Mount Diablo - Climate

Climate

The National Weather Service maintains a weather station at Mount Diablo Junction, 2,170 feet (661 m) above sea level. The warmest month at the station is July with an average high of 85.2 °F (29.5 °C) and an average low of 59.6 °F (15.3 °C). The coolest month is January with an average high of 55.6 °F (13.1 °C) and an average low of 39.3° (4.1 °C). The highest temperature recorded there was 111 °F (43.9 °C) on July 15, 1972. The lowest temperature on record was 14 °F (-10 °C) on February 6, 1989, and on December 14, 1990. (The San Francisco Chronicle reported that the temperature dropped to 10 °F (-12.2 °C) at the summit on January 21, 1962.) Temperatures reach 90 °F (32.2 °C) or higher on an average of 36.0 days each year and 100 °F (37.8 °C) or higher on 3.3 days each year. Lows of 32 °F (0 °C) or lower occur on an average of 15.4 days annually.

Annual precipitation averages 23.96 inches (609 mm). The most precipitation recorded in a month was 13.54 inches (344 mm) in February 1998. The greatest 24-hour precipitation was 5.02 inches (128 mm) on January 21, 1972. The average annual days with measurable precipitation is 65.3 days.

Snowfall at Mount Diablo Junction averages 1.2 inches (30 mm) each year. Prior to 2009, the most snowfall observed in a month was 17.0 inches (430 mm) in April 1975; that same month saw 6.0 inches (150 mm) in one day (April 4, 1975). The greatest snow depth was 3.0 inches (76 mm) on January 27, 1972. Measurable snowfall does not occur every year, so the annual average days with measurable snowfall is only .5 days. Snow is more common in the upper reaches of the mountain. On December 7, 2009 Mount Diablo received a rare snow event of 18.0 inches (460 mm), receiving more in one day than what it normally receives in one year.