During La Niña winter months (see La Niña story in Weather 101), Arizona usually has less precipitation than would normally be expected.
Those relatively dry winters bring into focus a weather phenomenon known as the Madden-Julian Oscillation because it's typically the only thing that brings rain into Arizona during La Niña's winter grip.
Scientists, Drs. Roland Madden and Paul Julian, discovered the pattern that oscillates, or develops, moves and falls apart, in 30-90 day cycles.
The wind along the equator blows from east to west.
Figure 1: Wind blows from east to west along the equator. Here: the eastern Pacific.
Figure 2: Wind blows from east to west along the equator. Here: the western Pacific.
Within that flow, especially in the very warm Indian Ocean region, clusters of thunderstorms form.
Figure 3: Clusters of thunderstorms form over the Indian Ocean.
These storms then fall apart and redevelop EAST of where they first formed. This is in essence against the wind flow.
Figure 4: The cluster of storms redevelops east of its original position.
Figure 5: (Showing progress from Figure 4.)The cluster of storms redevelops east of its original position.
This continues until the clusters of storms get in a position where the sub-tropical jet stream (steering wheel for storms) can tap into their moisture and send it into the southwestern United States.
Figure 6: The sub-tropical jet stream taps into Madden-Julian moisture and brings it into the southwestern United States.
7831 N. Business Park Drive