A National Weather Service cooperative weather station has been in operation in San Gregorio since June 1, 1954. San Gregorio enjoys mild weather throughout the year, except for some remarkably chilly mornings, even in the summer. Fog and low overcast are common in the night and morning hours, too, especially in the summer. The fog usually clears to the shoreline by the afternoon. Temperatures are often much warmer just a few miles inland than on the coast. Most of the rainfall falls from November through April.
Normal annual precipitation is 29.52 inches (74.98 cm). San Gregorio has an average of 92.3 days with measurable rain. The wettest year on record was 1983 when 57.86 inches (146.96 cm) of rain fell. The wettest month recorded was 12.13 inches (30.81 cm) in February 1998. The maximum 24-hour rainfall was 6.87 inches (17.44 cm) on December 23, 1955. Although snow is very rare in the coastal lowlands, 4.0 inches (10 cm) fell in San Gregorio on January 21, 1962.
January is the coolest month with an average high of 60.1 °F (15.6 °C) and an average low of 40.3 °F (4.6 °C). September is the warmest month with an average high of 72.5 °F (22.5 °C) and an average low of 49.3 °F (9.6 °C). The highest temperature on record was 99 °F (37 °C) on October 5, 1987. The lowest temperature on record was 20 °F (-6.6 °C) on December 20, 1998. Temperatures exceed 90 °F (32 °C) on an average of only 1.8 days, but have occurred in April, May, June, July, August, September, and October. Freezing temperatures (32 °F / 0 °C) or below are more common, occurring on an average of 13.1 days, and have been recorded in January, February, March, April, May, June, October, November, and December.
Named after Pope Gregory I (Saint Gregory the Great), San Gregorio was a booming town in the 1850s, when wealthy San Franciscans would travel to the San Gregorio House by stagecoach to enjoy fishing, hunting, sea bathing, and boat races. The building still stands, but is no longer a functioning hotel. However, the San Gregorio General Store has been thriving since 1889. It houses eclectic items for sale, from books to clothing to groceries. Its bar serves sarsaparilla and hot chocolate as well as alcoholic beverages, and is frequented by local musicians. The original stagecoach stop stands across Highway 84 from the store. Recently, San Gregorio has become a favorite among filmmakers looking for a "funky" historical backdrop.