall during intense, localized storms brought by the wind and pressure shifts that produce monsoons.

Since 2008, the start of the season has been set to June 15. That date corresponds to the onset of monsoon conditions in Mexico, not Arizona, said Ken Waters with the National Weather Service in Phoenix.

"We typically start seeing at least some influence of the monsoon usually about the first week of July. It's actually running a little behind schedule," Waters said. 

Waters said the service adopted fixed dates because the science behind using dew point was weak, and because the service wanted to focus more on monsoon threats and potential impacts.