A Gulf of California moisture surge, or simply gulf surge, is a meteorological event where a pulse of high humidity air is pushed up the Gulf of California. Gulf surges bring moisture to southern Arizona during the North American Monsoon. Prior to the 1970s, the consensus of meteorologists was the moisture that fueled the central and southern Arizona monsoon resulted from the movement of the Bermuda High to a more south and west position, which in turn transported water vapor to the region from the Gulf of Mexico. However, operational meteorologists in the 1970s described episodic surges of moisture that infiltrated the area that was thought to originate in the Gulf of California. It was noted that these episodes were likely to be associated with a convective system near the tip of the Baja peninsula such as a tropical cyclone or an easterly wave. North American monsoon The North American Monsoon is experienced as a seasonal reversal of the prevailing winds, which is usually accompanied by an increase in rainfall. Onset is usually in early July when the winds start to shift due to intense solar heating of the Southwest United States. During the winter months, the weather patterns in the Southwest United States are characterized by a semi-permanent high-pressure system with quasi-weekly weather systems moving through the area; a cold front will move through the area, followed by a gradual building of the ridge. During the monsoon months, the subtropical ridge moves northward due to the development of a thermal low from the intense solar radiation. The low develops over the Mexican Plateau and gradually moves northward towards the four corners region. Rains from the monsoon typically start in May or June along the western slope of the Sierra Madre Occidental and move northward, reaching southern Arizona sometime in July. The North American Monsoon is not as strong or persistent as its Indian counterpart, mainly because the Mexican Plateau is not as high or as large as the Tibetan Plateau in Asia. Dynamics There have been several proposed mechanisms for the development of gulf surges including gravity currents, ageostrophic flows, Kelvin waves, or Rossby waves. However, due to a lack of observations in the area, the exact cause is uncertain. The best data currently available that indicates moisture arriving from the gulf is from the NEXRAD radar in Yuma, Arizona. This instrument has the capability to measure wind speed