Jolting portent (Ultimate combat) and concentration check

[Brona_80]

If a caster fails the fortitude save for jolting portent he must make a concentration check? The spell damage applied before the caster action?

[MurphysParadox]

I believe the casting still occurs and then the fortitude save has to be made. It won't interrupt casting as it says it triggers on "casts a spell" not "starts casting a spell".