[3] Late summer and early autumn are typically the hottest times of the year with an average high of 78 °F (26 °C) in August and 77 °F (25 °C) in September.
San Diego experiences marine layer clouds, most often between May and August, which cause cool temperatures, cloudy weather and fog in the morning.
The local sayings "May gray" and "June gloom" refer to the way in which San Diego has the most trouble shaking off the early morning fog during those months, and cool, cloudy conditions often last into the afternoon or even all day.
This marks October as fire season for San Diego and the rest of Southern California due to low precipitation and sporadic heat waves that the average monthly temperatures do not immediately show.
Rainfall is strongly concentrated in the cooler half of the year, particularly the months December through March, although precipitation is lower than any other part of the U.S. west coast.
[11] Snow flurries were last seen in San Diego on February 14, 2008 around 1,700 to 1,800 feet (520 to 550 m), and the last measurable snowfall to hit various neighborhoods and suburbs around the city fell on December 13, 1967.
In winter, light snow is common in mountainous regions of east and north San Diego County (including Ramona and Julian) above 3,000–4,000 feet (910–1,220 m).
[12] Climate in the San Diego area often varies dramatically over short geographical distances, due to the city's topography (the Bay, and the numerous hills, mountains, and canyons), thus exhibiting microclimate: frequently, particularly during the "May gray / June gloom" period, a thick "marine layer" cloud cover will keep the air cool and damp within a few miles of the coast, but will yield to bright cloudless sunshine between about 5 and 15 miles (8 and 24 km) inland—the cities of El Cajon and Santee for example, rarely experience the cloud cover.
The Santa Ana winds are actually warm due to barometric pressure increases: As the air is pulled down to sea level from the higher altitudes of the Great Basin and Rocky Mountains to the east, it is compressed and its temperature rises.
[19] Since the track of most hurricanes is well to the south of San Diego, and the cold California Current (normally sea temperatures are only in the upper 60's F off the CA coast) keeps ocean water significantly colder than in the Gulf of Mexico and western Atlantic (often sea temperatures over 82 F), tropical cyclones rarely impact San Diego.
Most northwestward moving tropical cyclones dissipate over Baja California and fail to bring any precipitation to San Diego.
During the El Niño phase, San Diego receives more winter storms with warmer and more humid conditions.