I was driving west on interstate 90 nearing north bend when I noticed a strobe like white light coming g from over the mountains. The strobe flashed about every two seconds. The light would get lighter and darker like it was shifting east and west over the top of the interstate. My girlfriend was sleeping in the car next to me and after about two minutes I woke her up to make sure I wasn't going crazy. We had previously been attending a concert so I thought my eyes might be light sensitive. She woke up and saw the lights, about a minute later we rounded a corner that brought us around the side of a mountain and the light became much more visible. The stobing light seemed to go mostly up into the sky and was incredible bright. The source seemed to be behind trees on the left side of the freeway. Then suddenly for one strobe the source of the light came compleately into view. It was a perfect sphere and shone a brilliant white light. It was so bright that it appeared to have black lines on it but could have just been my eyes not able to handle the brightness. It felt similar on the eyes to staring into the sun. After about a two second strobe it disappeared and when it next appeared was partially behind some trees back off to my left. It continued to travel the same direction until going out of view behind me. I saw the flash/ strobe re appear in my rear view mirror momentarily about thirty seconds later. This craft was moving extremely speraticly east and west over the interstate and then changed coarse to the left of the interstate after coming into my view. The sphere seemed to disappear between stobes and then re appear at a slighly different altitude or on a different coarse next time it appeared. Me and my girlfriend were both shocked and agreed it was a ufo and most likely technology we would never see again in our lifetime. Also it began to rain as we were close to the craft. Stopped as soon as it went out of view. Only rain I've seen all summer in Washington