Tucson's Fifth Season, The Summer Monsoon

So what factors lead to this wind shift? Arizona's climate is dominated by westerly winds for most of the year. During late spring and early summer, the subtropical high pressure ridge (the Bermuda high) expands west and north across North America pushing the westerlies north. This process shifts the middle to upper wind pattern from predominately westerly to an east through south direction. At the surface, intense daytime heating of the desert creates rising air and surface low pressure (a thermal low) across northern Baja and the southwestern deserts. The two features combine to transport moisture northward from the eastern tropical Pacific, western Mexico and the Gulf of California. However, some speculation remains about the source of this moisture.

This "source of moisture" topic has been debated for years with early theory indicating the primary moisture source being the Gulf of Mexico. It was hypothesized in the 1950s and 1960s that the Gulf of Mexico moisture was advected across Mexico into the southwestern United States via the easterly middle to high level winds (Bryson and Lowry 1955). In the 1970s, researchers identified the Gulf of California as a role player in advecting moisture (via gulf surges) north across the deserts of the southwest United States (Hales 1972, 1974). Since the early 1990s, the research project SWAMP (SW Arizona Monsoon Project) has provided considerable evidence showing the Gulf of California is the moisture source of Arizona's summer thunderstorms.

Monsoon thunderstorms are convective in nature and form as intense surface heating is combined with sufficient moisture. This doesn't mean that thunderstorms occur every day in Tucson. There are peaks and lulls during the summer monsoon which have been dubbed as bursts and breaks (Carleton 1986). Bursts can last as long as several days while breaks can last for several weeks. Certain synoptic patterns are associated with breaks and bursts (Carleton 1986, Maddox et al. 1995).
Typically in Tucson, thunderstorms will develop over the mountains surrounding the metro area early in the afternoon and move across the city later in the afternoon. The monsoon thunderstorms generally travel from the east-southeast to the west-northwest associated with the mean middle and upper wind flow pattern.

On occasions, a line of thunderstorms will move off the Mogollon Rim and move south-southwest across the Tucson metro area during the late night and/or the early morning hours. Summer monsoon thunderstorms can pack a wallop with very strong gusty winds, heavy downpours, hail, blowing dust and dangerous lightning. Flash flooding associated with intense thunderstorms is fairly common across the Tucson metro area as dry washes tend to fill up quickly.
The monsoon season usually ends abruptly when the middle to upper westerly wind pattern starts to move south, surface pressures rise over the southwest deserts, thus decreasing the northward advection of monsoon moisture from Mexico.
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 of higher. The monsoon, on average, begins around July 7th and ends around September 15th.