Climate of California - Precipitation

Precipitation

Westerly winds from the oceans also bring moisture, and the northern parts of the state generally receive higher annual rainfall amounts than the south. California's mountain ranges influence the climate as well: moisture-laden air from the west cools as it ascends the mountains, dropping moisture; some of the rainiest parts of the state are west-facing mountain slopes. Northwestern California has a temperate climate with rainfall of 15 inches (380 mm) to 50 inches (1,300 mm) per year. Some areas of Coast Redwood forest receive over 100 inches (2,500 mm) of precipitation per year.

The Central Valley has a wide range of precipitation. The northern parts of the Central Valley receive substantially greater precipitation from winter storms which sweep down from the Pacific Northwest, while the southernmost regions of the Central Valley are near desert-like because of a lack of precipitation. Parts of the Valley are occasionally filled with thick fog (known locally as "tule fog").

The high mountains, including the Sierra Nevada, the Cascade Range, and the Klamath Mountains, have a mountain climate with snow in winter and mild to moderate heat in summer. Ski resorts at Lake Tahoe, Mammoth Lakes, and Mount Shasta routinely receive over 10 feet (3.0 m) of snow in a season, and some years, substantially more – leading, for example, to annual ski races on the Fourth of July.

On the east side of the mountains is a drier rain shadow. California's desert climate regions lie east of the high Sierra Nevada and Southern California's Transverse Ranges and Peninsular Ranges. The low deserts east of the southern California mountains, including the Imperial and Coachella valleys and the lower Colorado River, are part of the Sonoran Desert, with hot summers and nearly frostless mild winters; the higher elevation deserts of eastern California, including the Mojave Desert, Owens Valley, and the Modoc Plateau, are part of the Great Basin region, with hot summers and cold winters. During the summer months, especially from July through early September, the region is affected by the Mexican Monsoon (also called the "southwest monsoon"), which drives moisture from the tropical Pacific, Gulf of California, and/or Gulf of Mexico into the deserts, setting off brief, but often torrential thunderstorms, particularly over mountainous terrain.

Despite its long coastline, California is not vulnerable to tropical cyclones. Because of the cold California Current from the North Pacific Ocean and the fact that the storms tend to "steer" West, California has only been hit with two tropical storms in recorded history, a storm which came ashore in 1939 and dumped heavy rainfall on the Los Angeles Area and interior deserts and Tropical Storm Nora. The remnants of tropical systems will affect California more commonly, every several years.