In California, it is still warm in mid-September, with current temperatures around 77°F (25°C) and weather forecasts indicating continued warm to hot conditions for the next week. Typically, cold weather in California, especially in coastal and southern areas, starts arriving in late fall to early winter, around November to December, depending on the region. Northern and mountainous areas may experience cooler temperatures earlier. So, expect colder weather to begin gradually from late November onward in most parts of California.