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Water is much needed during summer, especially in the desert Southwest. Arizona and New Mexico are located on the northern fringe of the Mexican monsoon. For most of the year, winds aloft over the southwest U.S. are west to northwest. During the summer, winds turn to a more south to southeast direction, importing moisture from the Pacific Ocean, the Gulf of California, and the Gulf of Mexico.
As this moisture moves into the southwest...a combination of air being forced to rise by the mountains, daytime heating from the sun, and weak upper level disturbances moving across the region causes thunderstorms to develop across the region.
Learn more about our weather in JetStream - An Online School for Weather.