San Francisco to Barcelona is on sale with all three alliances. Oneworld and SkyTeam offer fares in the $409 to $438 price range for travel departures from mid-September all the way to July 2019 end of schedule dates. Star Alliance ticket prices drop in last week of October 2018 to March 2019.
Summer 2019 fare deals
The significance of this fare is availability of low fares to Europe past mid-June is not common. Typically airfares rise hundreds of dollars for departures to European destinations in mid-June for the summer season. The advance planner can lock in a low $400s ticket now for travel in summer 2019.
Google Flights shows fares for June 2019 as mostly phone call fares. When I see this happen, that is when I go to Momondo.com.
American Airlines San Francisco – Barcelona
FlightsBank online travel agency
San Francisco SFO – New York JFK – Barcelona BCN – Chicago ORD – SFO
American Airlines $413.39 round trip Tue June 25 – Tue July 9
Air France September 2018 Fare Deal
Air France SFO – Paris CDG – Barcelona BCN – CDG – SFO
All Delta flights can be ticketed for $423.41 round trip on these same dates.