10am departure from residence in Aberdeen to visit son and family in Tenino wa with stops in Olympia black lake area proceeding to Tenino via interstate 5 after. observed military helicopters (pair) in same area on outbound trip to Olympia (approx. 10:30am). my return trip to Aberdeen is when sighting occurred. visibility were restricted due to inclement weather, heavy to moderate rainfall. at time of observation of lights I had just passed the last exit's (overpass of hiway 12 and 8). moments after passing the area of Elma Wa something caught my attention in my drivers door mirror that caused me to turn my head and look. trailing behind, left of my vehicle as it/we were heading west was a bright blueish green light .. at that time it were paralleling the highway. there is a local airport for small planes at Elma near where the sighting first occurred. initially I thought it were a plane but the lights were constant and no flashing aviation lights were observed what so ever at any time. within a couple minutes it was directly adjacent to my drivers door window a few hundred yards south of my location at an elevation of from between 100 and 300 ft. unknown what the cloud ceiling was at the time but it remained under that. as my self and the other vehicles on the road were approaching the city of Montesano Wa the object veered north and crossed in front of my vehicle, between my own and the vehicle ahead which was at least 400 to 500 yards in front of me and was clearly visible for a matter of scant seconds. it was at this time I had the sense that there were more to it than merely a blue green light. there were discernable features but indistinct still due to weather? want to say, glass like, or silver, reflective. unknown as to whether my feeling I were looking at a light in the center of an inverted glass bowl were correct but that was my impression. the object passed directly in front of my vehicle at no less than 200ft and then was out of sight due to trees. in light of my statement earlier that weather is/was a factor it was indeed but, new vehicle, new glass/new windshield wipers, made for as clear a view as is possible from a motor vehicle.