The Flash Season 2: 11 Big Questions We're Asking After 'Trajectory'
8. Could This Tie Into Supergirl?
Speaking of Opal City, it's also a notable location because of another show that is (sort of) connected to the Arrow/Flash universe: Supergirl.
On the CBS show it is the home of Adam Foster, the son of Cat Grant who had a brief relationship with Kara. The much-anticipated Supergirl/Flash crossover is happening soon, and this could just be another subtle tease ahead of that.
It's expected that the explanation will be National City existing on another Earth, and that does seem the logical solution, but that in itself indicates doppelgängers of Kara Danvers and co. on Earth-1, and it would be fun if the show were to feature those in some way.