My first thought: "In the RAIN? But listening to the weather it sounds like it's going to ease off for now. Going on 5 or 101? Just make sure you are driving outfit is something comfortable. I don't even remember owning boots when we lived in LA, but I do remember rain.
Have a fun trip. From what I can tell it will be warmer there than in the Bay area.
How will your Mom do? Will she need to stop more frequently? I find as I am older now I can only stand to sit in a car for about 1 1/2 hours before I need to stop and just get out and walk around a bit.