Joshua Tree National Park - Rainshadow

The area of southeastern California is a rain shadow desert. The rain shadow effect is produced by the high mountains on the west, which block the movement of wet winter storms.

Coastal storms moving east collide with Mount San Jacinto (10,804 ft.) and Mount San Gorgonio (11,502 ft.) dropping most of their moisture on the west sides of these mountains. Land on the east side receives much less rain, which results in a desert environment. During late August or September occasional tropical storms move into southern California from the south. These storms end up on the east side of the Peninsular Ranges and can dump a considerable amount of water in a short time. Some five to 10 inches of rain may fall in a few hours, representing a large portion of Joshua Tree’s annual precipitation.