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In this study, the seasonal development of the North American monsoon system (NAMS), as simulated by a mesoscale model during a 22-yr simulation from 1980 through 2001, is assessed. Comparison between model simulations and observations shows that the model simulation reproduces the precipitation, skin temperature, and wind field patterns in the seasonal development (May-July) of the NAMS reasonably well and that the mesoscale features and spatial heterogeneity of the NAMS are described correctly. The onset of the monsoon in the central and southern Sierra Madre Occidental (SMO) in Mexico occurs on 20 June, about 2 weeks earlier than the onset in Sonora, Mexico (6 July), the Sonoran Desert, and central Arizona and New Mexico ( 8 July). The temperature in Mexico is highest after the onset of the monsoon and then decreases with the increasing monsoon rainfall. However, the temperature in the Sonoran Desert and central Arizona and New Mexico is highest just prior to the onset of the monsoon, and high temperatures may then persist throughout July. The lower-level (700 hPa) zonal wind field reverses from westerly to easterly over the central and southern SMO just before the onset of rain in these regions; this is associated with the abrupt northward movement of the subtropical high over this region. The progression of the subtropical high into central Arizona and New Mexico results in a local reduction in the westerly flow, and although the southwesterly flow weakens, atmospheric moisture is still mainly from the Gulf of California and the eastern Pacific Ocean.