monterey bay, ca: Annual Weather Averages. August is the hottest month in monterey bay, ca with an average temperature of 18°C (64°F) and the coldest is January at 11°C (52°F) .onterey bay, ca: Annual Weather Averages. August is the hottest month in monterey bay, ca with an average temperature of 18°C (64°F) and the coldest is January at 11°C (52°F) .