The season within a season, the "Southwest monsoon" is beginning to establish itself across the region. The word "monsoon" is defined as any seasnonal shift in the wind. Over the coming days high pressure centered over the four corners region will gradually transport moisture into Arizona from south.
The moisture source is the Gulf of Mexico and the Gulf of California. You probably have noticed the increase in our mugginess the past couple days as humidity levels are on the rise.
This monsoon is getting started about a week later than normal. Over the coming days chances for wetting rains will increase. NOAA's forecast calls for a slight tilt towards a wetter than average season. Forecast thinking is that it could be particlarly wet later in the season as tropical storms in the Pacific Ocean are enhanced by lingering El Niño conditions.
Monsoon rainfall accounts for nearly half of the annual precipitation received across the region, the other half being winter precipitation.