This definitely looks terrifying for the audience! The Google Translate of this page says:
The organization has been watching the weather. Around 9 employees saw a storm approaching storms on the radar.The organization had no indication of the unprecedented severity of the storm that came down on the festival grounds
Maybe Google translate's getting this wrong, but if not have to say I"m getting pretty sick of this pathetic "no warning" excuse. Doppler radar gives you the speed of the movement of the precipitation! Meteorologists and even amateur storm chasers like me can read this indication and give you at least a rough idea of the wind speed. Also, take a look at this:
This video is apparently of the same storm over the festival site. If this is the same storm, even to the untrained eye, this should look terrifying; the more experienced observer can see all kinds of signs of the power of this storm (lowering, wall cloud, raged-looking shelf cloud, etc).
I'm sorry to say that I've now posted so many of these tragic entries that I have now made a "show safety" category. Click that link to see all these entries in this sad, unnecessary series.
On the other hand, there are indications, after the Indiana tragedy, that some people are getting this right, like the recent evacuation of the Lollapalooza festival in Chicago.
Thanks to Johannes Mulder for the heads up!