The Flash will be visiting Supergirl's own National City next month and filming is under way for the crossover episode.
The Flash and Supergirl are both shows based on DC Comics characters and share some portion of their creative team — most notably executive producer Greg Berlanti — but Supergirl airs on CBS rather than the CW, so there were some hurdles to leap before locking down a crossover. Certainly more than previous teamups between The Flash and Arrow, which both air on the CW. The news that the two networks had worked out a deal that would allow Grant Gustin's Barry Allen to guest star on Supergirl was broken only last week.
Gustin has been posting pictures from the set of the shoot. They don't offer many details, but they do feature him in costume alongside Supergirl star Melissa Benoist.
I'm pretty sure the superpowers they have in common are speed and being totally adorable.