Weather / Clothing
August and September are the hottest months; highs can hit the mid 80s. In winter highs reach the upper 60s, but it can be rainy and cool (particularly at night). The average year-round LA 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, particularly the San Fernando Valley, which 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 intern 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!
Fall season: The average temperatures in the month of September are in the high 70s so be sure to pack lightweight clothing. As the season progresses, you will need mid-weight jackets and lightweight sweaters. You should think "layers of clothing" when packing, for the drearier days. A water proof jacket, umbrella and warm jackets are a plus for November. A winter coat is not necessary. Long sleeved shirts and sweaters work well for the end of fall term.
Winter: January through March are the coolest and wettest months in Los Angeles. Temps will run from the high 40s to the 60s. Bring warm jackets and umbrellas for the cold and rain but also bring layers for days when the sun decides to shine. Long sleeved shirts are always a plus because sleeves can be rolled up and down depending on how the day progresses. When coming to LA the best advice is to pack "layers". That way, any weather occurrence can be easily dealt with.
Summer: Temperatures will mostly be in the 70s and 80s. For the most part the days are warm, but nights can sometimes get cool. Pack shorts and daytime wear for weekends and lightweight professional clothing for your internship (follow the dress code). Sweatshirts and pants work well in the evenings, especially near the water.
Since you will spend many hours at your internship site, you should be prepared to dress more professionally than you might be accustomed to as a student.