Did Ethniu knock the power out in Chicago way back in Dead Beat?
We've suspected that Cowl is working with/for the fomor for a long time.
And in Dead Beat, Harry says he assumes Cowl did it -- but "If you'd have asked me yesterday, I wouldn't have thought this was possible. I don't know how he did it, but... his magic is stronger than mine. And from what I saw of his technique, he's a hell of a lot more skilled, too."
Perhaps... Cowl isn't quite as omnipotent as he appears. Like Harry, he cheats and gets huge effects by leveraging friends and favors.
Further support: When Harry's talking to Mab, by the lake, we get this:
Thunder rumbled over the lake. Off in the southwest, lightning leapt from cloud to cloud.
Mab turned her head to watch. "Interesting."
"Uh. What's interesting?"
"Powers at work, preparing the way."