Conditions vary greatly from winter to winter and from week to week in the winter; so it's really hard to tell you what conditions will be like. There might be a lot of snow or just a minimal amount of icy snow. In the winter, you need to prepared to boil snow for water, but sometimes you will find water available.
My favorite winter overnighter in Southern California is San Gorgonio via the north side. You may need snowshoes, axe, and crampons depending on conditions. The route is really gentle until the top part of the chutes on the north face and then it can gets a bit steeper and might be icy. You can camp at Dry Lake (rather cold there), or if you travel more directly toward the north face from South Fork Meadows and camp in the gentle area before you get to the chutes on the North Face at about 9200'. If there is enough snow to cover the bushes, the trek can be as short as 13 or 14 miles round-trip with about 4700' elevation gain. If snow conditions are not good and you find it is better to stick to the trail, the mileage can be significantly longer but the elevation gain remains the same. There is a good chance you can pick up the water you will need for the night at South Fork Meadows, but you have to worry about the water freezing overnight. Just make sure you pack a stove and fuel.
Here's a couple of photo albums with maps reflecting routes taken in the winter:
http://kathywing.smugmug.com/California ... -215-21613
http://kathywing.smugmug.com/California ... -via-South
http://kathywing.smugmug.com/California ... onio-41011
San Gorgonio can also be done as a day trip in the winter depending on conditions and how strong you are. There are also other great options on the north side of San Gorgonio such as Jepson Peak and Charlton Peak.
Another great winter climb in Southern California is Mount San Antonio (Baldy) via the Bowl if there is enough snow. It's easily done as a day climb and really enjoyable eventhough it can be crowded on the weekends.