Southern California is known for its sunny, warm summer beach weather, which draws millions to its sands adding to the region's economy. For the last 4 years, beachcombers in summer bring their sweaters and summer surfers keep their wetsuits on under gray skies. What causes these wild swings in coastal weather and can June gloom be forecasted? This study looks at the factors that are responsible for the variability in coastal temperatures and fog frequencies along the southern California coast.

The obvious factor is the Pacific, which directly impacts coastal temperatures and moisture. While several studies have shown relationships between SSTs in the tropics and U.S. weather, several other oceanic measurements and indices show even stronger connections to southern California temperatures. Using over 50 years of data, 1948-2001, the monthly Pacific Decadal Oscillation (PDO) cycles show up prominently in coastal temperatures and heavy fog frequencies. Other useful indices showing strong correlations with summer coastal temperatures include: Jan-Mar. Pacific North American Index (PNA), spring seasonal North Pacific Index (NP) values, and Apr-Jun Pacific Warm Pool Index values. Heavy fog frequencies in the Los Angeles coastal plain, since 1950, are also influenced by strong ENSO events and PDO cycles. However, it was also found that heavy fog has significantly declined over this period. Here, urban influences are also important, as the Los Angeles downtown temperatures have increased by approximately 5o F over this period. At the same time, particulate air pollution has declined by about half. Future urban growth, global warming and the PDO cycle may have significant impacts on coastal weather in southern California.