Road trip

Hi, I'm new to the forums. Read a lot. Searched some posts, but still need an advices from the pros.
I'm going on a little road trip along the west coast (US) covering WA, OR and CA.
I'm fairly new to photography and picked up my XSi this spring.
At the moment I'm planning to take my Canon 17-55/2.8 and 50mm/1.8 prime.
I want to take some landscapes, some city imagery and maybe some people. Given that it's going to be overcast at this time of the year or even rainy, do you think I can get away without a decent zoom?
I have a 100/2.8 prime (borrowed from a friend, just to play with macro), but that's about it.
Any suggestions?

Thank you in advance.

Sounds like you are fairly new to photography. Welcome. At this point, just use what you have and learn. Overcast days can produce wonderful images. Just avoid too many shots with the sky being predominant (unless it contains interesting elements). The 100 macro will enable you to focus on small things which benefit from flat lighting. You'll probably get posts that say you absolutely need more lenses and equipment. Ignore them.
The Oregon coast is one of the richest photographic environments on the planet. The haystacks, dunes, and bluffs are magnificent all along the coast. If you have a chance to go inland, don't miss Crater Lake.

Looks like you have all the lenses you need. I would suggest buying a circular polarizer for the 17-55. If you buy one make sure to get a good one, I like Tifen or Hoya.

Thank you guys for your suggestions. The trip was a blast, although we haven't spent too much time on Oregon coast. Oh, well, some other time.
I definitely need a circular polarizer as well as neutral density gradient filter. I've noticed so many times the sky was blown out whereas foreground was too dark.