We looked at daily maximum and minimum temperatures from 1906-2006 from the Department of Water & Power (DWP) downtown station and a suburban valley location, Pierce College in Woodland Hills, CA. The average annual maximum temperature in Los Angeles has warmed by 5.0 oF (2.8 oC) per century, while the average annual minimum temperature warmed 4.2 oF (2.3 oC) per century. By season, the greatest rate of change was during the summer months for both maximum and minimum temperature, with late fall and early winter having the least rates of change.
Heat wave events in Los Angeles have increased by over 3 events per century and nearly 26 heat days per century. Both have more than tripled over the last 100 years. Furthermore, along with the increase in heat waves there was an increase in heat wave duration. Heat waves lasting longer than six days occurred regularly after the 1970s but were non-existent until 9/24/1956. El Niño years had the highest number of heat wave events and extreme heat days. While heat days have increased dramatically in the last century, extreme cold days, where minimum temperature is below 45 oF (7.2 oC), show a decreasing trend. Pierce College, which is often one of the warmest parts of metro LA in summer, also showed marked increases in heat days, defined as days above 100 oF (38 oC) and decreases in cold days, defined as days below 32 oF (0 oC).
The summer of 2006 experienced an unusually high number of deaths due to an extreme heat event in California. Abnormally high sea surface temperatures (SSTs) off the California coast and an influx of monsoonal moisture combined with record heat causing extreme conditions. The trends in Los Angeles' heat waves indicate that the summer of 2006 may not be a singular event, but possibly more common in the future.