Lightning without thunder

I was watching something last night when I noticed some occasional flashes of light outside the window.

"Oh, are there fireworks for something in town? I don't remember there being anything on..."

Going to the bathroom window to look reveals that no, it's the clouds to the north being lit up by lightning. I can see the lines of some of the electical discharge as well as some general diffused flashes where the lightning was deeper into the clouds.

I can't hear a single roll of thunder though, so it's clearly some distance away.

Looking at a lightning tracker website reveals that it's happening at least forty miles away...