August and September are the hottest months; highs can hit the mid-80s. During the winter, highs can reach the upper 60s, but it can be rainy and cool (particularly at night). The average year-round temperature is around 70°F. While the weather is usually pleasant and sunny, at certain times of the year, it can be unpredictable. Offshore breezes keep the beach communities cooler in summer and warmer in winter than those farther inland; the San Fernando Valley, for example, is the hottest area in summer and the coldest in winter.
Be prepared for anything (except for snow—you shouldn't need to worry about that!).
Since you will be spending many hours at your internship site, you should be prepared to dress more professionally than you might be accustomed to as a student. Some internship sites are very business-like and some are more informal. You will eventually need to adapt to the environment you find yourself in. Do not, however, arrive for your first day at work in an old pair of jeans!
The average temperature in September is in the high 70s, so be sure to pack lightweight clothing. As the season progresses, you will need midweight jackets and lightweight sweaters. Be sure to pack layers of clothing for the drearier days. Bring a waterproof jacket, umbrella, and warm jackets for the end of the semester; however, a winter coat is not necessary.
January through March are the coolest and wettest months. Temperatures run from the high 40s to the 60s. Bring warm jackets and an umbrella for the cold and rainy weather, but also pack layers of clothing for days when the sun decides to shine.
Temperatures remain in the 70s and 80s. For the most part, the days are warm, but nights can sometimes get cool. Pack shorts and casual clothing for weekends and lightweight professional clothing for your internship (follow the dress code). Don’t forget sweatshirts and pants for the evenings, especially near the water.