In July the subtropical ridge normally migrates northward with its center located over west Texas and New Mexico. This shifts the flow to east to southeasterly in the mid-atmosphere and often brings onshore flow from the Gulf of California, causing a dramatic rise in humidity and thunderstorm activity over northwest Mexico and southern Arizona. The shift begins, on average, around July 6 in southern Arizona — give or take a week. Monsoon conditions then migrate northward over the following weeks.
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