You asked about day-trip skiing during a January visit to L.A. Your choices are the following: Mt.Baldy and Mountain High in the San Gabriel Mountains.

http://www.mtbaldy.com and http://www.mthigh.com/

And Snow Valley, Snow Summit, and Bear Mountain in the San Bernardino Mts.

http://www.snow-valley.com/ http://www.bigbearmountainresorts.com/

Snow Summit and Bear Mountain are owned and operated by the same company and are the largest of So. Cal. ski areas. Mountain High is the quickest access from a freeway (least amount of mountain driving). Mt. Baldy is closest to Pasadena, as the crow flies, but involves more mountain driving than Mountain High. Snow Valley is lowest in elevation and its snow coverage usually reflects that.

These areas have extensive snow-making capability so they offer quite a few runs (considering their relatively low elevations and latitudes). But Mother Nature can definitely impact the quality of skiing, particularly at that time of year. Also, be aware that the ratio of snowboarders to skiers in So. Cal. resorts can often approach 4 to 1.

John in San Diego