The Japanese government has estimated about 70 percent of the debris sank, but CNN reported nobody's certain how much of the remaining 1.5 million tons of debris is still floating in the Pacific Ocean.
Buoys, a soccer ball, an unmanned fishing trawler and other items that float on the surface and can be pushed by the wind have turned up.
Nancy Wallace, program director and division chief of the National Oceanic and Atmospheric Administration's Marine Debris Program, said models show the outer edge of the debris is at the West Coast and Alaska and most of the debris is north of Hawaii, moving east slowly.
Lynne Talley, a physical oceanographer with the Scripps Institution of Oceanography in San Diego, said the debris is now scattered over 4,000 miles in one direction and 1,000 miles in another area.
"The front edge of it has gotten here and the whole cloud should arrive at the end of another year," Talley said.
NOAA said it's unlikely radioactive material from the Fukushima nuclear power plant would make landfall.
Curt Hart, communications manager at the Washington state Department of Ecology, said oil or chemical drums or fuel cylinders could wash ashore.
U.S Coast Guard Lt. Regina Caffrey, a public affairs officer in Washington state, said the Coast Guard would focus on pollution threats and potential impediments to navigation.
Wallace said it's difficult to predict where debris would come ashore.
"We can only forecast a few days into the future because of wind and current," Wallace said. "We expect there to be an upswing in debris on the shore. The interesting thing is we don't know where it's going to be, and we think it is going to be spread out."
The March 11, 2011, earthquake and tsunami in Japan crippled the Fukushima nuclear plant, destroyed homes and killed thousands of people.