Yes Carole, I think it is Colossus !
For So. Cal in January, most likely in the mid 60's and possibly some rain.
You can get some basic weather info at weather.com and check on the 30 day forcast.
Unfortunately in the next couple of weeks they are predicting a few more storms every few days.
Magic Mountain is at the bottom of a mountain range that actually gets snow at times during the winter so bring both moderately warm and cool clothing.
Are you going to be spending quite a while out here? There are quite a few interesting parks just about an hour or two south of Valencia that are fun - only moderately "fun" coasters, but worth seeing.
Check out websites for Universal Studios, Knottsberry Farm, Disneyland and California Adventure. They are all between Hollywood and Anaheim. The beaches south of LA are nice also as is the drive on Highway 1 along the coast.
Not sure what you like to see or which way you will be going, but up here in No. Cal we have some "good stuff" also.