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There is a transition from generally dry conditions during the spring and early summer associated with the prevailing mid-latitude westerlies, to deep southeasterly or southerly winds that transport moisture from the Gulf of Mexico and the tropical eastern North Pacific. The change in air mass provides conditions favorable for the formation of deep convection that is typical of the summer in the southwest US. This phenomenon is referred to as the North American Monsoon.

During the winter, spring and early summer, the upper level winds are typically westerly over much of North America. Beginning in late June or early July, the winds shift and the western edge of the Bermuda High transports a deep moist air mass into northwest Mexico and the southwest US.

There is additional moisture transport near the surface from a thermal low or heat low that forms over the Colorado River Valley between Arizona and California.

There is also moisture transport into the low deserts from occasional Gulf Surges. Together, these features provided a favorable environment for thunderstorm formation in the desert areas of Mexico and the southwest US. A substantial fraction of the annual rainfall in these regions comes from the monsoon thunderstorms.

Additional Information on the monsoon available from the National Weather Service in Tucson, AZ

Mountain (orographic) thunderstorms The major mountain ranges in central Arizona (the Mogollon Rim) and the so called "Sky Islands" (Santa Catalina, Santa Rita and Rincon mountains) near Tucson serve as the initial locations for cloud and thunderstorm formation in Arizona.

The typical scenario over the sky islands involves convection beginning in clear air in the early to mid- morning. As the sun rises, it strikes the sides of the mountains more directly than the surrounding area and this provides a localized surface sensible heating.

Evaporation from the previous day's rain and evapotranspiration from the plants contribute to the flux of latent heat that also destabilizes the boundary layer over the mountains. On a typical day, the clouds begin to build over the Santa Catalinas at about 9am local time.

A measure of the amount of energy present in the environment for cloud and thunderstorm formation is called Convective Available Potential Energy or CAPE. CAPE is a measure of the temperature difference between a rising air and the environment into which the air is rising (vertically integrated positive buoyancy).

Soundings in southern Arizona typically have about 1500-2000 Joules/Kg of CAPE, which classifies them in the moderately unstable range. The area doesn't experience the severe, tornado producing supercell thunderstorms common in the Great Plains, but the monsoon thunderstorms can produce localized high winds and heavy rains.

In spite of there being sufficient CAPE to generate thunderstorms, the clouds develop slowly and in stages. Shallow convection in the form of convective plumes or turrets build, first into cumulus congestus and then, in many cases, into deep cumulonimbus. In general, the transition from the initial stages of convection to the appearance of a thunderstorm over the Catalinas is about 3 hours.

In many cases, the orographic thunderstorm dissipates or moves from the top of the mountain and a second episode of convection begins, with a transition from shallow to deep convection occurring on a similar time scale.

An example of one of these events is given in the Background and Goals section of this site.