PORTLAND, Ore.—Alaska Airlines canceled dozens of U.S. West Coast flights Friday as off-duty pilots picketed in several major cities over an impasse in nearly three years of contract negotiations, and the airline said more cancellations were possible through the weekend. More than 120 Alaska Airlines flights were canceled—about 9 percent of its operations—and about 15,300 passengers were affected, the airline said in a statement. Canceled flights included 66 in Seattle; 20 in Portland, Oregon; 10 in Los Angeles; and seven in San Francisco, according to the flight tracking website flightaware.com. At the same time, pilots held a rally and picket line in all those cities, according to a union website. Pilots with the Air Lines Pilots Association also protested last week in New York City outside the airline’s Investor Relations Day in a precursor of Friday’s picket. Those on the picket line Friday said they were off-duty and were not …