Something like 3 years worth of rain in 3 minutes. Maybe not quite that bad but an awful lot in a very short time. The ground is baked hard as concrete so all that rain doesn't seep into the ground. It runs off as it would on concrete or Tarmac. In an enclosed place like a stadium it was probably too much for the drainage system to handle and hence the layer of water.

Devon and Cornwall experienced a new phenomenon, namely, flash floods that came and went inside 10 minutes. Of course these wild extremes of weather have nothing whatsoever to do with climate change, now, do they?