Climate of The Los Angeles Basin

Climate Of The Los Angeles Basin

The Los Angeles Basin is noted for its moderate weather. It is characterized by a Mediterranean climate or seasonal changes in rainfall——dry summer and rainy winter——but relatively modest transitions in temperature. Under the modified Köppen climate classification, the coast is classified as Csb and the inland areas as Csa.

Summers are warm, hot and dry, with an average August high/low at LAX of 76.6 °F (24.8 °C) and 64.2 °F (17.9 °C). The same figures at the University of Southern California downtown campus are 84.8 °F (29.3 °C) and 65.6 °F (18.7 °C). However temperatures sometimes exceed 90 °F (32 °C) during the summer. Winters are mild and somewhat rainy with high temperatures at LAX of 65.5 °F (18.6 °C) and lows of 47.8 °F (8.8 °C). Spring and autumn bring mild days with cool evenings and are generally dry.

The Los Angeles area is also subject to the phenomena typical of a microclimate. As such, the daytime temperatures can vary as much as 36 °F (20 °C) between inland areas such as the San Fernando Valley and San Gabriel Valley versus the coast in the Los Angeles Basin.

Most rainfall occurs during winter and early spring, typical of a Mediterranean climate. February is considered the wettest month. The first rainfall of the water year usually occurs in October or November, with rain becoming more common December through February. The rain events then begin to taper off in March, and April and May usually see only one or two rains. However, the timing and volume of rains in the winter is far from predictable. Most cold fronts and depressions spun off by the strong winter seasonal low-pressure area in the Gulf of Alaska fail to carry significant precipitable moisture as far south as Los Angeles. This is subject to the position and strength of the jet stream and strength of the Gulf of Alaska low, as well as continental airmasses. Some years, however, a well-developed series of cold fronts moves into the area with relatively short separation between each, bringing an intermittently rainy period to Los Angeles for several weeks.

Sometimes rainfall can occur in the summer from monsoon thunderstorms or even from remnants of dissipating eastern Pacific hurricanes, but this is unusual. Los Angeles averages 15.1 inches (384 mm) of precipitation per year, but is lower at the coast and higher at the mountains. Tornadoes are rare, however, weak-to-moderate-strength waterspouts sometimes occur during warm winter storms, although this is unusual. Snow is extremely rare in the Los Angeles basin, but the nearby San Gabriel Mountains typically receive snow every winter season. The greatest snowfall recorded in downtown Los Angeles was 2 inches (5 cm) on January 15, 1932.