This process shifts the middle to upper wind pattern from predominately westerly to an east through south direction. 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. 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.
Bursts can last as long as several days while breaks can last for several weeks.
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. 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. The monsoon, on average, begins around July 7th and ends around September 15th.