My Olypmic National Park hiking vacation starts soon -- July 12. I've never been to ONP and I realize it's likely to rain during my 5 days there. I have a tiny umbrella and a plastic poncho that's no good in the wind. I've got jeans and hiking boots that I waterproofed. I won't start a hike if it's raining. I hate to pack or carry much with me but is there something I really should buy for the trip? Will I want to wear jeans or shorts? Thanks!

Depends on how much you mind being wet.

You might get a heavier-duty poncho as a start. If you're overnighting in Seattle or Portland before your trip a short stop at the REI co-op in either would be a fun place to get one (especially the Seattle HQ - amazing store.)

Jeans.

If its raining, jeans are a bad idea. Even in the summer, you could get chilled from wearing soaked cotton clothing. At the least, a poncho long enough to protect your upper legs is a good idea.

Are you going overnight, or just day hikes? That would make a huge difference in terms of clothing recommendations. If overnight, I think that you need rainpants and a decent rainjacket. If day hikes, then a nylon poncho might suffice.

No jeans...for any hiking, cotton or heavy clothes that don't dry well aren't the most comfortable. I've always found "tiny umbrellas" to be a pain b/c you still have to carry them somewhere. A good rain jacket with a hood is great and it's something you can always use on a day to day basis (depending on where you live). Agree that a stop to REI is a must if you don't have the right gear before you leave home.

I recommend that you buy a nice, lightweight rain jacket with a hood, that folds up into it's own pocket. I always take a daypack when I hike, and if there seems to be any chance of rain, I throw my rainjacket in - it's compact when folded up, and doesn't take up much space. A daypack is a good idea, because you can add or subtract layers as the temperature or weather warrents, and it gives someplace to carry them - also extra water, snacks or lunch, small first aid kit, etc.

I also agree that jeans are a bad idea if it's raining. Something that dries quickly, like nylon or microfleece, is much better. As for the shorts vs long pants question, I really like the convertible pants with the legs that zip off and turn into shorts. They are quick and easy to take on and off (there are zippers at the bottom, too, so you don't have to take off your boots). You can get those and the raincoat (and a daypack, if you don't have one) at REI.

Jeans, and all cotton clothing, are a bad idea in any weather for hiking in the PNW. When you get sweaty, they get wet, and cotton dries slooooowly. If caught in bad weather this actually can lead to hypothermia as you will be cold and wet.
Like others said, you need a water resistant parka and synthetic shirt and pants. And, wool or synthetic socks-water can come in the tops of your boots and you'll be miserable in cotton socks. Even in the rain with this stuff you'll be perfectly comfortable.