Not sure about the rain.
Just north of the border, at the Mt. Ashland/Hwy 273 exit. It's only about 5 miles from California.
Calahan's Restaurant sits very close to I-5 and is a great 'taste of civilization' if you've been out for a while (but won't be as impressive if you're just setting out). They are not open on Mondays.
Follow 273 down to 66 and you'll find a commercial campground - think it was a KOA last time I was there. Makes for an ok place to spend a night just prior to insertion.
Ashland itself is about 4 miles away (take 66 NW and cross I-5, you'll be on the south side of town), and has resources for backpackers (equipment, trailfood, etc), which makes it good for those "Oops, you know what I forgot?" emergencies.
I would take a new copy of this book: www.wildernesspress.com/book123.htm