Barcelona is a city I frequently find is the cheap spot for travel from San Francisco to Europe in 2019. Today there are summer fare deals available with American Airlines and oneworld alliance partners for travel departures SFO-BCN in June and July for as low as $483 round trip.
Google Flights shows these fares, but generally does not provide a deep link for summer dates I checked. I was able to pull up prices from $483 to $530 through Finnair’s website and Skyscanner.com searches of online travel agencies.
Skyscanner search with Fareboom.com ticket
American Airlines operated flights with Finnair codeshare
San Francisco SFO – Charlotte CLT – Barcelona BCN – Miami MIA – SFO
Finnair website ticket for AA-operated flights
San Francisco SFO – Charlotte CLT – Barcelona BCN – New York JFK – SFO
Summer 2019 is looking like a good year for discount airline tickets to Europe as long as you can manage with carry-on only bags.