Unfortunately, the simple answer is you need to be prepared for just about everything, as others suggest. It can be very chilly first thing in the morning, but then heat up to the 70s before cooling off again. You may also get rain at this time of year (of the persistent, drizzle variety, rather than the short, sharp showers which can be dodged), so a raincoat is also pretty much essential. December has already seen a real mixture (although it hasn't been that cold at any stage), and Jan/Feb are usually similar (and colder).