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In Arizona the operational criteria for the onset of “monsoon” conditions is a prolonged (three consecutive days or more) period of dew points averaging 55 degrees F or higher. The monsoon, on average, begins around June 15 and ends around September 30.
When dew points are below 50°, the air is generally too dry for widespread heavy downpours. When dew points are at or above 54° and conditions are favorable in the atmosphere for storm development, storms can form and produce heavy, flooding rains.
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